# BET surface area and gas sorption: isotherms, pore size and how to report them

> BET surface area explained: N₂ sorption at 77 K, IUPAC isotherm types, the BET range and Rouquerol criteria, pore size, degassing and limits for MOFs.

URL: https://www.labcanvas.com/techniques/bet

Gas sorption measures how much gas, usually nitrogen at 77 K, a degassed solid takes up as the pressure rises, giving an isotherm from which surface area, pore size and pore volume are calculated (Abid et al. 2023). The Brunauer–Emmett–Teller (BET) method, developed in the 1930s for open surfaces, turns the low-pressure part of that isotherm into a monolayer amount and a "BET area", and is now the most widely used surface-area metric for micro- and mesoporous materials (Osterrieth et al. 2022). For microporous solids such as MOFs, where pores fill rather than form layers, the result depends strongly on the pressure range chosen, so it should be calculated with consistency criteria (Gómez-Gualdrón et al. 2016; Osterrieth et al. 2022).

Characterisation techniques Published 27 October 2026 Last updated 27 October 2026 11 references

![BET: Surface area, pore size and pore volume (N₂ isotherm at 77 K)](https://www.labcanvas.com/materials/bet/bet-cover.webp)

**Contents**

**Contents**

- Gas sorption and BET surface area at a glance
- Gas sorption and BET surface area instrument drawing (high-resolution SVG)
- Gas sorption and BET surface area data: an example plot to edit
- How BET gas sorption works
- What the data looks like and how to read it
- Sample preparation: degassing and activation
- What BET tells you about materials
- Common mistakes and limits
- How to draw the BET setup and data in LabCanvas
- Probe gases for physisorption compared
- Related techniques
- More on LabCanvas
- Frequently asked questions
- References

## Gas sorption and BET surface area at a glance

| Probe | An adsorbing gas: N₂ at 77 K for pores larger than about 0.7 nm, Ar at 87.3 K for smaller pores, CO₂ at 273 K below 0.45 nm (Abid et al. 2023) |
| --- | --- |
| Signal measured | Amount of gas adsorbed and desorbed versus relative pressure P/P₀, by volumetric (manometric) or gravimetric methods (Abid et al. 2023) |
| Main information | Specific surface area, pore size and pore volume (Abid et al. 2023) |
| BET method | Developed in the 1930s for open surfaces; now the most widely used surface-area metric for micro- and mesoporous materials (Osterrieth et al. 2022) |
| Default BET range | Instruments often use about 15 points over P/P₀ ≈ 0.05–0.30, which can be wrong for strongly microporous solids (Morgan et al. 2026) |
| Range selection | Four consistency (Rouquerol) criteria guide the choice of pressure range (Gómez-Gualdrón et al. 2016) |
| Reproducibility | 18 isotherms analysed by 61 labs gave a wide range of BET areas from identical data (Osterrieth et al. 2022) |
| Pore classes | Micropores < 2 nm, mesopores ≥ 2 nm (Shulga et al. 2021) |
| Isotherm types | Six types in the IUPAC classification (Bingel and Walton 2023) |
| Total pore volume | From the N₂ uptake near P/P₀ ≈ 0.995 (Morgan et al. 2026) |
| Degassing (MOFs) | Below 75% of the decomposition temperature (Abid et al. 2023) |
| Typical values | Double-sided graphene 2620–2675 m² g⁻¹; MOFs 1000–10,000 m² g⁻¹ (Shulga et al. 2021) |

## Gas sorption and BET surface area instrument drawing (high-resolution SVG)

![Gas sorption and BET surface area instrument drawn as a LabCanvas Library object](https://www.labcanvas.com/materials/bet/bet-drawing.webp)

A gas sorption analyser as a LabCanvas Library instrument: the sample tube in a liquid-nitrogen dewar, the gas supply and an isotherm on screen. Change colours, labels and view in the figure editor.

