Reduce 1.5 : 2.5
1.5 : 2.5 = 15 : 25 = 3 : 5
Multiply both values by 10 to remove the decimal points. Then divide both integers by their greatest common divisor, 5.
The ratio stays exactly the same at every step.
Everyday math
Ratio Calculator
Simplify two values to an exact ratio, or find a matching value in a proportion.
Calculate a ratio or proportion
Simplified ratio
Enter your values, then select Calculate.
Calculation steps
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Formulas and examples
Find X in 3 : 5 = 12 : X
X = B × C ÷ A = 5 × 12 ÷ 3 = 20
The first value grows from 3 to 12, a factor of 4. The second value grows by the same factor: 5 × 4 = 20.
Equivalently, cross-multiplication gives A × X = B × C.
Simplify a 1920 by 1080 rectangle
1920 : 1080 = 16 : 9
Divide both dimensions by 120. The exact aspect ratio is 16 : 9, while width divided by height is approximately 1.777778.
A rounded decimal is an approximation; it does not replace the exact integer ratio.
Common questions
How are decimal ratios simplified exactly?
The calculator reads each decimal as an exact decimal value. Both sides are multiplied by the same power of 10 until they are integers. It then divides both integers by their greatest common divisor. The reduction uses integer arithmetic, so values such as 0.1 : 0.3 simplify exactly to 1 : 3.
Can one or both sides be zero?
Simplify mode compares quantities: 0 : 5 becomes 0 : 1, and 5 : 0 becomes 1 : 0. A ratio with a zero second part cannot be interpreted as a division by that part. The pair 0 : 0 is rejected because it has no unique simplified ratio. Solve mode requires A, B, and C to be greater than zero, so the fractions represented by A : B = C : X remain defined.
Can I solve for any missing value?
This calculator solves only for the fourth value, X, in A : B = C : X. Enter the original pair as A and B, then enter the new first value as C. The calculation is X = B × C ÷ A.
What if a proportion produces a repeating decimal?
The exact answer is shown as a reduced fraction, such as 1/3. A separate decimal is shown to at most 6 decimal places by default. Choose 0, 2, 4, 6, or 10 places; unnecessary trailing zeros are omitted. Exact halfway values round away from zero. The ≈ symbol means the displayed decimal differs from the exact fraction.
Can I use this for screen sizes or image dimensions?
Yes. Enter the width and height in the same unit to simplify their aspect ratio. You can also scale one dimension using Solve mode. A mathematically exact scaled dimension may be a fraction, while an image or screen may require a whole number of pixels. Rounding that dimension can slightly change the resulting aspect ratio.
Which input formats are accepted?
Enter nonnegative numbers in Simplify mode and positive numbers in Solve mode, with up to 15 digits before the decimal point and 10 after it. Correctly grouped thousands commas are allowed, such as 1,920. Use a period for decimals and include a leading zero, as in 0.5. Enter each value in its own field; fractions, ratio notation such as 3:5, units, and scientific notation are not accepted.