Gel Band Size Estimator
How to estimate a band size from a gel
- Measure how far each ladder band ran from the well, in millimetres on a printout or in pixels on a photo, and type the sizes with their distances.
- Measure your own bands the same way and type their distances.
- The size of each band is read off the fitted curve, with the R² of the fit.
Frequently asked questions
How does the fit work?
DNA moves at a rate that falls with the logarithm of its length, so a straight line through log10(size) against distance describes most of a gel well. The curved fit adds a squared term, which follows the bends at the very top and bottom of the gel.
Where do I measure from?
The same point on every band, usually the front edge, measured from the bottom of the well. Mixing the front of one band with the middle of another is the biggest source of error.
Can I measure in pixels?
Yes. The fit only needs the ladder and your bands measured in the same unit, so pixels from a gel photo work as well as millimetres from a printout. Make sure the photo is square on, not at an angle.
How accurate is it?
Within the range of the ladder, usually within 5% to 10%. Outside it the fit is extrapolating and can be out by a lot, which is why a band that ran past the smallest ladder band is flagged.
Why is my R² low?
Either a distance was mistyped, the gel ran unevenly or smiled, or the ladder sizes do not match the ladder you actually loaded. Check the extremes first; they pull the fit hardest.
Which ladder sizes should I enter?
Whatever the tube says. The default is a typical 1 kb ladder, but ladders differ between suppliers, so copy the sizes from your own product sheet.