Class colors have to be measured after you build them
A choropleth shows magnitude as shades of one color family. Once you have fixed the two end colors and the class count, though, there is no way to eyeball how far apart the middle steps really are. This tool divides the distance between your two colors into the number of classes you ask for and reports, per step, the contrast against the map background along with the contrast and lightness gap against the step before it.
How it is calculated
Colors are mixed in a linear-light space using the sRGB transfer function defined by W3C, then encoded back. Contrast uses the WCAG relative luminance formula and lightness is CIELAB L*. The non-text 3:1 verdict is applied only between a class color and the map background or border. Because a sequential ramp cannot reach 3:1 between adjacent steps once the class count grows, adjacent figures are printed for reference only. Written as of October 2026.
Limits
The tool does not decide how many classes are best, nor how to cut the data into them, whether by equal interval, quantile or natural breaks. Diverging scales and categorical palettes belong to other tools. Perceptually even steps are not guaranteed.
Frequently asked questions
There is no accepted optimal count. ColorBrewer publishes schemes from three to nine classes, but that is a range of choices rather than a rule. This tool takes the class count from you and reports the contrast of the result.
On a sequential ramp that is not achievable. Splitting the distance between two end colors into several steps drives the contrast between neighbors toward 1. Apply 3:1 between a class color and the map background or border, and read the adjacent figures as reference only.
The sRGB values are linearized with the transfer function, mixed in even steps and encoded back. That mixes by light quantity, which does not guarantee perceptually even steps.