Gel migration log size standard curve calculator

What this calculation answers

Gel migration often changes approximately with the logarithm of fragment or protein size over a limited working range. A straight line fitted directly to untransformed size can therefore produce a different and inappropriate interpolation.

This page fits log base ten of known ladder size against migration distance, then transforms the unknown estimate back into the entered size unit. It uses measured distances, not image recognition.

The Simple linear regression calculator fits an ordinary line in entered coordinates. This gel model applies that fit to log size and transforms the inferred value back.

Use the right input conventions

Enter at least three ladder bands from a comparable run and one consistent size unit. Distances are millimetres from the same reference position, and unknown distance must use that identical reference.

The fitted slope must be negative for the stated decreasing-size-with-distance model. Protein standards in kilodaltons and DNA standards in base pairs should never be mixed in one calibration.

Interpret the result

R squared is evaluated on logarithmic size, where the regression is performed. A perfect value indicates alignment in transformed coordinates, not proof that a real gel is linear over all molecular sizes.

The unknown receives an explicit range flag. Extrapolation beyond measured migration distances can be especially unreliable because real gel behavior may curve near the high- and low-size ends.

Work through the example

Ladder sizes one thousand, one hundred and ten at distances ten, twenty and thirty millimetres decrease one decade for each ten millimetres. At fifteen millimetres the inferred size is about 316.23.

That value is the geometric midpoint between one thousand and one hundred, not their arithmetic midpoint. The difference demonstrates why the logarithmic transformation matters for interpolation.

Where the model stops

The model does not identify a band, determine DNA conformation or correct anomalous protein migration. It assumes the chosen standards and unknown are compatible with the same local calibration relationship.

No image or raw gel data is uploaded. The result is a descriptive size estimate without an uncertainty interval, and should be interpreted with the actual gel conditions and ladder guidance.