[Edit the drawing in LabCanvas](https://www.labcanvas.com/editor.html?add=ch-bet)[Download SVG](https://www.labcanvas.com/materials/bet/bet-drawing.svg)[Download PNG (3000 px)](https://www.labcanvas.com/materials/bet/bet-drawing.png)

## Gas sorption and BET surface area data: an example plot to edit

![Gas sorption and BET surface area data plot drawn as a LabCanvas Library object](https://www.labcanvas.com/materials/bet/bet-plot.webp)

A nitrogen isotherm at 77 K as an editable LabCanvas plot: amount adsorbed against relative pressure p/p₀. Edit the data, labels and colours in the figure editor.

[Edit the plot in LabCanvas](https://www.labcanvas.com/editor.html?add=mof-chart-n2-iv)[Download SVG](https://www.labcanvas.com/materials/bet/bet-plot.svg)[Download PNG (3000 px)](https://www.labcanvas.com/materials/bet/bet-plot.png)

## How BET gas sorption works

A clean, degassed sample is cooled, usually to 77 K for nitrogen, and gas is dosed in steps. At each relative pressure P/P₀ (pressure divided by the saturation pressure of the gas) the instrument records how much gas is adsorbed, then reverses to record desorption. The result is an **adsorption–desorption isotherm**. Adsorption is measured either volumetrically (manometrically) or gravimetrically; volumetric instruments are preferred for nitrogen, argon and krypton at cryogenic temperatures such as 77.4 K and 87.3 K (Abid et al. 2023).

The **BET method** fits the low-pressure part of the isotherm to obtain the amount of gas in one monolayer and a BET constant C, and converts that monolayer amount into a surface area, the "BET area" (Gómez-Gualdrón et al. 2016; Morgan et al. 2026). The theory was derived for multilayer adsorption on flat, open surfaces (Walton & Snurr 2007; Osterrieth et al. 2022) and assumes a uniform surface (Shulga et al. 2021).

**Choice of gas.** N₂ at 77 K is the routine choice when molecules can reach the pores (wider than about 0.7 nm); argon at 87.3 K is preferred for pores below 0.7 nm, CO₂ at 273 K for ultramicropores below 0.45 nm, and krypton at 77 K for samples with very low surface areas (Abid et al. 2023). Because N₂ diffuses slowly at 77 K, equilibration can take a long time (Abid et al. 2023).

## What the data looks like and how to read it

An isotherm plots amount adsorbed against P/P₀ from 0 to 1. Its shape tells you the kinds of pores present (Abid et al. 2023), and the IUPAC classification, most recently updated in 2015, sorts isotherms into **six types** (Bingel & Walton 2023). The ones you meet most often:

- **Type I**: a steep rise at very low relative pressure, then a plateau. The steep uptake is the filling of micropores (Morgan et al. 2026). Extra uptake and a gap between adsorption and desorption at higher pressure show that a sample also has mesopores and is not purely Type I (Morgan et al. 2026).
- **Type IV**: uptake that continues at intermediate pressure with a hysteresis loop between adsorption and desorption, interpreted as mesoporosity; a Type IV isotherm with an H3 loop was reported for zeolite A crystals, for example (Madhu et al. 2024).
- **Types III and V**: Type III is seen, for example, for water on a hydrophobic carbon; Type V has a step, common in water adsorption and in flexible MOFs with gate-opening behaviour (Bingel & Walton 2023). For water, isotherms change from Type I for strongly hydrophilic solids to Type V for more hydrophobic ones (Bingel & Walton 2023).

Traditional isotherm models such as Langmuir, Freundlich and BET work well for Types I, II and IV (Bingel & Walton 2023). From the same isotherm, total pore volume is taken from the uptake near P/P₀ ≈ 0.995, the t-plot separates micropore area and volume, the Barrett–Joyner–Halenda (BJH) method gives mesopore sizes and the Horváth–Kawazoe method a model-dependent micropore size; density functional theory (NLDFT or QSDFT) with a suitable pore model gives a more rigorous **pore size distribution** (Morgan et al. 2026; Shulga et al. 2021). Advanced DFT and molecular-simulation methods now allow reliable pore size, volume and connectivity analysis, and mercury porosimetry adds macropores (Thommes & Schlumberger 2021).

## Sample preparation: degassing and activation

The pores must be empty before measurement. **Degassing** conditions, temperature and evacuation rate, are critical to keep a framework intact; for MOFs the degassing temperature should stay below 75% of the decomposition temperature, which [TGA](https://www.labcanvas.com/techniques/tga-dsc) provides (Abid et al. 2023). As one documented example, porous carbons were degassed under vacuum for about 24 h, ramping to 90 °C and then 200 °C (Morgan et al. 2026).

**Activation** removes solvent and unreacted precursor from the pores without collapsing them. Activation by heating alone can leave a lower surface area through partial breakdown or blocked pores, whereas solvent exchange works better: [HKUST-1](https://www.labcanvas.com/mof/hkust-1) rose from about 1100 m² g⁻¹ after heating to about 1700 m² g⁻¹ after methanol exchange (Abid et al. 2023). [FTIR](https://www.labcanvas.com/techniques/ftir) can confirm that coordinated solvent and linker residues have gone after activation (Abid et al. 2023).

Handle samples gently: [UiO-66](https://www.labcanvas.com/mof/uio-66) lost much of its surface area after mechanical compression at about 1.9 GPa (Abid et al. 2023).

## What BET tells you about materials

**MOFs.** Gas sorption is how the "trademark" porosity of metal–organic frameworks is reported (Abid et al. 2023). Synthesis matters: [MOF-5](https://www.labcanvas.com/mof/mof-5) made in diethylformamide gave 2766.5 m² g⁻¹ but only 1032.4 m² g⁻¹ when made in dimethylformamide (Abid et al. 2023). The highest reported values exceed 7000 m² g⁻¹, such as NU-110E at 7140 m² g⁻¹ (Abid et al. 2023). Molecular simulations show that BET areas from N₂ isotherms can match the geometric surface areas of MOF crystals (Walton & Snurr 2007), and comparing a measured BET area with one calculated from a simulated isotherm tests whether a sample is fully activated (Gómez-Gualdrón et al. 2016; Wang et al. 2015). Check crystallinity first with [XRD](https://www.labcanvas.com/techniques/xrd).

**Thin MOF films.** Films such as [ZIF-8](https://www.labcanvas.com/mof/zif-8) contain too little material for a standard measurement; krypton physisorption, quartz-crystal microbalance gravimetry, ellipsometric porosimetry and positron annihilation are used instead (Stassin et al. 2021).

**Carbons and graphene.** The surface area of double-sided graphene, 2620–2675 m² g⁻¹, is a useful yardstick: activated carbons above it must contain many nanoscale pores and inhomogeneities (Shulga et al. 2021). See [graphene](https://www.labcanvas.com/2d-materials/graphene).

**Zeolites.** For zeolite A, N₂ sorption at 77 K showed microporosity at low relative pressure together with a Type IV isotherm and H3 hysteresis loop, read as additional mesopores (Madhu et al. 2024); see [zeolites](https://www.labcanvas.com/zeolites) and [ZSM-5](https://www.labcanvas.com/zeolites/zsm-5). Pair surface area with [SEM](https://www.labcanvas.com/techniques/sem) for crystal size. All techniques are compared on the [characterisation techniques](https://www.labcanvas.com/techniques) hub.

