Percentage Calculator

How to use

1. Choose one of three modes: “What is B% of A?”, “A is what percent of B?”, or “Increase/decrease A by B%”.
2. Enter positive numbers for A and B (decimals such as 12.5 are fine). In the change mode, pick increase or decrease.
3. Press “Calculate” to see the answer and the steps.

Examples

• 25% of 300 is 75
• 12 is 30% of 40
• 2000 increased by 8% is 2160

When to use it (grade level)

Ratios and percentages are taught from around 5th grade and become the foundation for profit and loss, concentration, and probability later on. This tool suits checking home study, working out discounts and tax while shopping, and confirming your work before a test.

A percentage tells you how much of a whole you have when the whole is treated as 100. For example, “20% of 500” means 20 of the 100 equal parts of 500, which is 100. The percent sign (%) means “out of 100”.

How to work it out yourself

It is easiest to turn a percent into a decimal first by dividing by 100: 25% is 25 ÷ 100 = 0.25. To find “B% of A”, compute A × (B ÷ 100). For example, 15% of 60 is 60 × 0.15 = 9.

For “A is what percent of B?”, divide A by B and then multiply by 100. For 12 and 40, 12 ÷ 40 = 0.3, and 0.3 × 100 = 30%. For “increase or decrease by B%”, add or subtract the change from the original: A × (1 ± B ÷ 100). A 10% discount on 1000 is 1000 × (1 − 0.1) = 900.

Percent, decimal, and fraction table

The same ratio can be written as a percent, a decimal, or a fraction. Learning a few pairs helps you picture the size right away: 10% = 0.1, 25% = 0.25, 50% = 0.5, 8% = 0.08, and 1% = 0.01.

For everyday examples, a batting average of 12 hits in 40 at-bats is 12 ÷ 40 = 0.3, written as .300 (a 30% rate). Sales tax is an increase (2000 with 10% tax is 2000 × 1.1 = 2200), while a discount is a decrease (a 20% discount on 3000 is 3000 × 0.8 = 2400). They all use the same idea of a ratio.

Common mistakes

Three slip-ups are common. First, confusing “B% of A” with “A is what percent of B”. “12% of 40” (= 4.8) and “40 is what percent of 12” (≈ 333%) are completely different. Be clear about which amount is the whole (the base you treat as 100).

Second, forgetting to turn the percent into a decimal — multiplying by 25 instead of 0.25 makes the answer 100 times too big. Third, stopping at the change amount for “increase/decrease by %”: a 10% discount on 1000 subtracts 1000 × 0.1 = 100 to give 900, not 100.

Related topics and tools

Percentages bridge fractions and decimals: 25% = 1/4 = 0.25. Writing a percent as a fraction with the fraction calculator is good simplifying practice. Ratios also connect to speed and rates, so pairing this with the unit conversion tool broadens how you read numbers. Try each problem yourself first, then check the steps here.

FAQ

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