# Ratio Calculator

URL: https://speedycalculator.com/calculators/math/ratio/
Last updated: 2026-10-25

> 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.

## Inputs

- A
- B
- New A (C)

## Results

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

## Formula

```
A : B = C : D  →  D = B × C ÷ A
```

## Explanation

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

### How to use the Ratio Calculator

- **A**
- **B**
- **New A (C)**
- 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

FieldWhat to enterAA numberBA numberNew 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)9605401,4408101,9201,0802,4001,3502,8801,6203,8402,160

## 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.

## FAQ

**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).