How to Calculate a Percentage
Dev Nexus4 min read
Master the one percentage formula behind percent-of and what-percent questions, with worked examples you can check against an online calculator.
Percentages are everywhere - discounts, tips, tax, test scores, interest - yet plenty of people freeze the moment a percent sign shows up. The good news is that almost every percentage question comes down to one simple idea and a couple of short formulas.
This guide breaks it down step by step, with worked examples you can follow along with. By the end you will be able to find a percent of a number, work out what percent one value is of another, and check your answers with confidence.
The Problem
The trouble with percentages is rarely the arithmetic - it is knowing which numbers to divide, which to multiply, and where the decimal point belongs. Type the same values in the wrong order and you get a confidently wrong answer.
A classic example: you scored undefined on a test out of undefined and want the percentage. Do you divide undefined by undefined, or undefined by undefined? A basic calculator will happily give you both, but only one is right. Without a clear method, you are guessing.
The Solution
The fix is to remember that a percentage is just a fraction out of undefined. The word means "per hundred", so 50% is 50/100, or 0.5. Once you see every percent as a decimal, the formulas fall out naturally.
There are really only two questions to master. Finding a percent of a number and finding what percent one number is of another are two sides of the same relationship. If you would rather skip the mental maths entirely, the Percentage Calculator applies the right formula for you and shows the result instantly, right in your browser.
Step-by-Step Guide
- 1
Turn the percent into a decimal
Divide the percentage by undefined - or just move the decimal point two places left. So
25%becomes0.25,8%becomes0.08, and150%becomes1.5. This one conversion is the foundation of every percentage calculation. - 2
Find a percent of a number
Multiply the number by the decimal. To find undefined% of undefined: convert undefined% to
0.15, then compute80 × 0.15 = 12. So undefined% of undefined is undefined. This is the calculation behind discounts, tips and tax. - 3
Find what percent A is of B
Divide the part by the whole, then multiply by undefined. For a test score of undefined out of undefined:
47 / 60 = 0.783, then× 100 = 78.3%. The whole always goes on the bottom of the division - that is the number you are measuring against. - 4
Check your answer with a sanity test
Ask whether the result feels right. undefined% of undefined should be smaller than undefined - undefined passes. undefined out of undefined should be a bit under undefined% because undefined would be exactly undefined% - undefined.undefined% passes. If a percent-of result is bigger than the original number, your percentage was over undefined%.
- 5
Round only at the end
Keep full precision while you work and round just the final figure. Rounding
0.783to0.78mid-calculation, then multiplying, can shift the answer by a whole percent. Do the maths first, present the tidy number last.
Common Mistakes
Dividing the wrong way round
For "what percent is A of B", the whole (B) must be the denominator. Computing
60 / 47instead of47 / 60gives undefined% - obviously wrong for a score out of undefined. When in doubt, remember the result should be under undefined% whenever the part is smaller than the whole.Forgetting to convert the percent
Multiplying
80 × 15instead of80 × 0.15gives undefined, not undefined. Always turn the percent into a decimal first, or divide by undefined at the end - never both, and never neither.Confusing the part and the whole
The "whole" is the total you are measuring against, not always the bigger number. If sales rose to undefined from a target of undefined, the target is the base. Read the question carefully to spot which value is the reference.
Rounding too early
Rounding intermediate results and then multiplying compounds the error. Carry the full decimal through every step and round only the number you actually report.
Frequently Asked Questions
What is the basic formula for a percentage?
A percent is a fraction out of 100. To find a percent of a number, multiply the number by the percent written as a decimal (percent ÷ 100). To find what percent A is of B, divide A by B and multiply by 100.
How do I turn a percentage into a decimal?
Divide it by 100, or move the decimal point two places to the left. So 25% becomes 0.25, 5% becomes 0.05, and 120% becomes 1.2.
How do I calculate a test score as a percentage?
Divide your score by the total possible and multiply by 100. For 47 out of 60, that is 47 / 60 × 100 = 78.3%.
What if the percentage is more than 100%?
That is fine - it just means the part is larger than the whole. For example, 150% of 40 is 40 × 1.5 = 60. Percentages above 100% are common when measuring growth.
Do I need an app to calculate percentages?
No. You can do it with the formulas above and any calculator, or use an online percentage calculator that applies the right formula and runs entirely in your browser.
Try the Tool
Percentage
Skip the mental maths - find any percentage instantly, privately, in your browser.
Related Tools
Related Articles
How to Calculate Percentage Increase & Decrease
Use one percentage change formula to handle increases, decreases, discounts and growth rates - with worked examples and the traps that catch people out.
Read articleAre UUIDs Really Unique?
In practice UUIDs never collide - here is the math behind why, and the one thing people get wrong: a UUID is an identifier, not a secret.
Read articleencodeURI vs encodeURIComponent Explained
The clear difference between encodeURI and encodeURIComponent, with examples showing which one to use for a value versus a whole URL.
Read article