Everyday Life Calculators

For a 20% tip, take the total bill and move the decimal point one place left to get 10%, then double it. On a $45 bill: 10% is $4.50, doubled is $9 — your 20% tip. For 15%, take the 10% figure and add half: $4.50 + $2.25 = $6.75. For a rough 18%, find 20% and subtract 10% of that tip: $9 − $0.90 = $8.10. These shortcuts get you to a reasonable tip without a calculator, though the tool gives you the exact figure whenever precision matters.

A percentage discount scales with the original price — 30% off a $200 item saves $60, while 30% off a $50 item saves only $15. A fixed-amount discount is the same regardless of price — $20 off is always $20. Which is better depends on the item price: a percentage discount beats a fixed-dollar coupon when the item is expensive enough that the percentage produces a larger saving than the dollar amount, and the fixed coupon wins when the percentage would be smaller. The discount calculator shows both the saving and the final price so you can compare deals directly.

Unit price is the most objective measure of cost per unit of quantity, so it is reliable as long as you are comparing identical or equivalent products. The main caveats are quality differences between brands, waste before use (bulk buying of perishables that spoil before you finish them wipes out any unit-price saving), and storage — a lower unit price means nothing if you have nowhere to keep the larger size. Use unit price to compare, then factor in those practical constraints before deciding.