Everyday Money Maths
Tips, discounts, sales tax and what an hourly rate is really worth — four small calculations people get wrong in predictable directions.
The tools in this guide
All free, no signup. Each one has a printable sheet too.
- Tip CalculatorTip, total and the per-person split - plus the rounded-up figure, because the number you actually want is usually a whole one.
- Discount CalculatorWhat you actually pay after a discount - including stacked ones, which never add up to what they look like. 30% then 20% off is 44% off, not 50%.
- Sales Tax CalculatorBoth directions. Working backwards from a total is the one people get wrong — you divide by 1.08, you don't multiply by 0.92.
- Hourly to Salary CalculatorWhat an hourly rate comes to over a year once unpaid time off and overtime are counted - not the hourly rate times 2,080.
- Cost Per Serving CalculatorWhat a serving really costs once trim, peel and bone come out - and how much of a bulk pack you have to actually use for it to be cheaper.
- Laundry Detergent Cost CalculatorWhat a wash actually costs, and what habitually filling the cap instead of measuring adds up to over a year.
These are the calculations that come up in shops, restaurants and job offers — small enough that everyone does them in their head, and just awkward enough that the mental shortcut is wrong.
Each one fails in a consistent direction, which is what makes them worth knowing.
Stacked discounts never add up
The Discount Calculator carries the one that costs real money: 30% then 20% off is 44% off, not 50%.
The second discount applies to the already-reduced price, not the original. So stacked offers are always worth less than they look, and the gap widens as the numbers get bigger. This is not a trick — it is just how sequential percentages work — but it is relied upon in how offers get advertised.
Run it before deciding whether a stacked offer beats a single larger one. Sometimes a flat 40% is better than "30% plus an extra 20%".
Sales tax backwards is the one people get wrong
The Sales Tax Calculator works in both directions, and the backwards direction is where the error lives: you divide by 1.08, you don't multiply by 0.92.
Given a total that includes tax, taking 8% off it does not give you the pre-tax price, because the 8% was calculated on the smaller number. Multiplying by 0.92 gets you a figure that is close enough to look right and is wrong every time — and the error grows with the tax rate.
Useful for expenses, for splitting a receipt, and for any time you need to know what something actually cost before tax.
Tips: the number you want is a whole one
The Tip Calculator gives tip, total and the per-person split — plus the rounded-up figure, because the number you actually want is usually a whole one.
That is a small piece of design that reflects how people really pay. Nobody leaves $17.43. The useful output at a table is the round number just above the correct one, and the per-person share of it.
An hourly rate is not hours × 2,080
The Hourly to Salary Calculator is the one worth running before accepting a job.
It works out what an hourly rate comes to over a year once unpaid time off and overtime are counted — not the hourly rate times 2,080.
2,080 is 40 hours × 52 weeks, and it assumes you are paid for every one of them. If your time off is unpaid, those weeks come straight off the total, and the annual figure people quote themselves is meaningfully higher than what lands. Overtime moves it the other way.
Comparing an hourly role against a salaried one on the 2,080 figure compares two different things.
The short version
- Stacked discounts multiply. 30% then 20% is 44%.
- Reverse sales tax: divide by 1.08. Never multiply by 0.92.
- Tips are easier as the round number above.
- Hourly × 2,080 overstates it if any of your time off is unpaid.
Each has a printable, though these are the four most likely to live on your phone in a shop rather than on paper.
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