Quick answer
A ratio a : b converts to a decimal by dividing a by b. For 3 : 4 the decimal is 0.75, or 75%.
Enter the two parts of the ratio. The calculator returns the decimal and percentage.
Related tools: Decimal to Ratio Calculator: Convert a decimal to a ratio. Ratio Calculator: Simplify a ratio of two whole numbers. Fraction to Decimal Calculator: Convert a fraction to a decimal, showing repeating digits.
How to use the Ratio to Decimal Calculator
- Enter a.
- Enter b.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| A | A number | — |
| B | A number | — |
Formula and method
Formula
decimal = a ÷ b
IEEE-754 double precision; shown to at most 6 decimal places.
Divide the first term by the second; B must not be zero.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: A = 3; B = 4.
| Result | Value |
|---|---|
| Decimal | 0.75 |
| Percent | 75% |
Example 2
Inputs: A = 1; B = 8.
| Result | Value |
|---|---|
| Decimal | 0.125 |
| Percent | 12.5% |
The first line is the headline result (decimal). Below it the ratio to Decimal Calculator lists percent, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
What the decimal means
0.75 says A is three-quarters the size of B. This is part to part: for a part-to-whole share of 3 : 4, A is 3 ÷ 7 = 0.4286 of the total, a different number.
Conversions
1:2 = 0.5, 2:3 = 0.6667, 5:4 = 1.25. A decimal above 1 means the first quantity is larger. B cannot be zero.
A : B versus share of the total
The calculator divides A by B, so 3:4 is 0.75, meaning A is 75% of B. That is not A's share of the total: with 3 and 4 as parts of one whole, A is 3/7 ≈ 0.4286 of the sum.
Dividing by zero is undefined, so B must not be zero.
How the result changes with a
Holding every other input at the first worked example, this table changes only a. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| A | Decimal | Percent |
|---|---|---|
| 1.5 | 0.375 | 37.5% |
| 3 | 0.75 | 75% |
| 6 | 1.5 | 150% |
| 12 | 3 | 300% |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- B: entering "0" shows "B must not be 0." beside that field and no result.
Choosing between the Ratio to Decimal Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Decimal to Ratio Calculator](/math/decimal-to-ratio-calculator/) | decimal | Ratio |
| [Ratio Calculator](/math/ratio-calculator/) | first number and second number | Simplified ratio |
| [Fraction to Decimal Calculator](/math/fraction-to-decimal-calculator/) | numerator and denominator | Decimal |
| [Decimal to Percent Calculator](/math/decimal-to-percent-calculator/) | decimal | Percent |
| [Ratio Percentage Calculator](/math/ratio-percentage-calculator/) | part A and part B | Part A |
Background: rounding and basic arithmetic
Rounding to a place looks at the digit just to its right: 0 to 4 rounds down and 5 to 9 rounds up (the half-up convention, so 2.5 becomes 3). Rounding 838.274 gives 800 to the nearest hundred, 840 to the nearest ten and 838 to the nearest one.
Some systems use banker's rounding (half to even), which sends 2.5 to 2 and 3.5 to 4; check which convention your task requires, and round only at the end of a multi-step calculation.
Assumptions and limitations
- B ≠ 0
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for b to confirm this.
Frequently asked questions
What is 1:8 as a decimal?
1:8 means 1 ÷ 8 = 0.125.
What is 5:2?
5:2 means 5 ÷ 2 = 2.5.
How do I convert back?
Write the decimal as a fraction.
What is 5:8 as a decimal?
5 ÷ 8 = 0.625.
Why is 3:4 not 0.4286?
0.4286 is 3 out of the total of 7 parts. The ratio 3:4 as a decimal compares 3 with 4, giving 0.75.
What does the Ratio to Decimal Calculator calculate?
Convert a ratio a:b to a decimal. It returns Decimal and Percent.
What formula does the Ratio to Decimal Calculator use?
decimal = a ÷ b. Divide the first term by the second; B must not be zero.
What do I need to enter in the ratio to Decimal Calculator?
You enter A and B.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. B ≠ 0.
What is an example of a ratio to Decimal result?
With a 3 and b 4, the decimal is 0.75.
Related calculators
- Decimal to Ratio Calculator — Convert a decimal to a ratio.
- Ratio Calculator — Simplify a ratio of two whole numbers.
- Fraction to Decimal Calculator — Convert a fraction to a decimal, showing repeating digits.
- Decimal to Percent Calculator — Convert a decimal to a percentage.
- Ratio Percentage Calculator — Percentage share of each part of a ratio.
- Percent to Ratio Calculator — Express a percentage as a part-to-whole ratio.
Sources
- OpenStax, Prealgebra 2e (Rice University): fractions, decimals, percents, ratios and rounding
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.