Percentage Calculator: How to Figure Out Any Percent

This calculator works out any percentage problem you type in: find what percent one number is of another, calculate a percentage of a given number, or add/subtract a percentage. Enter the two or three numbers you have, press calculate, and you get the exact missing value with the formula shown step by step.

Formula

The calculator uses three standard percentage formulas, depending on what you are trying to find:

• Percent of a number: Result = Value A × Percent ÷ 100. For example, 20% of 50 = 50 × 20 ÷ 100 = 10.
• A as a percentage of B: Percent = (Value A ÷ Value B) × 100. For example, 50 is what percent of 200? (50 ÷ 200) × 100 = 25%.
• Percentage change: Change = (Value B − Value A) ÷ Value A × 100. For example, from 50 to 200 is (200 − 50) ÷ 50 × 100 = 300% increase.

Each variable is a plain number without units. The percent is entered as a number like 20, not 0.20. The result is a plain number (for “percent of”) or a percentage (for the other two).

Worked Example

Using the default values from the calculator:

1. Percent of a number: 20% of 50 = (50 × 20) ÷ 100 = 1000 ÷ 100 = 10.
2. A as % of B: 50 ÷ 200 × 100 = 0.25 × 100 = 25%.
3. Percentage change: (200 − 50) ÷ 50 × 100 = 150 ÷ 50 × 100 = 3 × 100 = 300%.

The calculator shows all three results at once, so you can check each formula against the same numbers.

How to Use the Calculator

You only need to fill in the values you have. The calculator is designed to be flexible:

• Find a percentage of a number: Type the number in Value A and the percent in Percent. The result is the part.
• Find what percent one number is of another: Type the part in Value A and the whole in Value B. The result is the percentage.
• Find the percentage change: Type the old value in Value A and the new value in Value B. The result is the increase or decrease as a percentage.

You can leave the Percent field at its default if you are not using it. The calculator always computes all three outputs, so you get extra context even if you only needed one.

When to Use Each Formula

Different everyday situations call for different percentage calculations:

• Percent of a number: Tips (15% of a bill), sales tax (8% of a price), discounts (30% off a $50 item = 30% of 50 = $15 off).
• A as % of B: Test scores (45 correct out of 60 = 75%), survey results (120 out of 400 people = 30%), or comparing parts to a whole.
• Percentage change: Price increases (from $2 to $2.50 = 25% increase), population growth, or any before/after comparison.

If you are not sure which one applies, ask yourself: “Am I finding a part, a ratio, or a change?” That will point you to the right formula.

Common Mistakes and How to Avoid Them

Percentage errors usually come from three places:

• Using the wrong base. “Increase by 20%” means the original number plus 20% of the original. Many people mistakenly add 20% of the new number. Always use the original value as the base.
• Mixing up percent and decimal. 20% is 0.20, not 0.2? Actually 0.20 and 0.2 are the same. The real mistake is using 20 in a formula that expects a decimal, or vice versa. In this calculator, you type 20 for 20%, and the formula divides by 100.
• Forgetting that percentage change can be negative. If Value B is less than Value A, the change is negative (a decrease). For example, from 200 to 50 is (50 − 200) ÷ 200 × 100 = −75%, a 75% decrease.

Also, rounding matters. If you round intermediate steps too early, the final answer can be off. Keep at least two decimal places until the end.

Limits of the Formula

The percentage formulas assume simple linear relationships. They do not account for compounding, which is a different calculation (used in interest, growth over multiple periods). For example, a 10% increase followed by another 10% increase is not a 20% total increase; it is 21% because the second increase applies to the larger base. This calculator is for single-step percentage problems only.

Also, the formulas require that Value B (the whole) is not zero. Division by zero is undefined, so the calculator will show an error if you enter 0 for Value B.

Frequently Asked Questions

How do I calculate 20% of a number?

Multiply the number by 20 and divide by 100. For example, 20% of 50 = (50 × 20) ÷ 100 = 10. In the calculator, put 50 in Value A and 20 in Percent.

What is the formula for percentage increase?

Percentage increase = (New Value − Old Value) ÷ Old Value × 100. If the result is positive, it is an increase; if negative, it is a decrease. For example, from 50 to 200 is (200 − 50) ÷ 50 × 100 = 300%.

How do I find what percent one number is of another?

Divide the part by the whole and multiply by 100. For example, 30 is what percent of 150? 30 ÷ 150 × 100 = 20%. Put 30 in Value A and 150 in Value B.

What is the difference between percentage and percentage points?

A percentage point is the arithmetic difference between two percentages. If a rate goes from 10% to 15%, it increased by 5 percentage points, but it increased by 50% (because 5 ÷ 10 × 100 = 50%). The calculator handles the percentage change, not percentage points.

Can I use this for reverse percentage problems?

Yes, with a small adjustment. If you know the result and the percentage, you can find the original number by rearranging: Original = Result ÷ (Percent ÷ 100). For example, if 20% of a number is 10, the number is 10 ÷ 0.20 = 50. The calculator does not have a dedicated reverse mode, but you can solve it by entering the known values and using the formula.

Why does the calculator show three results at once?

Because the three formulas use the same inputs. Showing all three gives you extra context and lets you verify your work. You can ignore the outputs you do not need.

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in a | Value A | - | 50 | 0
in b | Value B | - | 200 | 0
in p | Percent | % | 20 | 0
out result | Result | - | (a * p / 100) | 2
out pct_of | A as % of B | % | (a / b * 100) | 2
out change | % Change A→B | % | ((b - a) / a * 100) | 2