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APCA for W3C & WCAG3


APCA is the Accessible Perceptual Contrast Algorithm, a new way to predict contrast for text and non-text content on self illuminated displays.

The correct code to use is right here apca-w3 in this satellite repository, and is also available at npm i apca-w3 code here is the only code that should be used for any development purposes. Please post issues at the main repository.

The draft APCA Readability Criterion is up as a work in progress, still sections to be added and/or adjusted.

For comments or questions on the guidelines, use the ARC forum, over at Accessible Reading Technologies, as that is where we discuss the draft standards & guidelines, and discuss legal & conformance issues.

For comments or questions on theory, math, or the APCA algorithm itself, use the SAPC-APCA forum at the main repository, please post all comments, questions, or discussions regarding theory, math, code, third-party tools, third-party tools, and so forth, there and not here in this satellite repository, so they can be tracked and resolved.

Poster: a picture of crash test dummies crashing out of a car, and text that says don't be a dummy! Stop using low contrast text. At the bottom it says APCA the world is reading       Smokey the bear saying  ONLY YOU CAN STOP LOW CONTRAST      Uncle Sam saying I want you to use high contrast text


APCA is a contrast assessment method for predicting the perceived contrast between sRGB colors on a computer monitor. It has been developed as an assessment method for W3 Silver/WCAG3 accessibility standards relating to content for computer displays and mobile devices, with a focus on readability and understandability.

The APCA version in this repository is licensed to the W3/AGWG per the collaborative agreement with specific limitations and exclusions. Please see the license file for details.

Current Library Version: 0.1.9 (w3) (98G4g) beta

APCA The Revolution Will Be Readable



Visit git.myndex.com for a catalog of resources, including articles, third party and peer reviews, additional documentation, white-papers, and more.

Questions & Comments

For comments and discussions, please see the main SAPC-APCA repo, discussion area. This is also where we discuss draft guidelines and proposals. Please post all comments or discussions there and not in the satellite repositories so they can be tracked and resolved.


There is a working version with examples and reference material on the APCA demo tool site



    npm i apca-w3


    <script type="module">

    import { APCAcontrast, reverseAPCA, sRGBtoY, displayP3toY, adobeRGBtoY, alphaBlend, calcAPCA, fontLookupAPCA } from 'apca-w3';

    import { colorParsley, colorToHex, colorToRGB } from 'colorparsley';  // optional string parsing




If you need to parse a color string or 24bit number, use the colorParsley() function:

    let rgbaArray = colorParsley('aliceblue');


Intended for blending the foreground color into the background. Only the foreground has an alpha. There is no conversion to linear space, so blending takes place is the working colorspace, as is standard.

                       // Send 0-255 arrays alphaBlend(FG, BG)
    let alphaBlended = alphaBlend([0,0,0,0.6],colorParsley([255,255,255])),

                       // Send 0-1.0 float arrays for displayP3toY, 5th element
                      // is bool (false for floats): alphaBlend(FG, BG, false)
    let alphaBlended = alphaBlend([0.7,1.0,1.0,0.33],colorParsley([0,0,0]),false);


Send rgba INT array [123,123,123,1.0] to ` sRGBtoY() ` — this is a slightly different luminance Y that the IEC piecewise.

    let Ys = sRGBtoY([123,123,123,1.0]);

Note that for for sRGB and Adobe98, the values in the array must be 0.0-255.0 per CSS4 specs, but for displayP3toY(), the values must be 0.0-1.0, per Apple and CSS4 specs.


First color must be text, second color must be the background.

    let textColor = [17,17,17,1.0];
    let backgroundColor = [232,230,221,1.0];
    let contrastLc = APCAcontrast( sRGBtoY( textColor ), sRGBtoY( backgroundColor ) );

Example using everything together, including alphaBlend:

    let colorTEXT =  rgb(12,23,34,0.65);
    let colorBG =  #e6e0dd;

    let Lc = APCAcontrast(sRGBtoY( alphaBlend(colorParsley(colorTEXT), colorParsley(colorBG)) ),
                        sRGBtoY( colorParsley(colorBG) ));


The long complete line shown above is aliased into a function ` calcAPCA() `. Alpha for the text is automatically detected, and ignored if 1 or ‘’. The input type is also auto detected (string, king of string, number, array, object). By default returns a signed float -108.0 to 106.0 (approx)

    let Lc = calcAPCA(colorTEXT,colorBG);

NEW! Font Size Array

New in this version is an interpolated font size array. Send fontLookupAPCA(contrast) a contrast value, and it returns an array, with the contrast (Lc) as the zeroth element, then 9 font sizes in px corresponding to weights 100 thru 900:



fontArray = fontLookupAPCA(-68.541);

console.log(fontArray) // -68.541,67,40,28,20.5,18.5,16.5,15,416,418

Thus the 400 weight font is indicating 20.5px

NEW! Reverse APCA

New in this version is a reverse contrast lookup. Specify a contrast, and one color (i.e. bg) and it will try to find a color with that contrast.


  1. Currently only returns a greyscale color
  2. If a color can not fit the contrast, or other error, returns false.
    • A small overrun/underrun of a few percent is permitted.
  3. Can return a hex string (default) or an array of RGBA values, with the fifth element a string indicating if the color is for text or bg.
    reverseAPCA (Lc, knownY, knownType = 'bg', returnAs = 'hex')

String Theory

See the colorparsley package for documentation on the available string inputs.

colorParsley() is a dependency for the shorthand ` calcAPCA() `

Two Bonus Parameters

There are two extra parameters for calcAPCA(), and one extra for APCAcontrast.

    calcAPCA( text, BG, places, isInt )
    APCAcontrast ( txYs, bgYs, places = -1 )
    alphaBlend( txt, BG, round(bool) )

` places ` defaults to -1, but you can send it 0 and the Lc is returned as a rounded value, and instead of a minus sign for polarity, ‘WoB’ for white on black is returned.

