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About this tool
Test a logo colour against light, dark and photographic backgrounds using WCAG contrast ratios, and get the scrim opacity that fixes it.
The Logo On Background Tester computes the WCAG 2.1 contrast ratio between a logo colour and a set of light, dark, mid-grey and custom surfaces, then reports which ones miss the 3:1 threshold that WCAG SC 1.4.11 sets for graphical objects. For every failing surface it solves for the exact black or white scrim opacity that lifts the mark back over the target, and if you load a photograph it builds a luminance histogram and tells you what percentage of that image's pixels the logo would disappear against. Everything is computed locally from the relative-luminance formula; no image is uploaded.
Open Logo On Background Tester on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
Uses the linearised sRGB relative-luminance definition and the (L1+0.05)/(L2+0.05) ratio, not an eyeballed guess.
Bisects the scrim alpha to the smallest opacity that reaches your target instead of leaving you to trial and error.
Builds a luminance histogram of the image and reports the share of pixels that fail, plus the worst case.
3:1 is the working target. WCAG 2.1 SC 1.4.11 Non-text Contrast requires 3:1 for graphical objects a user needs to perceive, and that is the sensible bar for a logo on a surface. Body text next to it needs 4.5:1 under SC 1.4.3.
Not strictly. WCAG SC 1.4.3 explicitly exempts logotypes: text that is part of a logo or brand name has no minimum contrast requirement. Legibility still matters commercially, which is why 3:1 is a good self-imposed floor even where conformance does not demand it.
Each sRGB channel is divided by 255, linearised (c/12.92 below 0.03928, otherwise ((c+0.055)/1.055) raised to 2.4), then combined as 0.2126R + 0.7152G + 0.0722B to give relative luminance L. The ratio is (lighter L + 0.05) divided by (darker L + 0.05), which ranges from 1:1 to 21:1.
It depends on both colours, which is why this tool solves for it. The scrim is composited over the background in sRGB, the resulting luminance is recomputed, and the smallest opacity that reaches your target ratio is found by bisection. A white logo on a light grey backdrop, for example, typically needs a black scrim in the 25 to 30 percent range to clear 3:1.