Ratio Calculator
Three ways to work with ratios: simplify a ratio to its lowest terms, solve for a missing value in a proportion (A:B = C:D), or compare two ratios to see if they are equivalent. Results update instantly as you type.
1. Simplify a Ratio
2. Solve for a Missing Value (A:B = C:D)
Leave one field blank — the calculator fills in the missing value. The formula is A/B = C/D, so D = (B × C) ÷ A.
3. Compare Two Ratios
How Ratios Work
A ratio compares two quantities. It can be written as A:B, A/B, or "A to B". For example, a 3:4 ratio means for every 3 units of the first thing, there are 4 units of the second.
Simplifying divides both sides by their greatest common divisor (GCD). 24:36 simplifies to 2:3 because 24 ÷ 12 = 2 and 36 ÷ 12 = 3.
Proportions state that two ratios are equal: A:B = C:D. If three values are known, the fourth can be calculated by cross-multiplication: A × D = B × C.
Common Uses
- Scaling recipes — a 2:1 flour-to-water ratio for 4 people becomes 8:4 for 16 people.
- Aspect ratios — a 16:9 screen is 1.78× wider than it is tall; used for video, images, and monitors.
- Mixing paint, concrete, or chemicals — precise ratio matters for the final result.
- Financial ratios — debt-to-equity, current ratio, and other metrics expressed as ratios.
- Map scales — 1:100 means 1 cm on the map represents 100 cm in reality.
- Comparing prices — $3 for 500g vs $5 for 800g; the ratio tells you which is better value.
Frequently Asked Questions
What does simplifying a ratio do?
It reduces the ratio to the smallest whole numbers that preserve the same relationship. 24:36 and 2:3 describe the same proportion, but 2:3 is easier to work with. The simplification divides both values by their greatest common divisor.
How do I solve for a missing value in a ratio?
If you have A:B = C:D and one value is missing, cross-multiply and solve. For example, if 3:4 = 9:D, then 3 × D = 4 × 9, so D = 36 ÷ 3 = 12. The second calculator above does this automatically when you leave one field blank.
What is the difference between a ratio and a fraction?
A ratio compares two or more quantities of the same type. A fraction represents a part of a whole. 3:4 is a ratio meaning "3 for every 4." 3/4 is a fraction meaning "3 parts out of 4 total." A ratio does not always sum to a whole, while a fraction always does.
What is an aspect ratio?
An aspect ratio describes the proportional relationship between width and height. A 16:9 video is 16 units wide for every 9 units tall. Common aspect ratios: 16:9 (widescreen), 4:3 (old TV), 1:1 (square), 21:9 (ultrawide), 9:16 (vertical video). Use this calculator to simplify any width × height pair to its aspect ratio.
Can ratios have decimal or negative values?
Yes — ratios can use decimals (0.5:2) or negative numbers (-3:4), though negative ratios are rare in practice. This calculator accepts any numeric input and simplifies using the absolute GCD. For decimal ratios, the result may be shown as a decimal-to-decimal ratio unless both values convert cleanly to integers.
Is this calculator free and private?
Yes — completely free, no signup, no limits. All calculations run in your browser. No values you enter are sent to a server or stored.