Ratio Calculator

Simplify any ratio down to its lowest whole-number terms, or solve a proportion to find a missing fourth value when two ratios are equal.

Simplified ratio
2 : 3
Decimal equivalent
0.6667
Missing value

How it works

Simplifying a ratio divides both sides by their greatest common divisor (GCD), the same way a fraction is reduced to lowest terms. Solving a proportion uses cross-multiplication: if A:B equals C:D, then A x D = B x C, which rearranges to find whichever value is missing.

Formula
Simplify: A/GCD(A,B) : B/GCD(A,B) | Proportion: D = (B x C) / A

Example. 12:18 simplifies to 2:3. If 4:6 = 10:?, the missing value is 15.

Frequently asked questions

What is the GCD and why does it matter here?

The Greatest Common Divisor is the largest number that divides both parts of the ratio evenly. Dividing both sides by it guarantees the simplest possible whole-number form, the same idea as reducing a fraction.

How does cross-multiplication solve a proportion?

If two ratios are equal (A:B = C:D), then A x D must equal B x C - that's the definition of a proportion. Rearranging that equation for whichever value is unknown gives you the answer directly.

Related calculators

Percentage CalculatorFree percentage calculator. Find what percentage one number is of another, work out X% of Y, and calculate the increase or decrease between two values.Fraction SimplifierSimplify any fraction to its lowest terms using the GCD method. Get the simplified fraction, decimal equivalent and percentage.Area CalculatorCalculate the area of any shape: square, rectangle, circle, triangle or trapezoid. Results in square metres, centimetres, feet and inches.Business Days CalculatorFind the date N business days after or before a given date, skipping weekends. Useful for deadlines, shipping estimates and project planning.

Embed this calculator

Free to use on any site. Please keep the credit link.

<iframe
  id="letmecalc-ratio-calculator"
  src="https://letmecalc.com/embed/math/ratio-calculator"
  title="Ratio Calculator"
  width="100%"
  height="620"
  style="border:0;max-width:100%"
  loading="lazy"
></iframe>
<p><a href="https://letmecalc.com/math/ratio-calculator" target="_blank" rel="noopener">Ratio Calculator</a> by letmecalc</p>
<script>
(function () {
  var f = document.getElementById("letmecalc-ratio-calculator");
  window.addEventListener("message", function (e) {
    if (e.origin !== "https://letmecalc.com") return;
    if (!f || e.source !== f.contentWindow) return;
    var d = e.data;
    if (!d || d.source !== "letmecalc-embed" || d.type !== "resize") return;
    if (typeof d.height === "number" && d.height > 0) {
      f.style.height = d.height + "px";
    }
  });
})();
</script>