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Yellowness index (ASTM)

Get one number for yellowing: X 80, Y 85 and Z 90 under illuminant C give a yellowness index of 8.0282 by ASTM E313. Positive values lean yellow, and the same inputs always give the same index. Free in your browser.

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Worked example

If you enter

X: 80
Y: 85
Z: 90
Illuminant: C
Decimal places: 4

You get

Yellowness index: 8.0282

ASTM E313 yellowness index from XYZ. Search intent: yellowness index calculator. No network calls or random seeds are required. Failed jobs are not billed.

What the ASTM E313 yellowness index measures

The yellowness index condenses the color of a white or clear sample into one number that grows as the sample turns yellow. ASTM E313 defines how to compute it from the three tristimulus values X, Y and Z that a colorimeter or spectrophotometer reports. In plain words, the method weighs X against Z, divides the difference by Y, the lightness value, and scales the result by 100. With X 80, Y 85 and Z 90 under illuminant C, the index is 8.0282. A value near zero reads as neutral white, positive values lean yellow and negative values lean blue.

When to use it and mistakes to avoid

The index is useful whenever you need to track yellowing over time or compare batches against a reference, because one number is easier to log and chart than three color values. Keep the conditions identical: the weighting factors depend on the illuminant and observer, so an index computed under one illuminant cannot be compared with one computed under another. Enter X, Y and Z on the same scale your instrument reports, without converting them to RGB or hex first. Avoid rounding the inputs before calculating; if you need fewer decimals, ask for them in the result, where rounding happens only once. Finally, remember that the index summarizes appearance, not the cause of the yellowing.

What you can do with it

Teaching

Pinned goldens.

FAQ

What is the yellowness index?

It is a single number, defined in the ASTM E313 standard, that tells how yellow a white or clear sample looks. It is calculated from the sample's X, Y and Z color values.

What does a yellowness index of 8.0282 mean?

It is the result for X 80, Y 85 and Z 90 under illuminant C. A positive value means the sample leans yellow; the larger the number, the stronger the yellow cast compared with a neutral white.

Why does the illuminant matter?

The calculation weighs X and Z with factors that depend on the light source and observer, so the same sample can give a different index under another illuminant. Always compare values computed with the same one.

Can I choose how many decimals to keep?

Yes. Ask for the number of decimal places you need, such as 4 in the example, and the result is rounded once, half up, from the unrounded value, so rounding never drifts.

Related tools

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/color/yellowness-index

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/color/yellowness-index \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"X":80,"Y":85,"Z":90}'

const res = await fetch("https://api.kit.forhosting.com/color/yellowness-index", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "X": 80,
    "Y": 85,
    "Z": 90
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/color/yellowness-index",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "X": 80,
    "Y": 85,
    "Z": 90
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/color/yellowness-index", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"X":80,"Y":85,"Z":90}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"X":80,"Y":85,"Z":90}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/color/yellowness-index", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "X": 80,
  "Y": 85,
  "Z": 90
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "color.yellowness_index",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Technical notes

The calculator is free in the browser, deterministic, and identical on the API path. Inputs are validated before arithmetic so invalid payloads fail with invalid_input. Half-up rounding is applied once from raw IEEE values when decimals are requested. Use the same JSON from spreadsheets, CMS hooks, automation, or teaching demos.

How it works

ASTM E313 yellowness index from XYZ. No network calls or random seeds are required. Failed jobs are not billed. No network calls or random seeds are required. Failed jobs are not billed. YI=100*(Cx X − Cz Z)/Y. The calculator is free in the browser, deterministic, and identical on the API path. Inputs are validated before arithmetic so invalid payloads fail with invalid_input. Half-up rounding is applied once from raw IEEE values when decimals are requested. Use the same JSON from spreadsheets, CMS hooks, automation, or teaching demos. The calculator is free in the browser, deterministic, and identical on the API path. Inputs are validated before arithmetic so invalid payloads fail with invalid_input. Half-up rounding is applied once from raw IEEE values when decimals are requested. Use the same JSON from spreadsheets, CMS hooks, automation, or teaching demos.

Inputs, limits and rounding

Optional decimals use half-up once. Optional output fields are omitted, never null. No network calls or random seeds are required. Failed jobs are not billed. No network calls or random seeds are required. Provide required JSON fields with DOM-safe names. Invalid types raise invalid_input. The calculator is free in the browser, deterministic, and identical on the API path. Inputs are validated before arithmetic so invalid payloads fail with invalid_input. Half-up rounding is applied once from raw IEEE values when decimals are requested. Use the same JSON from spreadsheets, CMS hooks, automation, or teaching demos. The calculator is free in the browser, deterministic, and identical on the API path. Inputs are validated before arithmetic so invalid payloads fail with invalid_input. Half-up rounding is applied once from raw IEEE values when decimals are requested. Use the same JSON from spreadsheets, CMS hooks,

When to use it

Use when search intent matches. ASTM E313 yellowness index from XYZ. EN-only delivery; Nitro hydrates locales. No network calls or random seeds are required. Failed jobs are not billed. No network calls or random seeds are required. Failed jobs are The calculator is free in the browser, deterministic, and identical on the API path. Inputs are validated before arithmetic so invalid payloads fail with invalid_input. Half-up rounding is applied once from raw IEEE values when decimals are requested. Use the same JSON from spreadsheets, CMS hooks, automation, or teaching demos. The calculator is free in the browser, deterministic, and identical on the API path. Inputs are validated before arithmetic so invalid payloads fail with invalid_input. Half-up rounding is applied once from raw IEEE values when decimals are requested. Use the same JSON from spreadsheets, CMS hooks, automation, or teaching demos.

Integration use cases

Quick check

Free widget.

Automation

API same JSON.

Technical questions

Free?

Yes tier A.

Network?

No.

Bad input?

invalid_input.

API?

Yes.

Locales?

EN-only; Nitro hydrates.

Read the full KIT documentation →

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