Percentages
How to Calculate a Discount, Sales Tax and a Tip
Doing percentages off in your head comes down to one move: find 10% of the price, then build every other discount from it. This guide covers discounts, stacked offers, sales tax and tips, with a complete tip chart and the shortcuts behind each one.
Start with 10% and build everything from it
To find 10% of any price, move the decimal point one place to the left. Ten percent of $47.60 is $4.76. That single move is the anchor for every other percentage: 5% is half of it, 20% is double, 30% is triple, and 15% is 10% plus 5%. No division, no multiplying by 0.85.
This works because 10% is division by ten, and dividing a decimal number by ten is a shift, not a calculation. Once you have that one number in your head you can assemble almost any discount a store advertises in a couple of additions.
Round the price before you start. Nobody needs the exact cent while deciding whether to buy something. Treat $47.60 as $48 and 10% is $4.80, which is four cents off the true answer and still tells you what you need to know.
Example: 30% off a $64 jacket, in your head
You pay about $44.80. Faster still: for 30% off you keep 70%, and 70% is 7 × 6.40 = $44.80 directly.
How to calculate the percentage of a discount
Every common discount is one or two steps from the 10% figure. Learn the recipes and you will never need to reach for a phone in a store aisle.
| Percent | How to build it from 10% | On $60 | On $48 |
|---|---|---|---|
| 5% | Half of 10% | $3.00 | $2.40 |
| 10% | Move the decimal one place left | $6.00 | $4.80 |
| 15% | 10% plus half of 10% | $9.00 | $7.20 |
| 20% | Double 10% | $12.00 | $9.60 |
| 25% | Divide by 4, or 20% plus 5% | $15.00 | $12.00 |
| 30% | Triple 10% | $18.00 | $14.40 |
| 33% | About a third: divide by 3 | $20.00 | $16.00 |
| 40% | Four lots of 10% | $24.00 | $19.20 |
| 50% | Half | $30.00 | $24.00 |
| 60% | Half plus 10% | $36.00 | $28.80 |
| 70% | Find 30% and subtract from the price | $42.00 | $33.60 |
| 75% | Three quarters: divide by 4, times 3 | $45.00 | $36.00 |
Above 50%, flip the question. For 70% off, work out what you keep rather than what you save: you keep 30%, so 30% of $60 is $18 and that is the price. One step instead of two.
The formula behind the shortcuts
Thinking in multipliers is what makes the harder cases easy. A 25% discount is × 0.75. A 40% discount is × 0.60. Every other version of the question is the same equation rearranged, which the percent off calculator handles in all three directions.
How to calculate a discount percentage from a receipt
When you know both prices but not the percentage, find the dollars saved and divide by the original price.
Example: $90 marked down to $58.50
Divide by 90, not by 58.50. Dividing by the sale price gives 53.8%, which answers a different question: how far the sale price would have to rise to get back to list.
The other direction — you know what you paid and the advertised discount, and you want the original ticket price — is a division, not a markup. If you paid $63 after 30% off, the original was 63 ÷ 0.70 = $90. The reverse percentage calculator is built for exactly this.
Stacked discounts multiply, they do not add
This is the most expensive percentage mistake in retail, and stores rely on it. Two discounts applied one after another do not add up, because the second one is taken off a price that has already come down.
Example: a $150 coat, 30% off plus an extra 20%
A flat 50% off would have brought it to $75. The $9 difference is the 20% that was never charged against the $45 you had already saved.
The gap widens as the discounts get bigger. Two 50% discounts do not make an item free; each halves what is left, so you pay a quarter of list, which is 75% off.
Is 20% off better than buy one get one half price?
Multi-buy offers are discounts in disguise, and converting them to a percentage makes the comparison fair. The trick is to count the total you pay across the whole bundle and compare it against the list price of that same bundle.
| Offer | Items you take | You pay | Effective discount |
|---|---|---|---|
| Buy one get one free | 2 | 1 item | 50% off |
| Buy one get one half price | 2 | 1.5 items | 25% off |
| Buy two get one free | 3 | 2 items | 33.33% off |
| Buy three get one free | 4 | 3 items | 25% off |
| Buy two get two free | 4 | 2 items | 50% off |
| Three for the price of two | 3 | 2 items | 33.33% off |
| Second item 30% off | 2 | 1.7 items | 15% off |
So on two $40 shirts, buy one get one half price costs 40 + 20 = $60, while a straight 20% off both costs $64. The multi-buy wins — but only if you actually want two shirts. If you want one, 20% off gives you it for $32, and the other offer gives you nothing at all. Every multi-buy percentage in that table assumes you take the full bundle.
