Ratio to Percentage Calculator

How ratio to percentage works

Quick answer: add the parts of the ratio to get the total, divide each part by the total, then multiply by 100. For 2:3 the total is 5, so the shares are 2 ÷ 5 = 40% and 3 ÷ 5 = 60%.

How to convert a ratio to a percentage

1. 1Add the parts of the ratio to get the total number of parts.
2. 2Divide each part by the total to get its share as a decimal.
3. 3Multiply each share by 100 and add the percent sign.

A ratio compares parts. A percentage compares each part with the whole, and the whole is the sum of the parts. That is why the first step is to add them. The shares always add up to 100%.

Ratio to percentage formula

Share of a part = part ÷ (sum of all parts) × 100. The ratio to percent formula is the same for any number of parts, so a three-part ratio such as 3:5:2 uses a total of 3 + 5 + 2 = 10.

Three-part ratios

For three parts, divide each part by the same total, the sum of all three. The method does not change; there is one more division.

Part to part and part to whole

Ratios are normally read part to part, so 3:1 means three parts to one part, a total of four, and shares of 75% and 25%. It does not mean 300%. If you are given a ratio of a part to the whole, such as 3 to 4, the whole is already the second number and the share is 3 ÷ 4 = 75%. The calculator reads every ratio as part to part.

Percent to ratio

To go from percent to ratio, write the share and the remainder as a ratio, then simplify. A share of 25% leaves 75%, so the ratio is 25:75, which becomes 1:3 after dividing both numbers by their greatest common factor. For 40% the ratio is 40:60, which is 2:3, and for 50% it is 50:50, which is 1:1. Switch the direction in the calculator to convert a percentage into a ratio; it then works as a percentage to ratio calculator. A share with decimals, such as 33.33%, gives a long ratio (3333:6667), so a whole-number percentage gives a more readable one.

Ratio to percentage examples

Common ratios as percentages

Ratios that do not divide 100 evenly, such as 1:2 and 1:5, are rounded to two decimal places.

Ratio to percentage practice questions

Try these five before checking the answers. Round to two decimal places where needed.

1. 1Convert 3:7 to percentages.
2. 2Write 60% as a ratio in its simplest form.
3. 3Convert the three-part ratio 2:3:5 to percentages.
4. 4A drink is mixed 1 part syrup to 4 parts water. What percent is syrup?
5. 5A class has boys to girls in the ratio 7:8. What percent are girls?

Practice answers

Each answer starts with the total of the parts.

1. 1The total is 10, so the shares are 30% and 70%.
2. 260% leaves 40%, so the ratio is 60:40, which is 3:2.
3. 3The total is 10, so the shares are 20%, 30% and 50%.
4. 4The total is 5, so syrup is 1 ÷ 5 = 20%.
5. 5The total is 15, so girls are 8 ÷ 15 = 53.33% and boys are 46.67%.

Ratio to percentage practice questions

Ratio to percentage code

Both versions reproduce the worked examples above, reject non-numeric input and refuse a ratio whose parts are all zero.

JavaScript

1function finite(name, v) {2  if (typeof v !== "number" || !Number.isFinite(v)) throw new TypeError(`${name} must be a finite number`);3}4​5function places(decimals) {6  if (!Number.isInteger(decimals) || decimals < 0 || decimals > 10) throw new RangeError("decimals must be a whole number from 0 to 10");7  return decimals;8}9​10// [2, 3] -> [40, 60]; [3, 5, 2] -> [30, 50, 20]11export function ratioToPercent(parts, decimals = 2) {12  if (!Array.isArray(parts) || parts.length < 2) throw new TypeError("parts must be an array of at least two numbers");13  parts.forEach((p) => {14    finite("part", p);15    if (p < 0) throw new RangeError("parts must not be negative");16  });17  const d = places(decimals);18  const total = parts.reduce((a, b) => a + b, 0);19  if (total === 0) throw new RangeError("the parts must not all be 0");20  return parts.map((p) => Number(((p / total) * 100).toFixed(d)));21}22​23function gcd(a, b) {24  return b === 0 ? a : gcd(b, a % b);25}26​27// 25 -> [1, 3] (part to rest, simplified). Accepts up to 2 decimal places.28export function percentToRatio(percent) {29  finite("percent", percent);30  if (percent <= 0 || percent >= 100) throw new RangeError("percent must be above 0 and below 100");31  const p = Math.round(percent * 100);32  if (Math.abs(percent * 100 - p) > 1e-9) throw new RangeError("percent must have at most 2 decimal places");33  const rest = 10000 - p;34  const g = gcd(p, rest);35  return [p / g, rest / g];36}

