Western blot and SDS-PAGE molecular weight calculator
Upload a photo of your gel or blot, click the ladder bands (their sizes fill in from the ladder you choose), then click your sample bands. The calculator fits log₁₀(molecular weight) against migration distance and reports each band’s estimated size in kDa, with R², an annotated image and a CSV. It also works with distances typed in by hand.
Loading Western blot MW calculator…
What it does
How to use the western blot MW calculator
- Upload the imageDrop a PNG or JPEG of the gel or blot; it stays in your browser.
- Mark the ladderChoose your ladder and click its bands from top to bottom; correct any size in the table.
- Mark your bandsSwitch to sample bands and click each band of interest; read the estimated sizes and export.
How it works
log₁₀(MW) = a + b × distance
Over a gel’s resolving range, a protein’s migration distance is approximately linear in the logarithm of its molecular weight. The ladder bands give the calibration line by least squares; each unknown band’s distance is read off that line. Distances are measured from the top of the image, which gives the same result as using relative mobility (Rf) because Rf is a linear rescaling of distance.
Worked example
If the 70 kDa ladder band runs 210 px from the top and the 55 kDa band 262 px, a sample band at 236 px sits between them, at about 62 kDa, assuming the calibration line fits well (R² ≥ 0.98).
Frequently asked questions
How do you calculate the molecular weight of a protein from a gel?
Plot log₁₀ of the ladder sizes against their migration distances, fit a straight line, and read your band’s size from its distance. This tool does it from clicks on the image.
Why does my protein run at a different size than predicted?
Post-translational modifications (glycosylation, phosphorylation), unusual charge or structure, incomplete denaturation and SDS binding differences can all shift apparent molecular weight by 10 % or more.
Which ladder bands should I use?
Those that bracket your protein and are well resolved. The relationship is linear only over the gel’s resolving range, so drop bands compressed at the top or bottom if R² is low.
Is the image uploaded?
No. It is processed in your browser only.
Related free tools
Make the whole figure in LabCanvas
Vector figure editor, slides, 3D models, protein structures and Prism-style graphs, in your browser.
Get started