How to Calculate a Discount by Hand: Percentages Without a Calculator

Published on May 28, 2026 · 9 min read

A "30% off" sign looks generous on the shelf and confusing at the register. Most people pull out their phone, tap into a calculator, and still wonder whether the discount was applied before or after VAT. This guide gives you the three formulas that cover 95% of everyday percentage problems, plus the mental shortcuts that let you check the price in two seconds without any app.

The one formula to remember

Every percentage problem reduces to: part = whole × (percent ÷ 100). A 25% discount on €80 is 80 × 0.25 = €20 off, leaving €60 to pay. That is it. The rest is shortcuts.

Mental-math shortcuts

10% of anything: move the decimal one place left. 10% of €87.40 = €8.74.
5%: half of 10%. 5% of €87.40 ≈ €4.37.
1%: move the decimal two places left. 1% of €87.40 = €0.874.
15%: 10% + 5%. The classic tipping trick in the US.
20%: double the 10%. Faster than multiplying by 0.2.
25%: divide by 4. Quarter-off sales are the easiest.
50%: halve it. You knew that one.

Discount: what you actually pay

Shops advertise the saving; you care about the final price. There are two equivalent ways:

Subtract the discount: €120 with 30% off → 120 × 0.30 = €36 off → €84 to pay.
Multiply by what is left: 100% − 30% = 70%, so 120 × 0.70 = €84. Faster.

Use the second form whenever you can — one multiplication instead of a multiply-and-subtract.

Stacked discounts are not additive

A "20% off, then an extra 10% at checkout" promotion is NOT 30% off. The 10% applies to the already-discounted price. On a €100 item:

100 × 0.80 = €80, then 80 × 0.90 = €72. The real saving is 28%, not 30%. The combined multiplier is 0.80 × 0.90 = 0.72.

Adding VAT or sales tax

To add a 21% VAT, multiply by 1.21. To strip VAT out of a VAT-inclusive price, divide by 1.21 — do NOT subtract 21%.

Net €100 + 21% VAT → 100 × 1.21 = €121 gross.
Gross €121 → net = 121 / 1.21 = €100.
Subtracting 21% from €121 would give €95.59 — wrong by €4.41.

Percentage change: increase and decrease

To compare two numbers as a percentage change:

change% = (new − old) / old × 100.

Rent goes from €700 to €805: (805 − 700) / 700 = 0.15, so a 15% increase. A stock drops from €40 to €34: (34 − 40) / 40 = −0.15, a 15% decrease.

Warning: a 20% drop followed by a 20% rise does NOT return you to the start. €100 → €80 → €96. You need a 25% rise to recover from a 20% drop.

Worked examples

Black Friday jacket

Sticker price €189, sign says "40% off". Multiplier 0.60. 189 × 0.6 = €113.40. Mentally: 10% of 189 is 18.90, so 40% is 4 × 18.90 = €75.60 off, leaving €113.40. Both methods match.

Restaurant tip

Bill is $43.20. A 20% tip: 10% is $4.32, double it = $8.64. Total $51.84. A 15% tip: 10% ($4.32) plus 5% ($2.16) = $6.48.

Retail markup

You buy at €40 and want a 60% markup. Selling price = 40 × 1.60 = €64. Note: "60% markup" (on cost) is different from "60% margin" (on selling price). A 60% margin on a €64 sale means cost is 64 × 0.40 = €25.60.

FAQ

Is a 50% discount the same as half price?

Yes — that is the one case where the math is obvious. Multiply by 0.5.

Can a discount be more than 100%?

Not honestly. 100% off means free; anything above would mean the shop pays you.

Which is better: 25% off or buy one get one 50% off?

BOGO 50% averages to 25% off per item — they are equivalent, unless you only need one.

Bottom line

Multiply by the percentage that remains, not by the one that is taken away. Memorize 10%, 5%, and 1%, and you can build every other percentage from those three. For everything else — VAT, stacked promos, percentage change — keep the ConvertProf calculator open and double-check before you swipe the card.