How to use
Each box above is a separate mode — just fill in the one you need and the answer appears as you type. Use What is X% of a number for tips, taxes, and "how much is 15% of this". Use X is what percent of Y when you have two numbers and want the ratio, like a test score. The percentage increase / decrease mode compares an old and a new value — perfect for price changes, salary raises, follower counts, or anything that went up or down over time. And the discount mode works both ways: enter the original and sale price to get the discount percentage, or the original price and a discount percentage to get what you would pay.
The formulas
A percentage is just a ratio out of 100. These four formulas cover almost every everyday situation:
- X% of a number = number × (X ÷ 100)
- X as a percent of Y = (X ÷ Y) × 100
- Percentage increase / decrease = (new − old) ÷ old × 100 — positive means an increase, negative means a decrease
- Discount = (original − sale) ÷ original × 100, and sale price = original × (1 − discount ÷ 100)
One thing that trips people up with increases and decreases: the change is always measured against the old value. Rent going from $1,200 to $1,500 is a 25% increase, but coming back down from $1,500 to $1,200 is only a 20% decrease — same $300, different base.
The same asymmetry explains why stacked discounts disappoint. Take 20% off, then another 20% off the reduced price, and you have not saved 40%. The second cut applies to what is left, so 0.8 × 0.8 = 0.64 and the total is 36% off.
The calculator itself has no currency attached — the examples below use dollars, but pounds, euros or yen work exactly the same way.
Examples
What is 15% of $50? That's 50 × 0.15 = $7.50 — so a 15% discount on a $50 item saves you $7.50 and you pay $42.50. If you got 30 questions right out of 120, your score is (30 ÷ 120) × 100 = 25%. If your rent went from $1,200 to $1,500, that's (1,500 − 1,200) ÷ 1,200 × 100 = a 25% increase; if a stock fell from $50 to $42.50, that's (42.50 − 50) ÷ 50 × 100 = a 15% decrease. And if a $50 item is on sale for $39.90, the discount is (50 − 39.90) ÷ 50 × 100 = 20.2% off.
Tips work the same way. An 18% tip on a $64 restaurant bill is 64 × 0.18 = $11.52, bringing the total to $75.52. A quick check for 20%: move the decimal one place left and double it.
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percentage divided by 100. For example, 15% of $50 is 50 × 0.15 = $7.50.
How do I calculate percentage increase or decrease?
Take (new value − old value) ÷ old value × 100. Rent going from $1,200 to $1,500 is (1,500 − 1,200) ÷ 1,200 × 100 = a 25% increase. A negative result means a decrease. If the old value is 0, the percentage change is undefined — there is no base to compare against.
How do I calculate a discount percentage?
Take (original price − sale price) ÷ original price × 100. If a $50 item sells for $39.90, that's (50 − 39.90) ÷ 50 × 100 = 20.2% off. Going the other way, sale price = original price × (1 − discount ÷ 100). In the discount mode above, entering the original and sale price gives you the discount percentage, while entering the original price and a discount percentage gives you the sale price.
Is anything I type sent to a server?
No. All calculations run entirely in your browser, and nothing you type is ever sent to or stored on a server.