## Common mistakes and limits

- **Using the default pressure range on a microporous solid.** A fixed P/P₀ window of about 0.05–0.30 can be wrong for strongly microporous materials: for one porous carbon it gave a negative BET constant, while a range chosen by the Rouquerol criteria (P/P₀ = 0.0051–0.0597) gave a physically consistent area of about 944 m² g⁻¹ instead of 777.5 m² g⁻¹ (Morgan et al. 2026).
- **Trusting R² alone.** A high linear correlation coefficient does not make a BET fit physically valid; check that C is positive and the consistency criteria hold (Morgan et al. 2026).
- **Treating a MOF BET area as a true geometric area.** In microporous MOFs adsorption can occur by pore filling rather than layer formation (Walton & Snurr 2007). Even when all consistency criteria are met, BET can overestimate the monolayer loading, especially in MOFs that combine mesopores (≥ 20 Å) with large micropores (10–20 Å) (Gómez-Gualdrón et al. 2016); for NU-1103 the overestimate was about 16% (Wang et al. 2015).
- **Reporting without the analysis details.** When 61 labs calculated BET areas from the same 18 isotherms, the results spread widely; the BETSI software was developed to make the range selection unambiguous (Osterrieth et al. 2022). Report the pressure range, C value and criteria used.
- **Mixing BET and Langmuir areas.** The Langmuir model assumes a monolayer and gives larger values than BET, as for MOF-200 and MOF-210 (Abid et al. 2023); compare like with like.
- **Using N₂ for the smallest pores.** Below about 0.7 nm, argon at 87.3 K is preferred, and CO₂ at 273 K reaches pores below 0.45 nm that N₂ at 77 K cannot (Abid et al. 2023).

## How to draw the BET setup and data in LabCanvas

For a methods figure, show the sample tube in its liquid-nitrogen bath, the dosing manifold and the pressure gauge, and next to it the adsorption–desorption isotherm with the BET fitting range marked. In LabCanvas the figure editor’s Library has the gas sorption analyser as an editable object, including a cutaway of the gas path from manifold to sample, and an editable data plot for isotherms and pore size distributions. You can plot adsorption and desorption branches, shade the hysteresis loop, label the isotherm type and the BET area, and export the figure for a paper, a methods section or a graphical abstract.

## Probe gases for physisorption compared

| Probe gas | Temperature | Best for | Notes | Source |
| --- | --- | --- | --- | --- |
| Nitrogen (N₂) | 77 K | Surface area, pore size and pore volume when pores are wider than about 0.7 nm | Routine choice; slow diffusion means long equilibration | Abid et al. 2023 |
| Argon (Ar) | 87.3 K | Pores smaller than about 0.7 nm | Preferred over N₂ for narrow micropores | Abid et al. 2023 |
| Carbon dioxide (CO₂) | 273 K | Ultramicropores below about 0.45 nm | Reaches pores N₂ at 77 K cannot | Abid et al. 2023 |
| Krypton (Kr) | 77 K | Samples with very low surface area, including thin MOF films | Used for ZIF-8 thin films | Abid et al. 2023; Stassin et al. 2021 |

## Related techniques

[![XRD: Crystal structure, phase and crystallite size (nλ = 2d sin θ)](https://www.labcanvas.com/materials/xrd/xrd-cover.webp) Technique nλ = 2d sin θ X-ray diffraction (XRD) X-ray diffraction explained: Bragg’s law, how to read an XRD pattern, phase identification, Scherrer crystallite size, Rietveld refinement and common mistakes.](https://www.labcanvas.com/techniques/xrd)

[![TGA / DSC: Thermal stability, composition and phase transitions (Mass and heat flow vs temperature)](https://www.labcanvas.com/materials/tga-dsc/tga-dsc-cover.webp) Technique Mass and heat flow vs temperature TGA and DSC (thermal analysis) TGA and DSC explained: mass loss and DTG vs temperature, atmosphere, MOF thermal stability, and glass transition, melting and crystallisation in DSC.](https://www.labcanvas.com/techniques/tga-dsc)

[![SEM: Electron images of surfaces, plus EDS elemental maps (Surface morphology and composition)](https://www.labcanvas.com/materials/sem/sem-cover.webp) Technique Surface morphology and composition Scanning electron microscopy (SEM) How SEM works: secondary vs backscattered electron images, EDS/EDX elemental analysis, resolution, sample coating, low-vacuum mode and common mistakes.](https://www.labcanvas.com/techniques/sem)

[![FTIR: Functional groups from infrared absorption (A = log₁₀(I₀/I))](https://www.labcanvas.com/materials/ftir/ftir-cover.webp) Technique A = log₁₀(I₀/I) Fourier-transform infrared (FTIR) spectroscopy FTIR spectroscopy explained: infrared absorption by bond vibrations, the interferometer and Fourier transform, wavenumber regions, ATR and reading bands.](https://www.labcanvas.com/techniques/ftir)

