Percentage Calculator: How to Calculate Discounts, Tips, and Interest in Seconds

Percentages are everywhere — the 30% off sign at the store, the 15% tip you need to calculate at dinner, the 3.5% APR on your loan, and the 8% inflation figure in the news. Yet most people reach for their phone calculator and still get it wrong. Once you understand the three basic types of percentage problems, you can do most of them in your head.

Skip the mental math entirely with our free Percentage Calculator — handles percentage increase, decrease, and difference calculations instantly.

The Three Types of Percentage Problems

Every percentage question falls into one of these three categories:

Problem Type	Question	Formula	Example
Find the percent	What % of Y is X?	(X ÷ Y) × 100	What % of 500 is 75? → 15%
Find the part	What is X% of Y?	(X ÷ 100) × Y	What is 20% of 80? → 16
Find the whole	X is Y% of what?	(X ÷ Y) × 100	24 is 60% of what? → 40

Real-World Scenarios

Shopping Discounts

30% off $85 jacket: 30% of 85 = $25.50 off → pay $59.50
Buy one get one 50% off: Second item at half price = overall 25% discount on two items
Stacked discounts (tricky): 20% off + additional 15% off ≠ 35% off. It is 20% off the original, then 15% off the reduced price = 32% total. Always multiply successive discounts rather than adding them.
Final clearance: 50% off + extra 20% off = 60% total discount (not 70%).

Restaurant Tips

15% tip on $62 bill: 10% = $6.20, 5% = $3.10 → total tip = $9.30
20% tip (quick method): Move decimal left one ($6.20), double it ($12.40)
Splitting the bill: Use our Tip Calculator to divide the total including tip among any number of people

Interest Rates and Finance

3.5% APR on $20,000 loan: Annual interest = $20,000 × 0.035 = $700/year. Over 5 years that is $3,500 in interest.
5% APY on $10,000 savings: First year = $500. With compounding, year two earns interest on $10,500.
8% sales tax on $45 purchase: $45 × 0.08 = $3.60 tax → total $48.60

Mental Math Shortcuts (No Calculator Needed)

Percentage	How to Calculate It	Example: 250
50%	Half the number	125
25%	Half of half	62.5
10%	Move decimal one left	25
5%	Half of 10%	12.5
1%	Move decimal two left	2.5
20%	Double the 10%	50
15%	10% + half of 10%	37.5
33%	Roughly one-third	~83

Pro tip: For compound percentage changes (like a 20% discount followed by a 10% increase), multiply the decimal equivalents: 0.80 × 1.10 = 0.88 = 12% net decrease — not 10% decrease.

Percentage Change: Increase and Decrease

Percentage Increase: ((New Value − Original) ÷ Original) × 100

If rent went from $1,200 to $1,440: ($240 ÷ $1,200) × 100 = 20% increase.

Percentage Decrease: ((Original − New Value) ÷ Original) × 100

If a $50 item dropped to $40: ($10 ÷ $50) × 100 = 20% decrease.

Percentage Difference: |A − B| ÷ ((A + B) ÷ 2) × 100 — used when comparing two values without a clear “before” reference, like comparing prices across two stores.

Common Mistakes to Avoid

Adding percentages directly: A 20% discount + 15% discount = 32% off, not 35% off. Always apply discounts sequentially.
Confusing percentage points with percent: If interest rates rise from 4% to 6%, that is a 2 percentage point increase — but a 50% increase in the rate.
Forgetting to convert: 8% = 0.08, not 0.8. Move the decimal two places left.
Reverse percentage errors: If a price increased by 25% to $125, the original was $100 — not 25% less than $125 ($93.75). Use: Original = Final ÷ (1 + percentage).

For any percentage calculation — discounts, tips, tax, interest, or statistical data — use the free Percentage Calculator. And if you are shopping for a home, check our Mortgage Calculator to see how interest rates affect your monthly payment.