Python

1from decimal import Decimal, ROUND_HALF_UP2from math import gcd, isfinite3​4​5def _num(name, v):6    if isinstance(v, bool) or not isinstance(v, (int, float, Decimal)) or not isfinite(float(v)):7        raise ValueError(f"{name} must be a finite number")8    return Decimal(str(v))9​10​11def _round(x, decimals):12    if not isinstance(decimals, int) or isinstance(decimals, bool) or not 0 <= decimals <= 10:13        raise ValueError("decimals must be a whole number from 0 to 10")14    return x.quantize(Decimal(1).scaleb(-decimals), rounding=ROUND_HALF_UP)15​16​17def ratio_to_percent(parts, decimals=2):18    if not isinstance(parts, (list, tuple)) or len(parts) < 2:19        raise ValueError("parts must be a list of at least two numbers")20    nums = [_num("part", p) for p in parts]21    if any(n < 0 for n in nums):22        raise ValueError("parts must not be negative")23    total = sum(nums)24    if total == 0:25        raise ValueError("the parts must not all be 0")26    return [_round(n / total * 100, decimals) for n in nums]27​28​29def percent_to_ratio(percent):30    p = _num("percent", percent)31    if not 0 < p < 100:32        raise ValueError("percent must be above 0 and below 100")33    scaled = p * 10034    if scaled != scaled.to_integral_value():35        raise ValueError("percent must have at most 2 decimal places")36    a = int(scaled)37    b = 10000 - a38    g = gcd(a, b)39    return a // g, b // g

ratioToPercent([3, 5, 2]) returns 30, 50 and 20, the three-part example, and percentToRatio(40) returns 2 and 3. Round only for display. Rounded shares may not add to 100: ratioToPercent([1, 1, 1]) returns 33.33 three times, which is 99.99, so the calculator shows the exact total beside the rounded shares. The Python version rounds half up, which is the usual school convention, and returns Decimal values, so 40 prints as 40.00.

Privacy and input safety

• Values are calculated in your browser. The numbers you enter are not sent to a server or stored.
• Fields accept numbers only. Text, empty fields, NaN and Infinity are rejected with a message, and nothing you type is ever evaluated as code.
• A ratio needs at least two parts. Negative parts are rejected, because a part cannot be less than nothing.
• A part of 0 is allowed, as in 0:4, which gives 0% and 100%. A ratio with every part at 0 is refused, because its total is zero.
• Decimal parts are accepted, so 1.5:2.5 gives 37.5% and 62.5%.
• A percentage to convert must be above 0 and below 100, with at most two decimal places, so that the ratio stays readable.
• A comma followed by exactly three digits is read as a thousands separator, so 1,200 is 1200. Any other comma is a decimal point, so 12,50 is 12.50, and a number starting with 0, such as 0,500, is read as a decimal. Mixed formats such as 1.234,5 and ambiguous entries such as 1,2,3 are refused instead of guessed. Spaces, including no-break spaces, between digit groups are ignored.
• Each part has its own field, so a comma is never needed to separate the parts.

Common ratio to percentage mistakes

One split in five forms

The same split, 75% and 25%, written in the forms people ask about: