Gel Band Size Calculator — Read Sizes Off a Gel Photo
Mark the ladder on your gel photograph, click a band, get its size in bp.
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The opposite direction from a virtual gel: you already have the photograph, the ladder is in lane 1, and the question is what the band in lane 4 is. Drop the image in, click each ladder band, then click any band to size it. Migration distance is close to linear in log(size) over a gel's resolving range, so the fit interpolates between the rungs you marked — and says so when a band ran outside them, instead of quietly rounding it to the nearest rung. The per-rung residual is shown as well as R², because one mis-marked rung shifts every size on the gel and a single R² hides which. The image is read in your browser and never uploaded.
Drop a gel photograph here, or .
The image stays in this browser. It is not uploaded and no server sees it.
How to use the Gel Band Size tool
- 1Drop your gel photograph onto the page, or choose the file.
- 2Pick the ladder you ran, then click each ladder band from the top down — at least three.
- 3Click any other band to label it with its size; the ladder fit and its per-rung residuals appear below.
- 4Download the annotated image, or read the sizes off the labels.
Frequently asked questions
How is the size worked out?
A least-squares line is fitted through the ladder rungs you marked, of log10(size) against how far each ran. Any other band's size is read off that line. It is an interpolation between your marked rungs; a band that ran past them is labelled as extrapolated, because migration stops being log-linear outside a gel's resolving range and a number there is the least reliable thing on the report.
Why does it want three ladder bands, not two?
Two points define a line exactly, so R2 is 1 and every residual is 0 whatever you clicked — a fit that cannot be wrong is read as evidence the ladder was marked right, and it is not. Three is the minimum that can disagree with itself.
Is my gel image uploaded?
No. The file is read in your browser, drawn locally, and the annotated copy is produced from that same local canvas. Nothing about the image reaches a server.
Why is one of my ladder rungs off by 20%?
That rung is probably mis-marked — a doublet marked as one band, a click on the wrong rung, or a lane that ran crooked. It is worth fixing before reading any size off the gel, because the line is fitted through every rung and one bad one moves all of them.
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