Gel Electrophoresis Migration Calculator
Estimate the size of an unknown DNA band from its migration distance using a two-point log(size) standard curve.
How it works
log₁₀(size) = slope × distance + intercept · slope = [log(size₂) − log(size₁)] ÷ (dist₂ − dist₁)
On an agarose gel the distance a fragment migrates is roughly linear in the logarithm of its size — small fragments run fast and far, large ones lag near the well. Two ladder bands define a straight line on a log-size vs distance plot, and an unknown band is read off that line. Real ladders use many bands and a proper regression, but two well-separated points bracketing the unknown give a solid estimate.
Worked example
A 1,000 bp marker runs 20 mm and a 100 bp marker runs 50 mm, so the slope is (2 − 3) ÷ (50 − 20) = −0.0333 log units per mm and the intercept is 3.667. A band at 35 mm has log-size −0.0333 × 35 + 3.667 = 2.5, i.e. 10^2.5 ≈ 316 bp.
Sources: Gel electrophoresis — Wikipedia
Frequently asked questions
Why is migration linear in log of size, not size?
The gel matrix sieves molecules, and the mobility of DNA falls off logarithmically with length over the resolving range. Plotting log(size) against distance straightens the relationship so a line can be fit.
Can I trust an estimate outside the two markers?
Extrapolating beyond your bracketing markers is unreliable because the log-linear relationship bends at the extremes. Choose ladder bands that flank the unknown for the most accurate reading.
Should I use more than two points?
Yes, for best accuracy. A least-squares fit through many ladder bands averages out reading error; the two-point method here is a fast approximation.
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This calculator is for educational purposes. Double-check important results before acting on them.