Percentage Calculator

Related guide

Mortgage, Interest and Percentage Calculators, and the Numbers They Do Not Show You
The mortgage page recomputes as you type, the interest page waits for a button, and the percentage page answers a division by zero with 0%. Everything below was read out of each page's own script: the annuity formula, the rounding drift in the repayment table, the growth-curve point that lands a year past your term, and the axis label that is ten times off.

Four everyday percentage problems

The page has one panel per question type, and each shows both the answer and the formula it used, so you can follow the arithmetic rather than trusting a black box:

• Find a percentage of a number: value × percentage ÷ 100. (What is 15% of 200? → 30)
• Express one number as a percentage of another: part ÷ whole × 100. (30 is what percent of 200? → 15%)
• Percentage change between two values: (new − old) ÷ old × 100, shown with a + or − sign. (100 → 120 is +20%)
• Discount: price × discount ÷ 100 gives the amount saved, and the final price is price − saving. (20% off 300 saves 60, so you pay 240)

Percentage change versus percentage points

These two are easy to confuse and the difference matters most with rates. A fee going from 2% to 3% has risen by 1 percentage point, but that is a 50% increase in the fee itself. This page reports the relative change, because that is what the formula measures.

When someone quotes a "20% increase" in an interest rate, check which of the two they mean before you compare it with your own numbers.

Rounding, and the division-by-zero case

Results are rounded to two decimal places and trailing zeros are dropped, so 33.333…% displays as 33.33%. Nothing is rounded in the calculation itself — the displayed value is shortened only at the end, which means you can safely copy a figure out of one panel and use it in another.

If you ask what percentage one number is of zero, the answer is mathematically undefined, and the tool returns 0% rather than an error so the page keeps working. Treat that 0% as "not a meaningful question", not as a result.

FAQ

How do I work out a tip?
Use the first panel: enter the bill as the value and the tip percentage you want. 15% of 68 is 10.2, so you would leave 78.2 in total. For a quick mental check, 10% then half of that again gives you 15%.

Can I find the original price before a discount?
The discount panel works forward — from the original price to what you pay. To go backwards from a sale price, divide by (1 − discount): an item at 20% off that cost 80 was 80 ÷ 0.8 = 100.

Why does a change from 2% to 3% show as +50%?
Because percentage change is relative to the starting value: (3 − 2) ÷ 2 = 0.5. In everyday reporting that is often expressed as "1 percentage point", which is a different unit. Say which one you mean to avoid confusion.

Does the calculator handle negative numbers?
Yes. Negative inputs work in every panel, and the percentage-change panel shows a minus sign for decreases, so you can see the direction of the change at a glance.

How many decimal places should I use?
Two is right for money and for most reporting. If you are chaining calculations, keep the full precision between steps and round only at the end — the panels here do exactly that.

Is anything sent to a server?
No. Every panel calculates in your browser as you type; the numbers never leave your device.