Percentage Calculator: Every Percent Problem, Solved Free

By ZeroFee Tools · October 4, 2026 · 7 min read

Percent questions look simple until you sit down to solve one. Is 20% of 150 the same kind of problem as figuring out what percent 30 is of 150? What about measuring a price that moved from $80 to $100? They are three different questions, each with its own formula, and mixing them up is how discount math goes wrong at the register.

The free Percentage Calculator handles all three modes: X% of Y, X is what % of Y, and % change from A to B. Every answer displays the exact formula used, so you can trust and verify the math. It runs entirely in your browser with no signup and no limits, which makes it perfect for quick checks on shopping, tipping, and grades. For sale-price math specifically, pair it with the discount calculator, and for restaurant bills add the tip calculator to the mix.

How do you calculate X percent of Y?

Multiply Y by X and divide by 100. For example, 20% of 150 is (20 / 100) x 150 = 30. This single formula powers most discount, tip, and tax calculations in everyday life, from sale prices to restaurant bills.

This is the mode you will use most. A 30% off sale on an $80 jacket: (30 / 100) x 80 = $24 off, so you pay $56. An 18% tip on a $45 dinner bill: (18 / 100) x 45 = $8.10. A 6% sales tax on a $200 purchase: (6 / 100) x 200 = $12.

The mental shortcut is to find 10% first, then scale. Ten percent of 150 is 15, so 20% is 30. For 15%, take 10% plus half of 10%: 15 + 7.5 = 22.5. The calculator does all of this instantly and shows the formula it used, so you are never guessing.

Restaurant math rewards the same formula. Splitting a $120 bill three ways with a 20% tip: the tip is (20 / 100) x 120 = $24, the total is $144, and each person pays $48. Or reverse it: you want to leave $10 on a $54 bill, which is (10 / 54) x 100 = 18.5%, right in the standard tipping zone. Once the formula is automatic, you stop reaching for your phone.

What percent is one number of another?

Divide the part by the whole and multiply by 100. For example, 30 is what percent of 150? (30 / 150) x 100 = 20%. This mode tells you the share one value represents of a total, like test scores or budget shares.

This mode answers questions about scores and shares. Got 42 questions right out of 50? (42 / 50) x 100 = 84%, a solid B. Spent $320 of a $2,000 monthly budget on groceries? (320 / 2000) x 100 = 16%.

Note that the answer can exceed 100% when the part is bigger than the whole. If you sold 200 tickets against a goal of 150, that is (200 / 150) x 100 = 133.3% of target. And if the result looks wrong, check that you have part and whole in the right order: the classic mistake is dividing the whole by the part.

Polls and surveys live in this mode. If 1,240 of 2,000 respondents approve of a policy, that is (1240 / 2000) x 100 = 62% approval. Phone storage works the same way: 38 GB used of 128 GB is (38 / 128) x 100 = 29.7% full. Whenever the question contains the words "out of," you are looking at mode two.

How is percent change between two numbers calculated?

Subtract the old value from the new value, divide by the old value, and multiply by 100. Going from 80 to 100 is ((100 - 80) / 80) x 100 = a 25% increase. A negative result signals a decrease.

Percent change measures movement, and it is the mode behind stock swings, price hikes, and salary raises. A rent that went from $1,200 to $1,350: ((1350 - 1200) / 1200) x 100 = 12.5% increase. A weight that dropped from 190 to 171 pounds: ((171 - 190) / 190) x 100 = -10%, a 10% decrease.

Two traps to avoid. First, percent change has no meaning when the old value is zero, because you cannot divide by zero; going from 0 to 5 is not an infinite percent, it is simply undefined. Second, do not confuse percent change with percentage points: a rate rising from 20% to 25% climbed 5 percentage points but grew 25% in relative terms. News headlines mix these up constantly, and now you know the difference.

Watch out for chained changes too. A price that rises 10% then falls 10% does not return to the original: $100 becomes $110, then 110 x 0.90 = $99, a net change of -1%. Percentages multiply against moving bases, which is why "up 50% then down 50%" leaves you at 75% of where you started: 100 to 150, then 150 x 0.5 = 75. For money that compounds steadily over years rather than bouncing around, the compound interest calculator models growth period by period.

How to use the Percentage Calculator in 4 steps

1. Pick your mode. Choose "X% of Y" for discounts, tips and taxes, "X is what % of Y" for scores and shares, or "% change A to B" for increases and decreases.
2. Type your two numbers. Enter any values, including decimals, into the two input boxes for the active mode.
3. Read the result instantly. The answer appears immediately with the exact formula shown underneath, so you can see how it was computed.
4. Switch modes freely. Jump between modes to answer different questions about the same numbers without retyping everything.

6 practical tips

• Start with 10%. Ten percent of any number is just the decimal point moved one place left. Double it for 20%, halve it for 5%, and you can sanity-check the calculator in your head.
• Distinguish points from percent. When rates change (interest, tax, battery), ask whether the speaker means percentage points or relative percent. They differ by a lot.
• Watch the order in mode two. "X is what % of Y" divides X by Y. Swapping them gives the reciprocal, which answers a different question entirely.
• Remember the zero rule. Percent change from zero is undefined, not infinite. If your old value is zero, report the raw numbers instead.
• Use it for reverse discounts. A price of $56 after 30% off means 56 / 0.70 = $80 original. Mode two with X=56 and Y=80 confirms it is 70% of the original.
• Beware chained changes. Sequential percentage moves multiply against moving bases, so always recompute from the current value, never the original.

Frequently asked questions