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Ratio Calculator

Simplify ratios or solve cross-multiplication problems. Mode 1 expresses A:B as a decimal ratio. Mode 2 solves A/B = C/D for the unknown D.

Fill in the fields above and click Calculate to see your results.

How to use

Simplify ratios or solve cross-multiplication problems. Mode 1 expresses A:B as a decimal ratio. Mode 2 solves A/B = C/D for the unknown D.

How it's calculated

Ratio A:B (decimal)

value_a / value_b

A divided by B expressed as a decimal. To simplify A:B, divide both by their GCD manually.

D = B × C / A

value_b * value_c / value_a

Solves A/B = C/D for D: D = B × C ÷ A

Result

(mode == 1) * ratio_decimal + (mode == 2) * cross_multiply_result

Result for the selected mode

Examples

Simplify 12:8 → 1.5 (or 3:2)

  • Calculation Mode:1
  • Value A:12
  • Value B:8
  • Value C (cross-multiply mode):3

Cross-multiply: 12/8 = 3/? → 2

  • Calculation Mode:2
  • Value A:12
  • Value B:8
  • Value C (cross-multiply mode):3

Frequently Asked Questions

How do I simplify a ratio like 12:8?

Select mode 1 and enter A=12, B=8. The calculator shows the decimal ratio (1.5). To get the simplified whole-number ratio, divide both numbers by their GCD (4): 12÷4 = 3, 8÷4 = 2, so 12:8 simplifies to 3:2.

How does cross-multiplication work?

Select mode 2. If A/B = C/D and you know A, B, and C, then D = B × C ÷ A. For example, 12/8 = 3/D → D = 8 × 3 ÷ 12 = 2.

When would I use cross-multiplication?

Cross-multiplication is useful for scaling recipes, maps, and proportional problems. For example, if a recipe for 4 people needs 12 eggs, how many eggs for 8 people? Set up 12/4 = ?/8 → ? = 4 × 12 ÷ 4 = ... use the calculator to solve.

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