[![TEM: Atomic-scale images and diffraction from thin samples (HAADF intensity ∝ Z²)](https://www.labcanvas.com/materials/tem/tem-cover.webp) Technique HAADF intensity ∝ Z² Transmission electron microscopy (TEM) How TEM works: bright field, HRTEM lattice images, SAED diffraction, STEM-HAADF Z-contrast, EDS/EELS, sample thickness and beam damage of MOFs.](https://www.labcanvas.com/techniques/tem)

## More on LabCanvas

- [All characterisation techniques](https://www.labcanvas.com/techniques): XRD, XPS, BET, SEM, TEM, NMR, FTIR, Raman and more, explained with cited sources
- [Graphical abstract sizes and templates](https://www.labcanvas.com/graphical-abstracts): ACS, Wiley, Angewandte, RSC, Elsevier and Cell Press, with templates at the right size
- [Materials explained](https://www.labcanvas.com/blog/materials): MOFs, COFs, 2D materials, perovskites, batteries and zeolites, with cited facts
- [Journal figure size guide](https://www.labcanvas.com/blog/journal-figure-size-guide): Column widths and resolutions for 38 journals

## Frequently asked questions

### What is BET surface area?

The specific surface area calculated by the Brunauer–Emmett–Teller method from a gas adsorption isotherm, usually N₂ at 77 K: the monolayer amount from the BET fit is converted into an area. It is the most widely used surface-area metric for micro- and mesoporous materials (Osterrieth et al. 2022; Gómez-Gualdrón et al. 2016).

### Why is BET measured with nitrogen at 77 K?

Nitrogen at its boiling point, 77 K, is the routine probe for surface area, pore size and pore volume when the gas can enter the pores (wider than about 0.7 nm); argon at 87.3 K or CO₂ at 273 K are used for smaller pores (Abid et al. 2023).

### What relative pressure range is used for BET?

Instruments often default to about P/P₀ = 0.05–0.30, but for microporous materials the range must be chosen with the Rouquerol consistency criteria; for one microporous carbon the valid range was 0.0051–0.0597 (Morgan et al. 2026).

### What are the IUPAC isotherm types?

IUPAC classifies physisorption isotherms into six types (Bingel & Walton 2023). Type I, with a steep rise at low pressure, indicates micropore filling (Morgan et al. 2026); Type IV with a hysteresis loop indicates mesopores (Madhu et al. 2024); Type V shows a step, as in water adsorption or flexible MOFs (Bingel & Walton 2023).

### Is BET valid for MOFs?

With care. Simulations show BET areas of many MOFs agree with geometric surface areas (Walton & Snurr 2007), but BET can overestimate the monolayer when pore filling overlaps with monolayer formation, so the pressure range should follow the consistency criteria and results be compared with simulated BET areas (Gómez-Gualdrón et al. 2016).

### How do you get pore size distribution from gas sorption?

From the same isotherm: BJH for mesopores, Horváth–Kawazoe or the t-plot for micropores, and density functional theory (NLDFT or QSDFT) with a suitable pore model as the more rigorous option (Morgan et al. 2026).

### Why must samples be degassed before BET?

Solvent and adsorbed species block the pores and lower the measured area. Degassing and activation must empty the pores without collapsing them; for MOFs degas below 75% of the decomposition temperature (Abid et al. 2023).

### BET vs Langmuir surface area: what is the difference?

The Langmuir model assumes monolayer adsorption, while BET allows multilayers; Langmuir areas are larger than BET areas for the same MOF, as reported for MOF-200 and MOF-210 (Abid et al. 2023).

