ToolStack › Percentage Calculator
Math Tool

PERCENT
AGE

Six types of percentage calculation in one place. Discounts, tips, grade changes, tax — all instant, no calculator needed.

What is X% of Y?
01 / 06
e.g. "What is 15% of $80?" → "$12"
What is % of ?
12
15% of 80 = 12
X is what % of Y?
02 / 06
e.g. "45 is what % of 200?" → "22.5%"
is what % of ?
22.5%
45 is 22.5% of 200
Percentage Change
03 / 06
e.g. "From $50 to $65" → "+30% increase"
From to
+30%
30% increase (+15)
Discount / Sale Price
04 / 06
e.g. "20% off $120" → "$96 sale price, $24 saved"
% off
$96
Sale price · You save $24
Add Percentage (Tax / Tip)
05 / 06
e.g. "$85 + 18% tip" → "$100.30 total, $15.30 tip"
+ %
$100.30
Tip: $15.30
X% of what equals Y?
06 / 06
e.g. "25% of what = 40?" → "160"
% of what equals
160
25% of 160 = 40

Percentage Formula

Percentage = (Part ÷ Whole) × 100. To find a percentage of a number: multiply the number by the percentage and divide by 100. e.g. 15% of 200 = 200 × 15 ÷ 100 = 30.

Percentage Change

Change % = ((New − Old) ÷ Old) × 100. A positive result is an increase; negative is a decrease. e.g. from 80 to 100: ((100−80)÷80)×100 = 25% increase.

Reverse Percentage

To find the original value before a percentage was applied: Original = Final ÷ (1 + rate). e.g. if a price is $110 after 10% tax, the pre-tax price is $110 ÷ 1.10 = $100.

Common Percentages

10% = divide by 10 · 25% = divide by 4 · 20% = divide by 5 · 50% = divide by 2 · 33.3% = divide by 3 · 75% = multiply by 0.75. Mental math shortcuts for everyday use.

FAQ

Why Percentage Calculations Matter in Everyday Life

Percentages appear constantly in financial decisions, shopping, health metrics, business analysis, and academic performance. Yet many people struggle with percentage calculations — particularly the distinction between "percentage of" (what is 15% of $85?) versus "percentage change" (prices went from $80 to $95 — what percentage increase is that?) versus "percentage difference" (these two values differ by what percent?).

The Three Core Percentage Problems

Finding a percentage of a number: multiply the number by the decimal form of the percentage (15% of $200 = 0.15 × 200 = $30). Finding what percentage one number is of another: divide the part by the whole and multiply by 100 (30 is what percent of 200? = 30/200 × 100 = 15%). Finding percentage change: subtract the old value from the new, divide by the old value, multiply by 100 ((new-old)/old × 100).

Common Percentage Mistakes

A 50% price increase followed by a 50% decrease does not return to the original price — it leaves you 25% below it. This asymmetry trips up many people. Similarly, a 100% increase doubles a value, but a 100% decrease eliminates it. Understanding these relationships prevents costly errors in financial planning, negotiation, and business analysis.

How do I calculate a percentage in my head? ▾
For 10%, move the decimal one place left. For 5%, halve that. For 15%, add 10% and 5% together. For 20%, divide by 5. For 25%, divide by 4. Most everyday percentages (tax, tips, discounts) can be estimated using these building blocks.
What is percentage vs percentage points? ▾
These are different! If an interest rate goes from 3% to 4%, it increased by 1 percentage point, but the percentage change is 33% (1 is 33% of 3). Percentage points measure absolute differences between two percentages; percentage change measures relative change.
How do I calculate a tip? ▾
Use calculator #5 above. Or mentally: for 20%, divide the bill by 5. For 18%, calculate 20% and subtract 10% of that. For 15%, calculate 10% and add half. On a $60 bill: 20% tip = $12. 15% tip = $6 + $3 = $9.
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📋 Calculation History
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How to Use the Percentage Calculator

Calculate any percentage, percentage change, or reverse percentage instantly.

01
Choose your calculation type
Select from: 'What is X% of Y?', 'X is what % of Y?', '% change from X to Y', or 'X increased/decreased by Y%'. Pick the one that matches your question.
02
Enter your numbers
Fill in the two values you know. The third value calculates automatically as you type — no need to press Enter.
03
Read the result
The answer appears immediately with a plain-English explanation of what was calculated and why.
04
Use percentage change for comparisons
Great for comparing prices, salaries, or metrics year-over-year. A positive result is an increase; negative is a decrease.
💡
💡 Quick mental check: to find 15% of any number, find 10% (move decimal left one place) then add half of that. 15% of $240 = $24 + $12 = $36.