Solve a proportion by entering three known values. The calculator finds the missing fourth value and shows the cross-multiplication steps. It is useful for scale drawings, recipes, unit rates, maps and comparing quantities that change at the same rate.
Solve a / b = c / x for x.
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A proportion compares two ratios that describe the same relationship. Use this page when three values are known and the fourth is missing, such as scaling a recipe, reading a map scale, or comparing quantities at a constant rate.
This guide explains the underlying idea, walks through a practical example, and highlights situations where the result needs careful interpretation. Use the calculator above for your own values, then compare the displayed steps with the method below.
Write the relationship in matching order, for example 2/3 = 8/x. Cross multiplication gives 2 × x = 3 × 8, so 2x = 24 and x = 12. The answer is meaningful because both ratios now represent the same scale factor. Always keep corresponding units in the same positions. If the first ratio is cups per serving, the second must use cups per serving too.
The result should be interpreted in the context of the question, not just as a number on its own. Check the units, the starting assumptions, and whether the inputs describe the same kind of quantity. If you are comparing two different forms of a value, choose a tool that uses the appropriate definition rather than forcing the numbers into a formula that does not fit.
Keep full precision while working through intermediate steps. Round only when the final answer needs a stated level of precision, and make that choice clear when sharing the result.
A map uses 1 cm to represent 5 km. If two towns are 4 cm apart on the map, the real distance is 20 km. The proportion is 1/5 = 4/x, which gives x = 20. For recipe scaling, multiply every ingredient by the same factor. For example, doubling a recipe means multiplying each ingredient quantity by 2, not adding 2 to each amount.
After calculating, substitute the result back into the original question or reverse the operation where possible. A sensible answer should satisfy the original relationship and match the expected scale. If the answer seems unexpectedly large or small, check whether a decimal point, sign, unit, or input order was entered incorrectly.
Try a second example with simpler values. This is a useful way to check that you understand the process rather than relying only on the calculator output.
A proportion compares two ratios that describe the same relationship. Use this page when three values are known and the fourth is missing, such as scaling a recipe, reading a map scale, or comparing quantities at a constant rate. It can also support homework checks, quick estimates, and everyday decisions where a consistent numerical method matters. Write down what each input represents before entering values, especially when working with measurements or comparing results from different sources.
Do not swap numerator and denominator in only one ratio. Do not compare unlike units without converting them first. A denominator cannot be zero, because division by zero is undefined. A proportion is different from percentage change: proportions preserve a constant ratio, while percentage change measures how a value moves from an original baseline.
Choose the next tool based on what you need to do with the result. These links lead to specific calculations that often appear alongside this topic.
Enter values that match the labels shown in the calculator. Check whether the inputs should be whole numbers, decimals, fractions, or positive values before calculating.
Keep the full result for intermediate calculations. Round only when the task asks for a particular number of decimal places or significant figures, and use the same precision consistently.
Yes. Use the unrounded value where possible, and make sure the next calculation uses compatible units and assumptions. A related calculator above may help with the next step.
Different tools may use different rounding rules, precision limits, or definitions. Check the formula, input order, and displayed precision before comparing results.
Enter your values in the calculator above, review the result, and use the guide to understand the method.
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