Percentage Calculator

Overview

A five-panel percentage calculator that answers the questions people actually type into a search box: what is X percent of a number, what percentage one number is of another, how much a value changed between two points, what a number becomes after adding or subtracting a percentage, and what whole a given part is a given percentage of. Every panel recalculates as you edit the numbers, with no button to press and nothing sent anywhere.

How It Works

The percent-of panel computes 'what is A% of B' by multiplying A/100 by B. The share panel computes 'C is what percent of D' as C divided by D times 100. The change panel computes percentage change from an old value to a new one, colouring the result green for an increase and red for a decrease and labelling the direction. The add/subtract panel applies a rate to a number multiplicatively — enter a negative rate to subtract, so 200 at −10% gives 180. The reverse panel answers 'X is P% of what number' by dividing, so 30 being 15% of something means the whole is 200. All five panels run live from their own pair of number inputs, so you can work several problems side by side.

Step-by-Step Usage Guide

1. Pick the panel that matches your question.
2. Type the two numbers into that panel's inputs.
3. Read the live result beside the equals sign.
4. For percent change, put the earlier value first and the later value second; for add/subtract, type a negative rate to subtract.

Technical Specifications & Standards

The percentage-change panel divides by the absolute value of the starting number — ((new − old) / |old|) × 100 — so a fall from 100 to 50 shows as −50% and a rise from 50 to 100 shows as +100%; the change is always measured against the starting value, and the absolute value in the denominator merely keeps the sign meaningful for negative starting numbers. When the starting value is zero the change is mathematically undefined, and the panel shows a dash instead of a misleading result; the share panel shows the same dash when the base is zero, and the reverse panel shows a dash when the percentage is zero, because zero percent maps a part to no unique whole. The add/subtract panel multiplies by (1 + pct/100), so −10% of 200 is 180 exactly, and results are trimmed to two decimals. One habit worth noting: percentages of change compound, so a 50% loss followed by a 50% gain does not return you to the start — apply the change panel twice to see why. The inputs are not persisted across a page refresh; sensible defaults are pre-filled so each panel's layout explains itself.

Targeted Use Cases

• Working out a 15% tip or discount on a bill.
• Checking what share of a budget one expense represents.
• Measuring month-over-month growth or decline in a metric.
• Recovering the original whole from a known part and percentage, such as a price before a known margin was applied.

Notes & Gotchas

• Keep the direction right for change problems: old value first, new value second.
• Remember a percent change from zero is undefined, which is why the panel shows a dash.
• For successive changes, apply the panel twice instead of adding percentages.
• Round only at the end; the two-decimal display is for reading, not for further math.

Frequently Asked Questions