Anvil › Docs › /v1/percent

Percentage calculator API: of, is, change, apply, reverse

Five everyday percentage questions in one endpoint. Ask what 20% of 50 is, what percent 15 is of 60, how much 80→100 changed, what 200 becomes after +15%, or what the original was before a markup.

Parameters

Example

Request:

GET https://api.oddlathe.com/v1/percent?mode=of&percent=20&of=50

Response (generated by the real code when these docs were last built; random or time-based values will differ for you):

{
 "mode": "of",
 "percent": 20,
 "of": 50,
 "result": 10,
 "formula": "20% of 50 = 10"
}

Code examples

curl 'https://api.oddlathe.com/v1/percent?mode=of&percent=20&of=50'

const params = new URLSearchParams({
  mode: "of",
  percent: "20",
  of: "50"
});
const res = await fetch(`https://api.oddlathe.com/v1/percent?${params}`);
if (!res.ok) throw new Error((await res.json()).error.message);
const data = await res.json();
console.log(data);

import json, urllib.parse, urllib.request

params = urllib.parse.urlencode({"mode": "of", "percent": "20", "of": "50"})
url = "https://api.oddlathe.com/v1/percent?" + params
# Send a User-Agent: the edge network refuses Python's default one.
req = urllib.request.Request(url, headers={"User-Agent": "my-script/1.0"})
with urllib.request.urlopen(req, timeout=10) as r:
    data = json.load(r)
print(data)

When it's useful

• Checkout / invoice scripts that need markups and discounts without a spreadsheet
• Normalising "was £80, now £100" into a signed percent change for reports
• Recovering a pre-tax or pre-markup price from the final figure
• Teaching the difference between "20% of" and "percent change"

Good to know

• mode is required: of, is, change, apply or reverse.
• mode=of needs percent and of (the whole). Returns result.
• mode=is needs part and whole. Returns percent. whole must not be zero.
• mode=change needs from and to. Returns percent_change, absolute_change and direction. Percent change is (to − from) ÷ from × 100, so 80→100 is +25%. from must not be zero.
• mode=apply needs value and percent (positive increases, negative decreases). Returns result.
• mode=reverse needs result (the value after the change) and percent. Returns original. A −100% change cannot be reversed.
• A trailing % on number params is accepted and stripped. Pure maths on the query string; nothing is stored.
• Browser UI: Tooltern percentage calculator (same maths, nothing leaves your device).

Five percentage questions: what is X% of Y, X is what % of Y, percent change from A to B, apply ±% to a value, or recover the original before a % change. Pure maths; nothing is stored.

Errors

Errors are JSON like {"error": {"code": "…", "message": "…"}}. The ones this endpoint can return:

• missing_param (400): a required parameter is missing or empty
• bad_param (400): a value is the wrong type or out of range (the message says which)
• rate_limited (429): over 60 requests a minute; wait for Retry-After seconds

Questions

Related endpoints

• /v1/unitsConvert length, mass, temperature, data-size or data-rate units
• /v1/date/diffAge, duration and ISO week maths between two dates
• /v1/colorColour formats (hex, RGB, HSL, OKLCH) and WCAG contrast

Prefer a page to an API?

• Percentage calculator (of, is, change, apply, reverse) on Tooltern: same job, nothing leaves your browser.

Published 6 November 2026 · OpenAPI 3.1 spec · Limits and privacy