How do I calculate a percentage of a number?+
To find X% of Y, multiply Y by X and divide by 100: Result = (X ÷ 100) × Y. Example: 15% of $200 = (15 ÷ 100) × 200 = 0.15 × 200 = $30. In everyday language: "percent" means "per hundred," so 15% means 15 out of every 100. Mental math shortcut: find 10% first (move the decimal one place left), then adjust. 15% of $200: 10% = $20, 5% = $10, total = $30.
How do I find what percentage one number is of another?+
Divide the part by the whole and multiply by 100: Percentage = (Part ÷ Whole) × 100. Example: 45 is what percent of 180? → (45 ÷ 180) × 100 = 0.25 × 100 = 25%. Practical uses: your test score (18 correct out of 24 questions = 18/24 × 100 = 75%); your savings rate ($600 saved from $3,000 income = 20%); your down payment percentage ($10,000 down on $200,000 home = 5%).
How do I calculate percentage change (increase or decrease)?+
Percentage Change = ((New Value − Original Value) ÷ |Original Value|) × 100. If positive, it's an increase; if negative, it's a decrease. Example: Price rose from $80 to $100 → ((100 − 80) ÷ 80) × 100 = (20 ÷ 80) × 100 = 25% increase. Price fell from $100 to $80 → ((80 − 100) ÷ 100) × 100 = −20% decrease. Important: always divide by the original (starting) value, not the new value. Using the wrong base is one of the most common percentage mistakes.
What is the difference between percentage change and percentage difference?+
Percentage change compares a new value to an original value — there's a clear "before" and "after." It's directional (positive = increase, negative = decrease). Percentage difference compares two values where neither is definitively the "original" — it measures the relative difference using the average as the reference point: |(A − B)| ÷ ((A + B) ÷ 2) × 100. Example: Product A costs $40, Product B costs $60. Percentage difference = |40 − 60| ÷ ((40 + 60) ÷ 2) × 100 = 20 ÷ 50 × 100 = 40%. Use percentage change for "before/after" scenarios; use percentage difference for neutral comparisons.
What is a good tip percentage in the US?+
In the United States, tipping is customary in service industries and has a significant impact on workers' income, as servers' minimum wage can be as low as $2.13/hour federally before tips. Standard guidance for 2026: Restaurants (sit-down): 18–22% is now considered standard, with 20% being the most common default. Fast food / counter service: not required, but 10–15% is appreciated. Delivery: $3–5 minimum or 15–20% of order. Taxis / rideshare: 15–20%. Hotel staff: $1–5 per bag, $2–5 per night housekeeping. Hair salons / barbers: 15–20%. Coffee shops: 10–15% at the counter is becoming more common as tablet-based tip prompts normalize tipping there.
How do I calculate a discount and find the sale price?+
Sale Price = Original Price × (1 − Discount%) or equivalently: Original Price − (Original Price × Discount%). Example: $120 item at 25% off → $120 × (1 − 0.25) = $120 × 0.75 = $90. The discount amount = $120 × 0.25 = $30. To find the discount percentage from a sale price: Discount% = ((Original − Sale) ÷ Original) × 100. Example: Was $120, now $90 → ((120 − 90) ÷ 120) × 100 = 25% off. Tip: "% off" refers to what you save; the rest is what you pay. 30% off = you pay 70% of original price.
What is the formula for adding or subtracting a percentage?+
To add X% to a number: New Value = Original × (1 + X/100). Example: Add 20% to $50 → $50 × 1.20 = $60. To subtract X% from a number: New Value = Original × (1 − X/100). Example: Subtract 15% from $80 → $80 × 0.85 = $68. Common mistake: people calculate the percentage of the new number, not the original. If you add 20% and then want to "undo" it, you cannot subtract 20% of the new number — you'd subtract 20/120 = 16.67%. To reverse adding X%, divide by (1 + X/100).
What is the difference between markup and profit margin?+
Both measure profit, but with different bases: Markup = (Profit ÷ Cost) × 100, where Cost is the base. Margin = (Profit ÷ Revenue) × 100, where Revenue (selling price) is the base. Example: Item costs $60 to make, sells for $100. Profit = $40. Markup = ($40 ÷ $60) × 100 = 66.7%. Margin = ($40 ÷ $100) × 100 = 40%. A 66.7% markup ≠ 66.7% margin. Retailers typically talk in margin (% of sale price); manufacturers often use markup (% over cost). Confusion between these two numbers is extremely common in business — always clarify which metric is being discussed.
