Everyday math
Scale a ratio to find the missing equivalent amount.
Enter your numbers to see the answer.
Ratio calculator guide
A ratio such as 2:3 says that for every 2 units of the first quantity there are 3 units of the second. This calculator scales that relationship to a real amount: you enter the two ratio terms and a known amount of the first quantity, and it returns the matching amount of the second. Recipes (2 parts rice to 3 parts water), mixing (paint, fertiliser, fuel), map scales, and aspect ratios all use this step. The known amount always corresponds to the first term, so 10 of the first quantity paired with a 2:3 ratio gives 15 of the second. If you know the second quantity instead, swap the terms to 3:2 before entering it. The tool scales one ratio to one amount; it does not simplify ratios, split a total into parts, or compare two ratios, though the worked examples below show how to do those by hand.
Equivalent second amount = (second term ÷ first term) × known amount of the first quantity.
Worked example with the defaults: first amount 2, second amount 3, known amount 10. The scale factor is the ratio of the terms: 3 ÷ 2 = 1.5. Multiply the known amount by it: 1.5 × 10 = 15. The calculator displays 15 as the equivalent second amount, with rows showing the ratio 2:3 and the known amount 10. Read as a recipe, 10 cups of rice at 2:3 need 15 cups of water. If you knew the water (15) and wanted the rice, you would enter the ratio as 3:2 with 15 known, giving 10.
The two ratio terms are unitless counts of “parts”. The known amount carries the real unit (cups, litres, millimetres, dollars) and the answer comes back in that same unit. Ratio terms can be decimals, so 1.5:1 is fine. Results round to two decimals by default; a ratio of 3:7 applied to 10 shows 23.33.
The result follows directly from the numbers you entered, so its accuracy is the accuracy of those inputs. The most common source of a surprising answer is a mismatch between the question you meant to ask and the one the calculator answered, so re-read the field labels if the figure looks wrong. The formula is shown above so you can check the arithmetic yourself. On this page the figure rests entirely on first amount, second amount and known amount, so start there if the ratio calculator returns something you did not expect.
Good to know: the known amount must belong to the first term. Entering 10 as a known second quantity against 2:3 gives 15 when the correct first quantity is 6.67. To split a total into a ratio, add the terms first: a $50 total shared 2:3 has 5 parts of $10, so $20 and $30; this calculator does not do that step. Ratios keep their meaning when both terms are multiplied by the same number, so 2:3, 4:6, and 10:15 are the same ratio.
Results are planning estimates. Check the units, product coverage, carrier rules, or local rates before making a purchase or financial decision.
How it works
Scale a ratio to find the missing equivalent amount. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With First amount = 2 · Second amount = 3 · Known amount = 10 → 15 (equivalent second amount). Change an input above and this example updates with your numbers.
Common questions
15. Divide the second term by the first to get 3 ÷ 2 = 1.5, then multiply the known amount: 1.5 × 10 = 15. Ten cups of rice at a 2:3 rice-to-water ratio need 15 cups of water.
Divide the amount you have by the matching ratio term to find the size of one part, then multiply the other term by that. For 2:3 with 10 of the first, one part is 5, and 3 parts is 15.
Add the ratio terms to find the number of parts, divide the total by that to get one part, then multiply each term. $50 split 2:3 is 5 parts of $10: $20 and $30.
Set up the proportion 2:3 = 10:x and cross-multiply: 2x = 30, so x = 15. The calculator performs this step directly when you enter the known amount for the first term.
Yes. A mix of 1.5:1 with 6 of the first gives 1 ÷ 1.5 × 6 = 4 of the second. Decimal terms are common in fuel mixes and dilution ratios.