How to use the ratio calculator
- Enter both sides of the ratio, whole numbers or decimals.
- Read the simplified form, reduced using the greatest common divisor.
- Check the decimal and the percentage split beneath it.
- To scale, enter what the first value should become; the second follows proportionally.
- Copy the breakdown if you are handing the figures to someone else.
What this calculator does
- Reduces any ratio to its lowest whole-number terms.
- Handles decimal inputs by converting to whole numbers before simplifying.
- Scales a ratio so one side hits a target value, keeping the proportion exact.
- Shows the ratio as a decimal, which is what design software usually wants.
- Shows each side as a percentage of the total, for splitting amounts.
- Works with any two positive values, live as you type.
What a ratio expresses
A ratio compares two quantities without saying how large either is. A 16 : 9 screen is the same shape whether it is a phone or a cinema. A 2 : 1 mix is the same recipe whether you are making a cup or a bucket. That independence from absolute size is exactly why ratios are useful and why they need simplifying to be readable.
Simplification divides both sides by their greatest common divisor. 1920 : 1080 becomes 16 : 9 because both divide by 120. The relationship is unchanged; it is just easier to see and to remember.
Ratio, proportion or fraction?
- Ratio: 3 : 1 compares two parts to each other.
- Fraction: 3/4 compares one part to the whole.
- Proportion: a statement that two ratios are equal, such as 3 : 1 = 12 : 4.
The distinction matters and is a frequent source of error. A 3 : 1 ratio is not three quarters. It is three parts out of four total, which is 75 per cent of the whole for the first part. The percentage figure here does that conversion so you do not have to.
Scaling without distortion
Scaling means changing the absolute values while keeping the relationship identical. Enter a target for the first value and the second is calculated to match: 16 : 9 scaled so the first side is 1280 gives 1280 : 720, which is exactly why that resolution exists.
The same arithmetic applies to recipes, concrete mixes, paint tinting, model scales and map distances. Anywhere a proportion must survive a change in quantity, scaling rather than adding is the correct operation, adding the same amount to both sides changes the ratio, which is the mistake people make when doubling a recipe by adding rather than multiplying.
Where ratios turn up
Screen and image aspect ratios, gear ratios, financial ratios such as debt to equity, mixing ratios for chemicals and concrete, map scales, and the split of a shared cost or profit between people. In each case the useful question is either "reduce this to something readable" or "keep this relationship while changing the size", which are the two things this calculator does.
Decimals and rounding
Decimal inputs are multiplied up to whole numbers before the greatest common divisor is applied, so 2.5 : 5 correctly reduces to 1 : 2 rather than being rejected. Scaling can produce a fractional result, 16 : 9 scaled to 1000 gives 562.5, and the tool shows it rather than rounding silently, because whether to round is your decision.
For image dimensions specifically the aspect ratio calculator is purpose-built and includes presets. For dividing an amount by a ratio, the percentage calculator handles the resulting percentages. Everything runs in your browser.