How do I calculate sales tax and find the pre-tax price?+
To find the total price including tax: Total = Price × (1 + Tax Rate). Example: $100 item with 8.5% tax → $100 × 1.085 = $108.50. Tax amount = $100 × 0.085 = $8.50. To back-calculate the pre-tax price from a total: Pre-tax Price = Total ÷ (1 + Tax Rate). Example: $108.50 total, 8.5% tax → $108.50 ÷ 1.085 = $100. Never subtract the tax rate percentage directly from the total — you'd be calculating the wrong base. The US average combined state+local sales tax rate is approximately 7.0% (Tax Foundation, 2026). Five states have no sales tax: Oregon, Montana, New Hampshire, Delaware, and Alaska.
What does "basis point" mean?+
A basis point (bps) is 1/100th of a percentage point — or 0.01%. So 100 basis points = 1%. Basis points are used in finance and economics to avoid ambiguity when discussing interest rate changes. Example: "The Fed raised rates by 25 basis points" means rates rose by 0.25%. Why use basis points? If an interest rate rises from 2% to 3%, saying "it increased by 1 percentage point" is precise. Saying "it increased by 50%" is also technically correct (2 to 3 is a 50% increase) but misleading. Basis points eliminate this ambiguity in professional financial communication.
How do you convert a decimal to a percentage and vice versa?+
Decimal to Percentage: Multiply by 100 (or move the decimal two places right). 0.75 = 75%; 0.035 = 3.5%; 1.25 = 125%. Percentage to Decimal: Divide by 100 (or move the decimal two places left). 45% = 0.45; 7.5% = 0.075; 150% = 1.50. Fraction to Percentage: Divide numerator by denominator, then multiply by 100. ¾ = 0.75 = 75%; ⅜ = 0.375 = 37.5%. These conversions are fundamental because multiplication with decimals is far easier than percentages, which is why calculators and computers always work with decimals internally.
What is a percentage point vs. a percentage change?+
A percentage point is an absolute difference between two percentages. A percentage change is a relative change. Example: Unemployment rises from 4% to 6%. The increase is 2 percentage points (absolute). The percentage change is ((6 − 4) ÷ 4) × 100 = 50% increase (relative). These are dramatically different — and frequently confused in news coverage. If your savings account APY goes from 1% to 1.5%, that's a 0.5 percentage point increase (small in absolute terms) but a 50% increase in your interest earnings (large in relative terms). Always specify which you mean to avoid misunderstanding.
What percentage of my income should I save?+
The standard guidance from financial advisors in the US for 2026 is: Emergency fund first: save 3–6 months of expenses. Retirement savings: 15% of gross income (including any employer match) is the widely recommended target. The "50/30/20 Rule" budget: 50% of take-home pay for needs, 30% for wants, 20% for savings and debt repayment. Minimum for 401(k): at least enough to get the full employer match (typically 3–6% of salary) — this is "free money." The younger you start, the lower the percentage you need to save, due to compound interest. Use our Compound Interest Calculator to model your specific savings goal.
How do I calculate percent error?+
Percent Error = (|Measured Value − True Value| ÷ |True Value|) × 100. It measures how far off an experimental measurement or estimate is from the actual/accepted value. Example: You estimated a project would take 40 hours; it took 55 hours. Percent error = |40 − 55| ÷ 55 × 100 = 15/55 × 100 = 27.3%. Lower percent error = better accuracy. In science, percent error less than 5% is generally considered acceptable. In everyday use, percent error helps you evaluate how accurate your estimates, surveys, polls, or predictions are compared to actual outcomes.
What is 100% of something? Can percentages exceed 100%?+
100% of something means the entire thing — all of it. Percentages absolutely can exceed 100% and commonly do. Examples: If a company's revenue grew from $1M to $3M, that's a 200% increase (it tripled). If a product's price doubled, that's a 100% increase. If you give 110% effort, that's a motivational expression meaning maximum effort beyond normal. In finance, returns regularly exceed 100% (a stock that doubles is a 100% gain). If a measurement is 150% of the original, the amount is 1.5 times the original. There's no mathematical restriction on percentages — they simply represent a ratio expressed per hundred, and any ratio can be greater than 1 (i.e., 100%).
How do I calculate a percentage increase and then reverse it?+
Adding X% and then subtracting X% does not return you to the original number. Example: $100 + 50% = $150. $150 − 50% = $75 (not $100). To reverse a percentage increase, you must divide by (1 + X/100). To reverse a 50% increase: $150 ÷ 1.50 = $100. To reverse a 20% increase: divide by 1.20. To reverse a 10% discount: divide by 0.90. This is because the second percentage is calculated on the new (higher) base, not the original. In retail, this explains why "50% off, then another 50% off" is not the same as "100% off" — it's 75% off total (50% leaves 50%, then 50% off that leaves 25% of original price).
