Speedy Calculator

Ratio Calculator

Math Updated October 25, 2026

Quick answer

The ratio calculator scales a ratio A : B up or down to find a missing value, for example resizing an image or a recipe. Enter the original ratio and one new value to get the other.

New B (D)
—
Simplest form of A : B
—
… : B
—

Equivalent ratios keep the same proportion between quantities, whatever their size.

How to use the Ratio Calculator

  1. A
  2. B
  3. New A (C)
  4. Want to work backwards? Type a value into New B (D) and New A (C) is calculated for you.

Results update as you type, and every field accepts the units shown in its menu.

How it works

Cross-multiply: the missing value equals B times the new A, divided by the original A.

Inputs explained

Field What to enter
A A number
B A number
New A (C) A number

What each result means

  • New B (D) = [B] × [New A (C)] ÷ [A]
  • Simplest form of A : B = [A] ÷ gcd([A], [B])
  • … : B = [B] ÷ gcd([A], [B])

Example values

How the main result changes when New A changes and everything else stays at the example values above:

New A (C) New B (D)
960 540
1,440 810
1,920 1,080
2,400 1,350
2,880 1,620
3,840 2,160

Formula

A : B = C : D → D = B × C ÷ A

Worked example

Inputs — A: 16; B: 9; New A (C): 1,920.
Results — New B (D) = 1,080; Simplest form of A : B = 16; … : B = 9.

Frequently asked questions

How do I simplify a ratio?

Divide both parts by their greatest common factor.

Is a ratio the same as a fraction?

Related: A : B corresponds to the fraction A/B.

What formula does the Ratio Calculator use?

A : B = C : D → D = B × C ÷ A.

Can I work backwards from the result?

Yes. Type the result you want into New B (D) and the calculator finds the matching new a (c).

Cite this calculator

Speedy Calculator. (2026). Ratio Calculator. Speedy Calculator. https://speedycalculator.com/calculators/math/ratio/