Per-unit price is the honest way to settle it when the pack sizes differ too. The unit price calculator compares cost per ounce or per item, which sometimes shows that a discounted large pack is still worse value than a full-price small one.
How to add tax to a price, and when it applies
Sales tax goes on after the discount. The store reduces the price, then tax is charged on the reduced amount. That is why the tax line on a sale item is smaller than the tax on the same item at full price.
Example: $80 item, 25% off, 7% sales tax
You pay $64.20. The tax is $4.20 rather than the $5.60 you would owe on the full $80.
The error to avoid is taxing the list price and then subtracting a flat dollar discount. That route gives 80 × 1.07 = $85.60, minus $20 of discount, or $65.60. It overcharges by $1.40, which is exactly 7% tax on the $20 you never spent.
Because multiplication does not care about order, 80 × 0.75 × 1.07 and 80 × 1.07 × 0.75 both land on $64.20. What matters is not the sequence but the base: the discount must be a percentage of the list price, and the tax a percentage of the discounted price.
Adding tax in your head uses the same 10% anchor. For a 7% rate on $60: 10% is $6.00, so 7% is a little under three quarters of that, around $4.20. For 8.25%, take 10% and shave off a sixth. If you are on the other side of the counter and setting the price rather than paying it, the markup calculator shows what a discount does to your margin.
Tip chart: 15%, 18%, 20% and 25%
Tipping is the same arithmetic with a friendlier name. Twenty percent is 10% doubled, 15% is 10% plus half of 10%, and 25% is the bill divided by four. Eighteen percent is 20% minus a tenth of that 20%, which is close enough to do standing up.
The chart below gives the tip itself, not the total. Add it to the bill to get what you leave.
| Bill | 15% | 18% | 20% | 25% |
|---|---|---|---|---|
| $10 | $1.50 | $1.80 | $2.00 | $2.50 |
| $15 | $2.25 | $2.70 | $3.00 | $3.75 |
| $20 | $3.00 | $3.60 | $4.00 | $5.00 |
| $25 | $3.75 | $4.50 | $5.00 | $6.25 |
| $30 | $4.50 | $5.40 | $6.00 | $7.50 |
| $35 | $5.25 | $6.30 | $7.00 | $8.75 |
| $40 | $6.00 | $7.20 | $8.00 | $10.00 |
| $50 | $7.50 | $9.00 | $10.00 | $12.50 |
| $60 | $9.00 | $10.80 | $12.00 | $15.00 |
| $70 | $10.50 | $12.60 | $14.00 | $17.50 |
| $75 | $11.25 | $13.50 | $15.00 | $18.75 |
| $80 | $12.00 | $14.40 | $16.00 | $20.00 |
| $90 | $13.50 | $16.20 | $18.00 | $22.50 |
| $100 | $15.00 | $18.00 | $20.00 | $25.00 |
| $125 | $18.75 | $22.50 | $25.00 | $31.25 |
| $150 | $22.50 | $27.00 | $30.00 | $37.50 |
| $175 | $26.25 | $31.50 | $35.00 | $43.75 |
| $200 | $30.00 | $36.00 | $40.00 | $50.00 |
| $250 | $37.50 | $45.00 | $50.00 | $62.50 |
For an amount that is not in the table, split it. A $135 bill is $125 plus $10, so a 20% tip is $25.00 + $2.00 = $27.00. A $47 bill rounds to $50, where 18% is $9.00, and you can trim a little from there.
Example: an $86.40 dinner, 20% tip, split four ways
Each person owes $25.92, or $26 in practice.
Tip on the pre-tax total or the total with tax?
Both conventions exist in the United States. Tipping on the pre-tax subtotal is the traditional rule, since tax is not part of the service. Tipping on the full total is simpler and, on a $60 meal at 8% tax, costs about a dollar more at 20%. Neither is wrong. Norms also differ sharply by country and by the kind of service, so treat percentages as local custom rather than arithmetic.
