What it does
Densitometry turns the darkness of a band into a number, so you can say that a protein is three times more abundant in one lane than another. This page does what a gel analysis program does, inside your browser:
- Lanes. It splits the lane area into equal lanes and reads the middle part of each (60 % of the lane by default), so the gaps between lanes and smiling edges are left out.
- Profiles. For every lane it takes the average signal across the lane, row by row down the gel. Dark bands on a light gel or film and light bands on a dark fluorescent scan both work.
- Background. Straight line under each band draws a line from the start of the band to its end and counts only what sits above it, the classic method that copes with a background that gets darker down the gel. Rolling window rolls a flat window along the underside of the whole profile and suits an even background. None counts the raw signal.
- Bands. Band rows are found from the average profile of all lanes. You can rename them, move their top and bottom, or type your own.
- Amounts. Each band’s volume (signal added up over the band’s rows and the lane’s width) is shown as a percentage of a reference lane. Pick a loading control, such as actin, tubulin or GAPDH on a western blot, and each lane’s band is divided by that lane’s control first.
- Saturation check. A band whose pixels hit the top of the photo’s range reads too low. The page warns you when more than 1 % of a band’s pixels are saturated.
How to use it
- Photo. Open the original image from the scanner or imager. A phone photo of a gel works for rough numbers, but a flat, evenly lit scan is better. Do not adjust contrast or brightness first.
- Lanes. Tap Set the lane area, then tap two opposite corners around your lanes. Type the number of lanes. Blue boxes show what is read.
- Profiles. Check that the green band rows sit around each band. Change Find bands above or type the rows by hand if needed.
- Amounts. Pick the reference lane and, for a blot, the loading control. Read the bar chart and table, and download the CSVs or a PNG, PDF or Word report.
Worked example
The example is a made-up blot with six lanes (untreated, then 1 to 24 hours of a treatment). The target band was drawn with amounts 1, 2, 4, 6, 3 and 1.5, and the loading control with 1, 1.1, 0.9, 1, 1.05 and 0.95, on a background that darkens down the film, with a little noise.
With the straight-line background, the target comes back as 100, 200, 400, 601, 302 and 150 % of the untreated lane, within 1 % of the truth. Divided by the loading control, the 2 h lane is about 444 %, because that lane was loaded with a little less protein. Switch to the rolling window and the ratios come out up to 10 % high, because the background slopes under the bands and the faint untreated band loses the most: a good reason to look at the profiles before trusting a number.
Tips
- Keep every band in the linear range of the stain or film. Run a dilution series of one sample once to check that twice the protein gives twice the volume.
- Compare bands only within one gel or blot. Different exposures are not on the same scale.
- Choose the lane width so that the boxes stay inside the bands; reading into the gaps adds background.
- Report the loading-control-normalised values for blots, with the number of repeats.
FAQ
Are the volumes in real units? No. Volume is the stain signal (0 to 1 for each pixel) added up over pixels. Only ratios between bands on the same image mean anything, which is why the page shows percentages.
Why do my numbers differ from another program? Programs differ in how they draw the background and where a band ends. Use one method for a whole study.
Can I measure a DNA gel? Yes. Pick light on a dark background for an ethidium or SYBR photo, unless it has been inverted. To size bands instead, use the gel band size estimator.
Where the numbers come from
- Lane profiles, the straight-line and rolling-window backgrounds, band finding and volumes are written by fibub and checked in the tests against NumPy on a random image.
- The example blot is drawn by the page from made-up band amounts (target 1, 2, 4, 6, 3 and 1.5; loading control near 1), and the tests check that the page gets those amounts back.
- A photo you open is read on the page only. It is never uploaded or stored.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.