Online Tool Store
Ratio Calculator
Solve missing values and scale two-part proportional ratios with live formulas and equivalent-ratio steps. Free and runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Example shown — replace the values with your own ratio.
Solve a proportion
Choose which term is unknown, then enter the other three values.
Missing value
How the ratio calculator works
- Choose whether to solve one missing term or scale a two-part ratio.
- Replace the worked example with your values and, when solving, choose the unknown term.
- Read the equivalent ratio, scale factor, and substituted formula as they update instantly.
The proportion formula
Two ratios are proportional when their fractions are equal: A/B = C/D. Cross multiplication gives A × D = B × C, which can be rearranged to find whichever term is missing.
A = (B × C) ÷ D
B = (A × D) ÷ C
C = (A × D) ÷ B
D = (B × C) ÷ A
In the example, 4:6 = 10:D, so D = (6 × 10) ÷ 4 = 15. To scale a ratio, divide the target by its corresponding original term to find a scale factor, then multiply both terms by that factor. Scaling 4:6 so the first term becomes 10 uses a factor of 10 ÷ 4 = 2.5 and produces 10:15.
FAQ
Can the calculator solve for any of the four ratio terms?
Yes. Choose A, B, C, or D as the missing term in A:B = C:D. Enter the other three positive values and the missing value updates instantly.
Can I use decimals in a ratio?
Yes. The inputs accept integers and decimals. Enter a decimal value rather than an expression such as 1/2; for one-half, enter 0.5.
Why must every ratio term be greater than zero?
This calculator is designed for proportional quantities such as recipes, maps, models, and dimensions. Positive terms keep those real-world scales meaningful and avoid division by zero.
How are long decimal results rounded?
Calculations use full JavaScript number precision, while results are displayed with up to 10 decimal places. Very large or very small values use scientific notation.
Are my ratio values sent to a server?
No. The calculation runs locally in your browser. Your values are not uploaded or sent to a calculation service.
How we compare
| Feature | Online Tool Store | CalculatorSoup | Calculator.net | Omni Calculator |
|---|---|---|---|---|
| Solve a missing term in A:B = C:D | Any term | Any term | Any term | Any term |
| Scale to a chosen ratio term | Dedicated mode | Included | Via proportion | Via proportion |
| Shows the scale factor | With every result | Shows calculation work | Scaling mode | Explained in method |
| Formula with your values substituted | Updates instantly | Shows calculation work | Method and examples | Method and examples |
CalculatorSoup, Calculator.net, and Omni Calculator all solve standard ratio proportions. Choose this Ratio Calculator when you want a focused, live workspace that combines missing-term solving and target-based scaling while keeping the scale factor and substituted formula visible.