Calculator Input
Example Data Table
| Marker | Known MW | Distance | Dye Front | Rf | log10(MW) |
|---|---|---|---|---|---|
| L1 | 150 kDa | 18 mm | 82 mm | 0.2195 | 2.1761 |
| L2 | 100 kDa | 23 mm | 82 mm | 0.2805 | 2.0000 |
| L3 | 75 kDa | 29 mm | 82 mm | 0.3537 | 1.8751 |
| L4 | 50 kDa | 37 mm | 82 mm | 0.4512 | 1.6990 |
| Unknown | Estimate | 34 mm | 82 mm | 0.4146 | From curve |
Formula Used
Rf = corrected band distance ÷ corrected dye front distance
log10(MW) = intercept + slope × Rf
MW = 10^(intercept + slope × Rf)
The calculator builds a straight line on semi-log paper. The x value is Rf. The y value is log10 of molecular weight. Unknown bands are then read from that line.
How to Use This Calculator
- Measure the dye front distance from the loading origin.
- Enter known ladder molecular weights and migration distances.
- Enter unknown band names and their migration distances.
- Select the unit used by your ladder values.
- Use origin offset only when your distance scale starts before the well.
- Press the calculate button to view results above the form.
- Download CSV or PDF for reporting and lab notes.
Protein Gel Molecular Weight Log Paper Guide
Why Semi Log Analysis Helps
Protein gel bands rarely form a simple linear scale against molecular weight. Larger proteins move slowly. Smaller proteins move farther through the gel matrix. Because of this behavior, laboratory workers often plot migration on semi log paper. The ladder creates a reference curve. Unknown bands are compared with that curve. This calculator follows the same idea, but it performs the regression for you.
How The Curve Is Built
Each standard band needs two values. The first value is known molecular weight. The second value is migration distance. The tool divides each corrected distance by the corrected dye front distance. This gives Rf. It then converts each known weight into log10 molecular weight. A least squares line is fitted through the ladder points. The equation becomes a working log paper ruler for the gel.
Reading Unknown Bands
Unknown bands are handled with the same distance method. Their Rf values are placed on the fitted line. The predicted log value is converted back into molecular weight. Results are best when the unknown band lies between the smallest and largest useful ladder Rf values. Values outside that range are extrapolated. They can still be useful, but they need careful review.
Better Measurement Practice
Use one image for all distances. Measure from the same origin point. Keep the ruler straight along the lane. Avoid distorted lanes, overloaded bands, and smiling gels. Choose clear ladder bands. Remove bands that are blurred or poorly separated. More reliable input creates a stronger R² value and a better estimate.
When To Use The Export Options
CSV export is useful for spreadsheets and records. PDF export is useful for sharing a quick summary. Include the regression equation, R² value, and ladder range in reports. These details help another reader understand whether the estimate was interpolated or extrapolated.
FAQs
What does this calculator estimate?
It estimates protein molecular weight from gel migration distance, ladder standards, dye front position, and a semi-log regression curve.
What is Rf in protein gel analysis?
Rf is relative mobility. It equals corrected band distance divided by corrected dye front distance. It normalizes migration across gel runs.
Why is log10 molecular weight used?
Protein migration is often near linear when molecular weight is plotted on a logarithmic scale against relative mobility.
How many ladder standards should I enter?
Use at least two standards. More clear standards usually improve the regression and make the estimate more dependable.
Can I use Da instead of kDa?
Yes. Select the same unit used for all ladder values. The calculator returns unknown estimates in that selected unit.
What does extrapolated mean?
It means the unknown band lies outside the entered ladder range. The estimate may be less reliable than interpolation.
What is origin offset?
Origin offset corrects measurements when your ruler begins before the actual loading origin. Most users can leave it at zero.
Why is my R² value important?
R² shows how closely ladder points fit the regression line. A value near one usually indicates stronger calibration.