Ratio Calculator and Proportion Calculator
Our ratio calculator simplifies any ratio, solves proportions and splits an amount in a given ratio. It also works as a proportion calculator for recipes, maps and screen sizes, with an aspect ratio mode for images. Pick a mode and type your numbers to get the answer.
What a Ratio Is
A ratio compares two or more amounts of the same kind. The ratio 2 : 3 means that for every 2 units of the first thing there are 3 units of the second. You can write it as 2 : 3, 2 to 3 or the fraction 2/3.
Ratios describe the size of each part compared with the others, not the actual amounts. A class with 12 boys and 18 girls and a club with 2 boys and 3 girls share the same ratio. That idea of equal comparison sits behind every ratio calculator.
How to Use the Ratio Calculator
- Simplify a ratio: type terms separated by colons, such as 12:18 or 2:3:5.
- Solve a proportion: fill three of A, B, C and D and leave the fourth empty.
- Divide an amount: enter the total and the ratio to split it in.
- Aspect ratio: enter the original width and height, then one new size.
Results update as you type, and the note under them explains the method. Decimal terms work in every mode. You can use up to six terms when you simplify or split.
How to Calculate a Ratio in Simplest Form
To simplify a ratio, divide every term by the greatest common factor of all the terms. For 12 : 18 the common factor is 6, so the ratio becomes 2 : 3. With decimals, multiply every term by 10 or 100 first, so 1.5 : 2.5 becomes 15 : 25 and then 3 : 5.
The tool also shows a unit ratio, where the first term is 1. For 12 : 18 that is 1 : 1.5, which is handy when you compare prices or speeds. You also see each term as a share of the total, so 2 : 3 means 40% and 60%.
Equivalent Ratios
Equivalent ratios describe the same relationship with different numbers. Multiply or divide both terms by the same number and the ratio does not change, so 2 : 3, 4 : 6 and 20 : 30 are all equal. Checking this by hand gets slow with large numbers, and a ratio calculator does it in a moment.
Solving a Proportion
A proportion says that two ratios are equal, written A : B = C : D. If you know three of the values, cross multiplication finds the fourth, because A × D always equals B × C. That rule is all a proportion calculator needs.
Take 3 : 4 = 12 : D. Cross multiply to get 3 × D = 4 × 12, so D = 48 ÷ 3 = 16. Enter 3, 4 and 12 in the proportion calculator, leave D empty and it returns 16 with the check line 48 and 48.
Recipe Example
A recipe uses 250 g of flour for 4 people and you need to feed 10. Set up 4 : 250 = 10 : D and the proportion calculator gives 625 g. The same step scales every other ingredient in the recipe.
Map Scale Example
On a 1 : 50,000 map, 4 cm on paper stands for 4 × 50,000 = 200,000 cm on the ground. That is 2 km, found with the same proportion. Enter 1, 50000 and 4 in the proportion calculator to check it.
Dividing an Amount in a Ratio
To split an amount, add the ratio terms to find the total number of parts. Divide the amount by that total to get the value of one part, then multiply by each term. Sharing 1,000 in the ratio 2 : 3 gives 5 parts of 200 each, so the shares are 400 and 600.
Three way splits work the same way. Sharing 900 in the ratio 1 : 2 : 2 gives parts of 180, so the shares are 180, 360 and 360. Business partners, heirs and roommates use this to divide profits, estates and bills.
Aspect Ratio for Screens and Images
An aspect ratio is the width of a screen or image compared with its height. A 1920 × 1080 video simplifies to 16 : 9, while an Instagram portrait at 1080 × 1350 is 4 : 5. To resize without stretching, keep the same ratio for the new size.
Enter 1920 and 1080 as the original size and 1280 as the new width. The ratio calculator returns a new height of 720, which keeps the picture in proportion. Designers and video editors use this step every day when they export files for different platforms.
| Aspect ratio | Common use | Example size |
| 16 : 9 | HD video and most monitors | 1920 × 1080 |
| 4 : 3 | Older screens and iPad | 1024 × 768 |
| 1 : 1 | Square social posts | 1080 × 1080 |
| 4 : 5 | Portrait social posts | 1080 × 1350 |
| 9 : 16 | Stories, Reels and Shorts | 1080 × 1920 |
Part to Part and Part to Whole
A part to part ratio compares one group with another, such as 12 boys to 18 girls. A part to whole ratio compares one group with the total, such as 12 boys out of 30 students. The first gives 2 : 3 and the second gives 2 : 5, or 40%.
Everyday Uses of Ratios
- Cooking and baking, where a ratio scales recipes up or down.
- Building, where concrete is often mixed at 1 : 2 : 3 cement, sand and gravel.
- Finance, where debt to income and price to earnings ratios guide decisions.
Why Use Our Ratio Calculator
It handles simplifying, proportions, splitting and resizing in one place, with up to six terms. Every result shows its working, and decimals are welcome. It works as a proportion calculator on phones and desktops, and nothing you type is stored. Pick a mode and solve your ratio now.
Frequently asked questions
Is this ratio calculator free?
Yes, it is free with no sign up and no limit.
How to calculate a ratio from two numbers?
Divide both numbers by their greatest common factor to get the simplest form.
What does the proportion calculator need?
Any three of A, B, C and D, with the fourth box left empty.
What is a unit ratio?
It is the ratio rewritten so the first term equals 1.
How does cross multiplication work?
In A : B = C : D, the product A × D always equals B × C.
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