Direct Proportion Calculator
Calculate a new output under direct proportion from a known input-output pair and a new input.
How to use Direct Proportion Calculator
1. Enter Known Input, Known Output, New Input in the stated order. 2. Follow nonzero denominator and whole-number requirements where they apply. Keep corresponding quantities on the same unit scale. 3. Select Calculate or press Enter to see new output. 4. Copy result copies the output; Reset clears the inputs and answer.
Why use this tool?
Scale a quantity when output changes in the same proportion as input and the model passes through zero.
How the calculation works
Constant k = known output ÷ known input; new output = k × new input. Known input must be nonzero. Numeric outputs use browser floating-point arithmetic, with six decimal places at most. Smaller nonzero outputs use six significant digits in scientific notation. Unrepresentable intermediate values or numeric outputs beyond the supported magnitude are rejected. Numeric input magnitudes, including each fraction or ratio component, must not exceed 9,007,199,254,740,991. Mathematical references: https://openstax.org/books/prealgebra-2e/pages/4-key-concepts and https://openstax.org/books/prealgebra-2e/pages/6-5-solve-proportions-and-their-applications.
Example
Known Input = 10, Known Output = 25 and New Input = 5 give k = 2.5 and New Output = 12.5.
Practical tip
Check that the relationship is directly proportional. Fixed setup costs, offsets and changing rates do not fit y = kx.
Frequently asked questions
Does this work for any linear relationship?
No. A directly proportional relationship passes through zero. A line with a nonzero fixed offset needs a different formula.
Do signed inputs always have a practical interpretation?
These forms perform the stated arithmetic. Negative values, rates above 100% or a negative baseline can be mathematically valid while being unsuitable for a particular real-world quantity. Check the task and model before applying the result.
Does the tool choose units or round my input first?
Enter corresponding measurements in matching units; the maths forms do not convert units automatically. Exact fraction and ratio text is kept for integer arithmetic. Other numeric operations use browser floating-point values and the display precision stated above.