Solve proportions with the rule of three: enter three values and the fourth is calculated as you type. Covers direct and inverse proportionality, shows the formula with your numbers plugged in, and explains the result in a sentence.
Everything runs locally in your browser. Your data never leaves your device.
Setup
How the two quantities relate, which value to solve, and how results round.
Proportionality
More of one means more of the other in the same ratio. Twice the apples cost twice as much.
Solve for
Auto solves the field you leave empty.
Decimal places
Values
Fill in three values. The highlighted one is calculated for you.
If
A
applescorrespond to
B
€
then
C
applescorrespond to
X
€
Result
The formula with your numbers plugged in, the solved value, and the answer in words.
X = B × C ÷ A
X = 2 × 7 ÷ 3 = 4.67
Result (X)
4.67€
In words
If 3 apples correspond to 2 €, then 7 apples correspond to 4.67 €.
About the Rule of Three Calculator Tool
This tool solves proportions with the rule of three. Fill in three of the four values in the sentence and the fourth is calculated as you type, whichever one you leave out. It handles both direct proportionality, where the values grow together, and inverse proportionality, where one shrinks as the other grows.
The formula is always shown with your numbers plugged in, so you can see exactly how the answer was reached, and the result is also written out as a plain sentence. Optional unit labels like apples and € make that sentence read naturally.
What you can do
Solve a proportion for any of the four positions, not just the last one.
Switch between direct and inverse proportionality with one toggle.
See the cross multiplication formula with your own numbers filled in.
Read the answer as a full sentence, with your own unit labels.
Type decimals with a point or a comma, both are understood.
Copy the result or the sentence with one click, and set the decimal places.
How to use the Rule of Three Calculator
1Pick direct or inverse proportionality. The one line explanation tells you which fits.
2Type three of the four values into the sentence and leave the one you want to find empty, or pick it under Solve for.
3Optionally name the two quantities, like apples and €, so the answer reads as a sentence.
4Read the result next to the formula and copy it with the button.
Direct or inverse?
Direct proportionality means the two quantities move together. If 3 apples cost 2 €, then 7 apples cost more, and the tool computes X = B × C ÷ A. Prices, distances and recipe amounts usually work this way.
Inverse proportionality means one quantity shrinks as the other grows. If 4 workers need 6 hours, then 8 workers need less time, not more, and the formula becomes X = A × B ÷ C. Workers and time, speed and travel time, or pump count and fill time are typical inverse cases. If your answer looks upside down, you probably need the other mode.
Where the formula comes from
The rule of three is cross multiplication in disguise. In a direct proportion A : B = C : X the products across the equals sign match, so A × X = B × C, and dividing by A gives X. In an inverse proportion the products of each pair match instead, so A × B = C × X. The tool picks the right rearrangement for whichever value you left empty and shows it with your numbers, so you can follow the math or copy it into homework.
Some questions have no answer. When the value you solve for would require dividing by zero, the tool says so in plain words instead of showing an error code. For percentage questions like discounts and tips, the Percentage Calculator asks them directly, and for switching between measurement units there is the Unit Converter.
Enter a resolution to get its simplified aspect ratio, or start from a ratio and one side to get the missing dimension. Knows the common ratios like 16:9, 4:3 and 21:9 and shows how close your size is to each.
Calculate AND, OR, XOR, NOT and bit shifts with a live binary view of every operand and the result. Enter numbers in decimal, hex, octal or binary and see each bit line up as you type.
Convert Unix file permissions between checkboxes, octal like 755 and symbolic notation like rwxr-xr-x, all kept in sync as you edit any of them. Covers setuid, setgid and the sticky bit, and shows the matching chmod command ready to copy.
Convert between px, rem, em, percent, pt and viewport units with a configurable root font size. Type into any field and the others update as you go, set the parent font size for em math, and read a conversion table for common sizes.
Work with dates: count the days between two dates, add or subtract a duration, get someone's exact age, and look up the ISO week number, day of year and other facts about any date.
Work out how long a download or file transfer takes from the file size and your connection speed. Handles all the common size and speed units, breaks the duration down readably, compares typical connections and can also solve for speed or file size.