APCA Contrast Calculator
Enter a text and background hex color to compute the APCA Lc contrast.
Result
How it is calculated
Each colour is converted to relative luminance Y = 0.2126729 R2.4 + 0.7151522 G2.4 + 0.0721750 B2.4 with R, G, B scaled to 0 to 1; very dark values are softly clamped. If the background is lighter than the text (dark text), Lc = ((Ybg0.56 − Ytxt0.57) × 1.14 − 0.027) × 100. If the text is lighter, it uses exponents 0.65 and 0.62 and the result is negative. Colours may be given as 3 or 6 hex digits, with or without #. The tool prints the signed Lc and its absolute value. It returns one score only: it does not look up minimum Lc values for font size or weight, so you compare the number with an APCA table yourself. Differences below a tiny luminance gap return 0.
Worked example
Take mid-grey text #767676 on a white background. Each channel is 118/255 = 0.4627, raised to 2.4, so the luminance is Y = 0.1573; white is 1.0. Dark text on light: (10.56 − 0.15730.57) × 1.14 = (1 − 0.3485) × 1.14 = 0.7427. Subtract the 0.027 offset: 0.7157, times 100 gives 71.6 Lc. Swapping the colours (white on #767676) gives −77.0 Lc, a larger magnitude, because APCA treats light-on-dark differently.
Common mistakes
- Comparing an Lc value with the WCAG 2 ratio thresholds such as 4.5:1: they are different scales, and 71.6 Lc is not a ratio.
- Ignoring the sign: a negative Lc just means light text on a dark background; judge it by its absolute value.
- Assuming swapping text and background gives the same score: the two polarities are calculated differently, so the numbers differ.
- Using one Lc pass mark for all text: small or thin text needs a much higher Lc than large bold text, and this tool does not give those thresholds.
- Typing a colour name such as red or an rgb() value: only hex codes work, and anything else returns no result.
About this calculator
The APCA (Accessible Perceptual Contrast Algorithm) is a next-generation contrast metric being developed for WCAG 3, designed to replace the older WCAG 2.x contrast ratio formula, which is known to over- and under-estimate perceived readability for many color combinations. Instead of a simple luminance ratio, APCA accounts for text polarity (light-on-dark vs. dark-on-light), font weight, and font size, producing an Lc (Lightness Contrast) value roughly on a 0–106 scale that better matches how legible text actually looks to the human eye.
Because required Lc thresholds shift with font size and weight — small thin text needs a much higher Lc than large bold text to stay readable — APCA is paired with a font-size/weight lookup table rather than one fixed pass/fail number like WCAG 2's 4.5:1. Web accessibility auditors, UI designers, and design-system teams use APCA scores to choose text and background color pairs that will remain legible under the upcoming WCAG 3 guidelines, especially for typography sizes and weights the older ratio handled poorly.
Was this helpful?
You can pick up to 3 reactions.
Popular calculators
Loan Calculator
Monthly payment, total interest and full amortization schedule.
Percentage Calculator
Percent change, percent of, and ratio calculations.
Discount Calculator
Calculate final price, savings, and discount amount.
BMI Calculator
Body mass index from height and weight (metric or imperial).
Age Calculator
Compute your exact age in years, months and days from a birth date.
Compound Interest Calculator
Calculate the final amount and interest earned with compounding.
All Calculators
No calculators found
Comments (0)