Ratio Calculator

Effortlessly simplify ratios, solve proportions, scale, and convert with ThisCalc's Ratio Calculator.

Last reviewed: June 2026
Built and maintained by · Last updated June 2026

Simplify a Ratio

Enter a ratio A:B to reduce it to its simplest form.

:
Simplified Ratio
3:2
12:8 simplified by dividing both parts by GCD 4 = 3:2
Fraction
3/5
Decimal
1.5
A as % of total
60%

Quick Answer

To simplify a ratio, divide both numbers by their greatest common divisor — for example, 12:18 divides by 6 to become 2:3. To scale a ratio, multiply both sides by the same factor. Enter your ratio above to simplify it, find an equivalent ratio, or solve for a missing value in a proportion like 2:3 = 4:x.

Planning estimate: This calculator performs arithmetic ratio, proportion, scaling, and aspect-ratio calculations from the values provided. It does not judge whether a ratio is appropriate for a financial, medical, legal, engineering, or code-compliance decision.

Solve a Proportion (A:B = C:?)

Enter any three values to find the fourth.

:
=
:
Missing Value
12
3:4 = 9:12 (cross-product: 3 x 12 = 4 x 9 = 36)

Scale a Ratio

Multiply or divide both parts of a ratio by a scale factor.

:
x
Scaled Ratio
12:8
3:2 scaled by 4 = 12:8

Three-Part Ratio (A:B:C)

Simplify a three-part ratio and split a total into three shares.

:
:
Simplified Ratio
1:2:3
Already in simplest form. 6 total parts.
Share A
100
Share B
200
Share C
300

Aspect Ratio Calculator

Find the missing dimension or the simplified aspect ratio for screens and images.

x

Scale to new width:

x
Aspect Ratio
16:9
1920x1080 is a 16:9 aspect ratio. Scaled to 1280 wide: 1280x720.
Common aspect ratios:

What this ratio calculator does

This ratio calculator handles five common ratio tasks on one page: reducing a two-part ratio, solving a missing proportion value, scaling both sides by the same factor, simplifying a three-part ratio, and preserving an image or screen aspect ratio while changing width.

For a two-part ratio such as 12:8, the calculator finds the greatest common divisor and divides both parts by it. Because 12 and 8 share a factor of 4, the simplified ratio is 3:2. The relationship is unchanged; only the notation is smaller and easier to compare.

How to use each section

  • Simplify a ratio: enter A and B to reduce A:B, see A/B as a fraction, see A divided by B as a decimal, and see A as a percent of A+B.
  • Solve a proportion: choose which value is missing in A:B = C:D. The calculator uses cross multiplication so matching proportions have equal cross-products.
  • Scale a ratio: multiply both parts by the same factor when resizing a recipe, mix, drawing, batch, or other proportional relationship.
  • Three-part ratio: reduce A:B:C and optionally split a total into shares that follow those parts.
  • Aspect ratio: simplify width:height and compute the matching height for a new width without stretching the image or layout.

Worked examples

With the default simplify inputs, 12:8 becomes 3:2. The fraction view is 3/2, the decimal view is 1.5, and A is 60% of the total because 12 divided by 20 equals 0.6.

With the default proportion inputs, 3:4 = 9:D solves to D = 12. The cross-products match because 3 x 12 and 4 x 9 both equal 36.

With the default three-part ratio, 1:2:3 has six total parts. Splitting 600 by that ratio gives 100, 200, and 300. For the default aspect-ratio inputs, 1920x1080 simplifies to 16:9, and scaling to 1280 pixels wide gives 1280x720.

Common ratio mistakes

  • Mixing part-to-part and part-to-whole: in 3:2, A divided by B is 1.5, but A as a percent of the total is 60%.
  • Scaling only one side: multiplying just A changes the relationship. To preserve the ratio, multiply every part by the same factor.
  • Ignoring zero and negative values: ratios used for quantities, shares, and dimensions usually need positive values. A zero part may be meaningful in algebra, but it cannot describe a usable aspect-ratio dimension.
  • Using rounded dimensions too early: aspect ratios can produce fractional scaled dimensions. Round only at the final pixel or measurement step.

Sources and further reading

For math background, see OpenStax on ratios and rates and solving proportions. For web layout context, see MDN's guide to the CSS aspect-ratio property.

Frequently Asked Questions

A simplified ratio divides every part by a common factor until the parts have no shared whole-number factor left. The relationship is unchanged: 12:8 and 3:2 describe the same comparison.
Use the proportion solver when one ratio should equal another and one value is missing. For A:B = C:D, the calculator uses cross multiplication so A x D equals B x C.
A as percent of total converts A:B into A divided by A plus B. For 12:8, A is 12 out of 20 total parts, so the displayed percent is 60%.
The calculator adds the three ratio parts, then assigns each share as total multiplied by that part divided by the sum. A 1:2:3 split of 600 produces 100, 200, and 300.
The aspect ratio section simplifies width:height with the greatest common divisor. When you enter a new width, it multiplies that width by original height divided by original width to preserve the same shape.