` isInt ` or round defaults to true, as we assume the RGB tuples are 0-255. If you are sending float such as for displayP3, then set ` isInt = false ``

NOTE: neither of these are “official” and may change, move, or vanish.

APCA-W3 Basic Math (sRGB)

the math for the basic APCA

Accessibility Statement for Beta Sites

Beta testers may wish to include the following statement in their site’s boilerplate:

This website is beta testing the APCA guidelines for determining text and non-text contrast. It is well known that WCAG 2 contrast maths are not accurate regarding human perception, and automated testing using WCAG 2 math may incorrectly indicate errors in some color pairs on this site. The APCA guidelines are a substantial improvement in accessibility over WCAG 2 contrast criterions, and therefore compliant with the ADA, and also 508 rules under the alternative methods section. Any automated or manual testing done to determine conformance of this site needs to be conducted using a perceptually accurate method such as APCA.


Current Algorithm Version: 0.0.98G-4g (February 15, 2021) (w3)

This is the base algorithm version. The other version numbering as listed below are for the overall library file, relating to features and functions are added to aide in integration. These added features do not impact the base algorithm which is stable and undergoing public beta validation.

Current Library Version: 0.1.9 (w3) (98G4g) beta

Font Lookup Table

Current as of May 27, 2022

Notes on the lookups:

APCA Lookup Table
APCA Lookup table legend
APCA Lookup Table

Current APCA Constants

( 0.0.98G - W3 last changed Feb 15, 2021 )
These constants are for use with the web standard sRGB colorspace. These are the current constants for use with current library version 0.1.9+

/////  0.0.98G - W3 constants (W3 license only):                       /////
////   These constants remain unchanged for apca-w3                    ////
///    versions 0.1.0 (initial npm package) and later                  ///
//                                                                     //

  exponents =  { mainTRC: 2.4,    normBG: 0.56,    normTXT: 0.57,  revTXT: 0.62,  revBG: 0.65, };

  colorSpace = { sRco: 0.2126729, sGco: 0.7151522, sBco: 0.0721750, };

  clamps =     { blkThrs: 0.022,  blkClmp: 1.414,  loClip: 0.1,  deltaYmin: 0.0005, };

  scalers =    { scaleBoW: 1.14,  loBoWoffset: 0.027, 
                 scaleWoB: 1.14,  loWoBoffset: 0.027, };	

// Note: loClip && deltaYmin do not affect lc in range & only clamp low Lc values

/////   0.1.1 Color space coefficients for P3 and Adobe                /////
////    These are derived from the 1931 CIE standard observer          ////
///     Using the equations found at BruceLindbloom.com                ///
//      And using the current CIE D65                                  //

/////  Display P3: /////

const sRco = 0.2289829594805780, 
      sGco = 0.6917492625852380, 
      sBco = 0.0792677779341829; // displayP3 coefficients

// Derived from 1931 CIE xyY:
// xW       yW        K     xR    yR    xG    yG    xB    yB
// 0.312720 0.329030  6504  0.680 0.320 0.265 0.690 0.150 0.060

///// AdobeRGB: /////

const mainTRC = 2.35; // Pending further evaluation:
                     // 2.35 exponent to emulate actual monitor perception

const sRco = 0.2973550227113810, 
      sGco = 0.6273727497145280, 
      sBco = 0.0752722275740913; // adobeRGB coefficients

// Derived from 1931 CIE xyY:
// xW       yW        K     xR    yR    xG    yG    xB    yB
// 0.312720 0.329030  6504  0.640 0.330 0.210 0.710 0.150 0.060

///// 0.1.14G MAGIC NUMBERS for UNCLAMP, used only with reverseAPCA() /////
////  for use with blkThrs: 0.022 & blkClmp: 1.414                    ////

  const mFactor = 1.94685544331710;
  const mFactInv = 1/mFactor;
  const mOffsetIn = 0.03873938165714010;
  const mExpAdj = 0.2833433964208690;
  const mExp = mExpAdj / blkClmp;
  const mOffsetOut = 0.3128657958707580;


0.1.9 - July 3, 2022

0.1.8 • June 5, 2022

0.1.7 • June 5, 2022

0.1.4 • May 27, 2022

0.1.3 • May 17, 2022

0.1.2 • April 23, 2022

CHANGE for 0.1.1: Jan 12, 2022

CHANGE for 0.1.0: Jan 10, 2022

0.0.98G-4g-4: Dec 21, 2021

0.0.98G-4g-3: Dec 13, 2021

0.0.98G-4g-2: Dec 11, 2021

0.0.98G-4g-betafish: Initial npm publish Dec 2, 2021

This moved over the base APCA and G4g constants (from Feb. 15th, 2021) to an npm package.

APCA is the Accessible Perceptual Contrast Algorithm


Disclaimer: APCA is being evaluated as a replacement for WCAG 2 contrast math for future standards and guidelines, however, standards that will be incorporating APCA are still developmental. Because WCAG 2 contrast math does not accurately model human visual perception nor visual impairments, there will be discrepancies between WCAG 2 contrast math, and perceptually uniform models such as APCA. It is up to the end user to determine suitability of purpose for their region and conformance requirements.