# Western blot and SDS-PAGE molecular weight calculator

> Upload a gel or blot, click the ladder and sample bands, and estimate protein sizes in kDa from a log-linear standard curve with R². Free, in-browser.

URL: https://www.labcanvas.com/tools/western-blot-mw-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.

## Quick answer

- **What it does:** Estimates protein sizes in kDa from a gel or western blot image: click the ladder and sample bands and it fits a log-linear standard curve with R².
- **Formula:** log₁₀(MW) = a + b × migration distance, fitted by least squares to the ladder bands.
- **Example:** With the 70 kDa band at 210 px and the 55 kDa band at 262 px, a sample band at 236 px is about 62 kDa.

Free, in the browser, no account needed. Updated 2026-11-08.

## Features

- Click bands on the image
- Ladder presets: PageRuler, PageRuler Plus, Precision Plus
- Log-linear standard curve with R²
- Flags extrapolated bands
- Annotated image, plot and CSV export

## How to use it

1. **Upload the image**: Drop a PNG or JPEG of the gel or blot; it stays in your browser.
2. **Mark the ladder**: Choose your ladder and click its bands from top to bottom; correct any size in the table.
3. **Mark your bands**: Switch to sample bands and click each band of interest; read the estimated sizes and export.

## At a glance

| | |
| --- | --- |
| Input | An image of your blot; click the ladder and sample bands |
| Ladders | PageRuler, PageRuler Plus, Precision Plus |
| Model | \(\log_{10}(\mathrm{MW})\) linear in migration distance, fitted with R² |
| Flags | Bands outside the ladder range (extrapolated) |
| Output | Annotated image and calibration plot (PNG), CSV |

## How it works

$$\log_{10}(\mathrm{MW}) = a + b \times \text{distance}$$

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 (\(R_\mathrm{f}\)) because \(R_\mathrm{f}\) 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 (\(R^2 \ge 0.98\)).

## Questions

### How do you calculate the molecular weight of a protein from a gel?

Plot \(\log_{10}\) 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.