Math Tools
Everyday calculators for quick number problems - no formula memorization required.
These calculators handle the math problems that come up most often - working out percentages, splitting restaurant bills, calculating discounts and tips. All results are instant and run entirely in your browser.
About these calculators
Everyday math is full of small, repetitive calculations - the kind nobody enjoys doing by hand and nobody trusts themselves not to fumble at the dinner table. Working out a tip on a $73.40 bill, splitting it five ways, calculating the discount on a $129 jacket marked 35% off, figuring out what percentage one number is of another - none of it is hard, but doing it correctly while half-distracted is harder than it looks.
The tools here exist because spreadsheets are overkill, calculator apps are slow to type into, and asking your phone is unreliable. Open the page, type the numbers, see the answer. Everything runs locally in your browser; nothing is sent anywhere.
Common scenarios
- Restaurant bills. Tipping 18% on $87 for a group of four - use the tip calculator for both the tip amount and the per-person split, including how to handle uneven splits or pre-tax vs post-tax tipping.
- Sales and discounts. A $1,200 laptop marked 25% off comes out to $900. The percentage calculator handles "X% of Y," "X is what percent of Y," and percentage change - the three flavors that cover almost every real situation.
- Budgeting and growth. Calculating what percentage of your income goes to rent, or how much your salary grew year-over-year, is the same percentage math viewed from two angles.
- Quick sanity checks. Before signing a contract or accepting a quote, running the numbers through a clean calculator catches off-by-a-decimal mistakes that everyone makes.
The math, briefly
Percentages are just fractions with a fixed denominator of 100. "15% of 80" means 15/100 × 80 = 12. "25 is what percent of 200" means 25/200 × 100 = 12.5%. Percentage change between two numbers is (new − old) / old × 100 - note the divisor is the original value, not the new one. Tipping is the same operation: a 20% tip on $50 is 0.20 × 50 = $10.
Where people most often go wrong: dividing by the wrong number for percentage change (using the larger value when calculating a decrease, for example), confusing percentage points with percent (a rate going from 4% to 6% is a 2-percentage-point change, but a 50% relative increase), and rounding too early in multi-step calculations.
Frequently asked questions
Is the tip calculated on the pre-tax or post-tax total?
Customary practice in the US is pre-tax, but most diners simply tip on the bottom-line bill for convenience. The difference on a $50 meal with 8% tax is roughly 80 cents on a 20% tip - small enough that nobody but the IRS cares.
How do I split a bill unevenly?
Calculate each person's share of the subtotal, add their proportional share of tax and tip. Or: agree on a common per-head charge for shared items and add individual extras separately.
What's the difference between markup and margin?
Markup is the increase over cost as a percentage of cost. Margin is profit as a percentage of selling price. A 50% markup gives a 33% margin - they're not interchangeable.
How do I undo a percentage discount?
If a price after a 20% discount is $80, the original was $80 / 0.80 = $100. Don't add 20% back to the discounted price - that gives you $96, which is wrong.