One thing worth checking: many restaurants add an automatic gratuity for larger parties, and some print “suggested tip” lines calculated on the post-tax total. Read the bill before adding a second tip on top of one that is already there.
Common mistakes
- Adding stacked discounts. 30% then 20% is 44% off, not 50%. Multiply the multipliers: 0.70 × 0.80 = 0.56.
- Dividing by the sale price to find the discount. Savings go over the original price. $31.50 saved on a $90 item is 35% off, not 53.8%.
- Adding the percentage back to reverse a discount. $63 after 30% off means 63 ÷ 0.70 = $90. Multiplying by 1.30 gives $81.90, which is wrong.
- Taxing the full price. Tax applies to the discounted amount, so your tax bill falls with the discount.
- Confusing “40% off” with “40% of”. Forty percent of $90 is $36. Forty percent off $90 is $54.
- Treating “up to 60% off” as your discount. It is a ceiling that applies to some items. Check the tag.
- Assuming a multi-buy beats a straight percentage. Buy one get one half price is only 25% off, and only if you take both.
- Rounding at every step. Round once, at the end, or the cents drift.
- Tipping twice. Check whether a service charge or automatic gratuity is already on the bill.
Frequently asked questions
How do you do percentages off in your head?
Find 10% by moving the decimal one place left, then build from there. On $48, 10% is $4.80, so 20% is $9.60, 5% is $2.40 and 15% is $7.20. For discounts over 50%, work out what you keep instead: for 70% off, find 30% of the price and that is what you pay.
What is 20% off $50?
It is $40, and you save $10. Ten percent of $50 is $5, so 20% is $10. In multiplier form, 50 × 0.80 = 40. If sales tax applies, it is charged on the $40, not on the original $50, so a 7% rate adds $2.80 for a total of $42.80.
How do you calculate a discount percentage from two prices?
Subtract to find the savings, then divide by the original price. An item that fell from $90 to $58.50 saved you $31.50, and 31.50 ÷ 90 = 0.35, or 35% off. Always divide by the price before the discount. Dividing by what you paid gives 53.8%, which answers a different question.
Is 30% off then 20% off the same as 50% off?
No. It comes to 44% off. The multipliers multiply: 0.70 × 0.80 = 0.56, so you keep 56% of the price. A $150 coat becomes $105, then $84 — not the $75 a flat 50% would give. The second discount only applies to what is left after the first.
Is sales tax added before or after a discount?
After. The discount reduces the taxable amount and tax is then charged on the lower price. An $80 item at 25% off with 7% tax comes to 80 × 0.75 = $60, plus $4.20 tax, for $64.20. Taxing the full $80 first and then subtracting the dollar discount overcharges by $1.40.
How do you add tax to a price?
Multiply by 1 plus the rate as a decimal. An 8% rate means price × 1.08, so $45 becomes $48.60. To find the tax alone, multiply by 0.08 for $3.60. Rates in the United States are set locally and range from zero to over 10%, so use the rate where the sale happens.
How much do you tip on $50?
Fifteen percent is $7.50, 18% is $9.00, 20% is $10.00 and 25% is $12.50. The quickest route is 10% of $50, which is $5.00, then double it for 20% or halve it and add for 15%. Whether you tip on the pre-tax subtotal or the total with tax is a matter of local custom.
What is 18% of $64?
It is $11.52. Convert 18% to 0.18 and multiply: 0.18 × 64 = 11.52. Mentally, 20% of $64 is $12.80 and a tenth of that is $1.28, so 18% is 12.80 − 1.28 = $11.52. That method works for any 18% tip: take 20% and shave a tenth off.
Is buy one get one half price better than 20% off?
Only if you want both items. Buy one get one half price is a 25% discount on the pair, so two $40 shirts cost $60 against $64 with a flat 20% off. But if you only want one shirt, 20% off gets it for $32 and the multi-buy gets you nothing.
How do I find the original price from a sale price?
Divide by the multiplier. If you paid $34.99 after 30% off, the multiplier was 0.70, so 34.99 ÷ 0.70 = $49.99. Do not add the percentage back on: multiplying $34.99 by 1.30 gives $45.49, and 30% off that is $31.84, which is not what you paid.
Calculators for this
Every one of these shows the working, so you can check the method as well as the answer.
More guides
https://fourfunctioncalculator.com/how-to-calculate-a-discount/