How do I calculate a percentage of a number?

Multiply the number by the percentage and divide by 100. Example: 25% of 80 = 80 × 25 ÷ 100 = 20.

How do I find what percent one number is of another?

Divide the part by the whole and multiply by 100. Example: 20 is what percent of 80? 20 ÷ 80 × 100 = 25%.

How do I calculate percentage increase/decrease?

Subtract old from new, divide the change by the old value, and multiply by 100. Increase if the result is positive, decrease if negative.

Percentage Calculator

Free percentage calculator to solve every percent problem: what is X% of Y, X is what percent of Y, percentage increase/decrease, and add/subtract a percent. Fast, mobile-friendly, with formulas and worked examples.

Full original guide (expanded)

Percentages

Percentage Calculator

Four mini-tools in one: What is X% of Y, X is what percent of Y, Percentage increase/decrease, and Add/Subtract a percent. This covers almost every real-life percent problem (discounts, VAT, tips, markups, grades, analytics).

Percent (X)

Of (Y)

Result

Answer

—

Steps

—

Percentage formulas used here

1. What is X% of Y?
\(\text{Result} = \dfrac{X}{100} \times Y\)

2. X is what percent of Y?
\(\text{Percent} = \dfrac{X}{Y} \times 100\)

3. Percentage change
\(\text{Percent change} = \dfrac{\text{New} - \text{Old}}{\text{Old}} \times 100\)

4. Add/Subtract percent
\(\text{New} = \text{Original} \times (1 \pm \dfrac{p}{100})\)

When to use which mini-tool

Shopping, VAT, tips → “What is X% of Y”.
Exam scores, KPIs → “X is what percent of Y”.
Price changes, analytics, growth → “% increase/decrease”.
Markup / discount directly → “Add/Subtract %”.

FAQ

Can I enter decimals?

Yes, enter 12.5, 0.5, 99.99 — the calculator will handle them.

What if the denominator is 0?

You can’t divide by zero — the tool will show an error for those cases.

Audit: Complete

Formula (LaTeX) + variables + units

This section shows the formulas used by the calculator engine, plus variable definitions and units.

Formula (extracted LaTeX)

\[','\\]

','\

Formula (extracted text)

1. What is X% of Y?\(\text{Result} = \dfrac{X}{100} \times Y\) 2. X is what percent of Y?\(\text{Percent} = \dfrac{X}{Y} \times 100\) 3. Percentage change\(\text{Percent change} = \dfrac{\text{New} - \text{Old}}{\text{Old}} \times 100\) 4. Add/Subtract percent\(\text{New} = \text{Original} \times (1 \pm \dfrac{p}{100})\)

Variables and units

No variables provided in audit spec.

Sources (authoritative):

NIST — Weights and measures — nist.gov · Accessed 2026-01-19
https://www.nist.gov/pml/weights-and-measures
FTC — Consumer advice — consumer.ftc.gov · Accessed 2026-01-19
https://consumer.ftc.gov/

Changelog

Version: 0.1.0-draft
Last code update: 2026-01-19

0.1.0-draft · 2026-01-19

Initial audit spec draft generated from HTML extraction (review required).
Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
Confirm sources are authoritative and relevant to the calculator methodology.

Verified by Ugo Candido on 2026-01-19
Profile · LinkedIn

Formulas

(Formulas preserved from original page content, if present.)

Version 0.1.0-draft

Citations

Add authoritative sources relevant to this calculator (standards bodies, manuals, official docs).

Changelog

0.1.0-draft — 2026-01-19: Initial draft (review required).