## References

- H. R. Abid, M. R. Azhar, S. Iglauer, Z. H. Rada, A. Al-Yaseri, A. Keshavarz. Physiochemical characterization of metal organic framework materials: a mini review. *Heliyon* 2023, 10, e23840. [doi:10.1016/j.heliyon.2023.e23840](https://doi.org/10.1016/j.heliyon.2023.e23840)
- J. W. M. Osterrieth et al.. How reproducible are surface areas calculated from the BET equation?. *Adv. Mater.* 2022, 34, e2201502. [doi:10.1002/adma.202201502](https://doi.org/10.1002/adma.202201502)
- K. S. Walton, R. Q. Snurr. Applicability of the BET method for determining surface areas of microporous metal-organic frameworks. *J. Am. Chem. Soc.* 2007, 129, 8552–8556. [doi:10.1021/ja071174k](https://doi.org/10.1021/ja071174k)
- D. A. Gómez-Gualdrón, P. Z. Moghadam, J. T. Hupp, O. K. Farha, R. Q. Snurr. Application of consistency criteria to calculate BET areas of micro- and mesoporous metal-organic frameworks. *J. Am. Chem. Soc.* 2016, 138, 215–224. [doi:10.1021/jacs.5b10266](https://doi.org/10.1021/jacs.5b10266)
- T. C. Wang et al.. Ultrahigh surface area zirconium MOFs and insights into the applicability of the BET theory. *J. Am. Chem. Soc.* 2015, 137, 3585–3591. [doi:10.1021/ja512973b](https://doi.org/10.1021/ja512973b)
- H. M. Morgan, C.-H. Tsai, W.-T. Tsai. Production of highly porous carbon materials from coastal driftwood. *Materials* 2026, 19, 3629. [doi:10.3390/ma19173629](https://doi.org/10.3390/ma19173629)
- L. W. Bingel, K. S. Walton. Surprising use of the business innovation Bass diffusion model to accurately describe adsorption isotherm types I, III, and V. *Langmuir* 2023, 39, 4475–4482. [doi:10.1021/acs.langmuir.3c00147](https://doi.org/10.1021/acs.langmuir.3c00147)
- J. Madhu, A. Santhanam, D. Velauthapillai. Synthesis of "ice cube" shaped zeolite A type and Cu²⁺ ion-exchanged zeolites and study of their CO₂ adsorption performance. *ACS Omega* 2024, 9, 45926–45942. [doi:10.1021/acsomega.4c05487](https://doi.org/10.1021/acsomega.4c05487)
- Y. M. Shulga et al.. The concentration of C(sp³) atoms and properties of an activated carbon with over 3000 m²/g BET surface area. *Nanomaterials* 2021, 11, 1324. [doi:10.3390/nano11051324](https://doi.org/10.3390/nano11051324)
- M. Thommes, C. Schlumberger. Characterization of nanoporous materials. *Annu. Rev. Chem. Biomol. Eng.* 2021, 12, 137–162. [doi:10.1146/annurev-chembioeng-061720-081242](https://doi.org/10.1146/annurev-chembioeng-061720-081242)
- T. Stassin et al.. Porosimetry for thin films of metal-organic frameworks: a comparison of positron annihilation lifetime spectroscopy and adsorption-based methods. *Adv. Mater.* 2021, 33, e2006993. [doi:10.1002/adma.202006993](https://doi.org/10.1002/adma.202006993)

## Draw Gas sorption and BET surface area figures for your paper

LabCanvas is a browser workspace for scientific figures: instruments with their beam and light paths, editable data plots, crystal structures, graphs and graphical abstracts at journal sizes.

[Draw Gas sorption and BET surface area in LabCanvas](https://www.labcanvas.com/editor.html?add=ch-bet)

**Contents**

- Gas sorption and BET surface area at a glance
- Gas sorption and BET surface area instrument drawing (high-resolution SVG)
- Gas sorption and BET surface area data: an example plot to edit
- How BET gas sorption works
- What the data looks like and how to read it
- Sample preparation: degassing and activation
- What BET tells you about materials
- Common mistakes and limits
- How to draw the BET setup and data in LabCanvas
- Probe gases for physisorption compared
- Related techniques
- More on LabCanvas
- Frequently asked questions
- References