Percentage Calculator

About the Percentage Calculator

This calculator covers the three percentage questions that come up most often, without you having to remember which number goes where:

• X% of Y — a share of a total. Working out a 15% tip on a $80 bill, 20% off a marked price, how much 8% commission is on a sale, or GST/VAT on a subtotal.
• X is what % of Y — turning two raw numbers into a proportion. Your score of 47 out of 60, 340 MB used of a 2 GB plan, or how much of a monthly budget one bill represents.
• Percentage change — how much something moved from an old value to a new one. A price rise, a salary bump, a drop in website traffic, or quarter-over-quarter growth.

It's aimed at anyone who does quick maths day to day — students checking marks, shoppers comparing discounts, freelancers invoicing, and anyone reconciling figures — rather than at spreadsheet-heavy financial modelling.

How it works — the actual formulas

Each mode uses the standard percentage identity, part = rate × whole, rearranged for the unknown:

• X% of Y → (X ÷ 100) × Y. So 20% of 150 = 0.20 × 150 = 30.
• X is what % of Y → (X ÷ Y) × 100. So 30 out of 150 = (30 ÷ 150) × 100 = 20%.
• Percentage change → ((New − Old) ÷ Old) × 100, using the old value as the base. From 150 down to 120 = ((120 − 150) ÷ 150) × 100 = −20%; a negative result is labelled a decrease, positive an increase.

Everything recalculates as you type — there's no button to press.

Assumptions and rounding

• Results are rounded to two decimal places for display, then formatted with your device's locale (so you may see thousands separators like 1,250). The underlying maths uses full precision; only what you see is rounded.
• Percentage change always uses the first (old) value as the base. Going from 100 to 150 is a +50% change, but going from 150 back to 100 is −33.33% — the two directions are not symmetric, because the base differs. That's mathematically correct, not a bug.
• Inputs accept decimals and negative numbers.

Limitations

• Percentage change from zero is undefined. If the old value is 0, dividing by it has no answer, so the result shows a dash rather than infinity. There's genuinely no meaningful "percentage increase from nothing."
• It calculates a single percentage relationship at a time — it isn't a compounding or amortisation tool. For repeated growth (e.g. interest year after year) or loan repayments, a change of X% applied once is not the same as X% applied repeatedly.
• Rounding to two decimals means a long chain of calculations done by hand from the displayed values can drift by a cent or two from the exact figure.

Quick reference

A few relationships worth remembering:

• Percentage points ≠ percent. Going from 5% to 7% is a rise of 2 percentage points, but a 40% increase in the rate itself. They're different statements about the same move.
• A % increase then the same % decrease doesn't return you to the start. +10% then −10% of 100 gives 99, because the second percentage is taken from a larger number.
• To reverse a discount: if a price of $80 is after 20% off, the original was 80 ÷ 0.8 = $100, not 80 + 20%.

Privacy

Every calculation runs in your browser with JavaScript. No numbers are sent to a server, logged, or stored.