Ratio Calculator
Simplify a ratio, find the missing number in a proportion, or split an amount between any number of shares. Nothing is uploaded.
Use any number of parts, separated by colons —
for example 2:3 or 1:1:2.
| Share | Parts | Amount | Percent |
|---|
How to use it
- The first box reduces a ratio to its simplest form. Screen sizes, map scales and recipes all live here.
- The second fills in the missing fourth number when you are scaling something up or down.
- The third divides money, land or ingredients between any number of shares.
What simplifying really does
A ratio compares two quantities, and the same relationship can be
written many ways. 1920:1080, 1280:720 and
16:9 all describe exactly the same shape of screen. The
simplest form is the one where no whole number divides both sides any
further.
To get there you divide both sides by their highest common factor. For 1920 and 1080 that factor is 120, which is why the answer is 16:9. This page shows you the factor it used, so the arithmetic is not a mystery.
Solving a proportion
The second calculator answers questions of the form “3 is to 4 as 15 is to what?” You cross-multiply: multiply 4 by 15, divide by 3, and the answer is 20.
This is the single most useful piece of ratio arithmetic in ordinary life. A recipe for four people scaled to seven, a map scale converted to real distance, a photograph resized without stretching — all the same sum.
Splitting an amount
To divide 5,000 in the ratio 2:3, add the parts to get 5, divide 5,000 by 5 to find that one part is worth 1,000, then multiply out: 2,000 and 3,000.
The third calculator accepts as many parts as you need, so
1:1:2 splits three ways, and it shows each share as a
percentage as well. Where the division does not come out exactly, the
amounts are rounded to two decimal places and the tool tells you when the
rounded shares no longer add back to the original total — which
matters when the thing being split is money.
Frequently asked questions
How do I simplify a ratio?
Divide both sides by the largest number that goes into each of them exactly, which is called the highest common factor. For 1920 to 1080 that number is 120, giving 16 to 9. The first calculator on this page does it for you and shows which factor it used.
Can it handle decimals in a ratio?
Yes. Decimals are scaled up to whole numbers before being simplified, so 2.5 to 1 becomes 5 to 2. Very long decimals are rounded sensibly rather than producing an absurdly large pair of whole numbers.
How do I split an amount in a ratio?
Add the parts of the ratio together, divide the amount by that total to find the value of one part, then multiply by each side. The third calculator does this, and it takes as many parts as you need, not only two.
Is anything I type sent anywhere?
No. The arithmetic happens in JavaScript inside your own browser. Nothing is uploaded and nothing is saved — close the tab and it is gone.