How to Calculate Percent Off

You’re standing in a store. A jacket is $89, tagged “35% off”. Is that a good deal? What does it actually cost? The math is doable in your head if you know the trick, and impossible if you don’t. This is a short guide to calculating percent off in three ways: the mental shortcut, the formula on paper, and a calculator when the numbers get ugly.

The formula (the long way)

The math behind any percentage discount is the same:

Sale price = Original price × (1 − Discount percentage / 100)
Savings    = Original price × (Discount percentage / 100)

So for a $89 jacket at 35% off:

Sale price = 89 × (1 − 35/100) = 89 × 0.65 = $57.85
Savings    = 89 × 0.35 = $31.15

That’s the textbook version. Nobody actually does it that way at checkout.

The 10% trick (the mental shortcut)

The fast way to calculate any percent off in your head is to start with 10%, because finding 10% of any number is trivial — just move the decimal point one place to the left.

• 10% of $89 = $8.90
• 10% of $250 = $25.00
• 10% of $1,499 = $149.90

Once you have 10%, you can scale to almost any percentage in a couple of steps:

To get	Do this	Example ($89)
5%	Half of 10%	$8.90 ÷ 2 = $4.45
15%	10% + half of 10%	$8.90 + $4.45 = $13.35
20%	10% × 2	$8.90 × 2 = $17.80
25%	20% + 5%, or just price ÷ 4	$89 ÷ 4 = $22.25
30%	10% × 3	$8.90 × 3 = $26.70
33%	Price ÷ 3	$89 ÷ 3 ≈ $29.67
40%	10% × 4	$8.90 × 4 = $35.60
50%	Half the price	$89 ÷ 2 = $44.50
60%	50% + 10%	$44.50 + $8.90 = $53.40
66%	Two-thirds of price	$89 × 2 ÷ 3 ≈ $59.33
70%	10% × 7, or 100% − 30%	$89 − $26.70 = $62.30
75%	Three-quarters of price	$89 × 3 ÷ 4 = $66.75
80%	10% × 8, or 100% − 20%	$89 − $17.80 = $71.20

So for the jacket: 35% off is “30% + 5%” = $26.70 + $4.45 = $31.15 off, leaving $57.85. Matches the formula exactly, but you can do it standing in the aisle.

The common percentage cheat sheet

For the percentages you see most often on sale tags:

10% off → move decimal left one place, subtract from price 15% off → 10% + half of 10% 20% off → 10% × 2, then subtract 25% off → divide price by 4, that’s your savings 33% off → divide price by 3, that’s your savings 50% off → halve the price 70% off → price × 0.3 (you pay 30%) 75% off → price ÷ 4 (you pay a quarter)

The “you pay” framing is faster than “you save” once you’re used to it. At 75% off, you pay 25% — divide by 4. At 80% off, you pay 20% — divide by 5. At 60% off, you pay 40% — multiply by 0.4.

Reverse calculation — “what was the original price?”

You see a clearance tag: “$24.99, was 60% off.” What was the original price? The math:

Original = Sale price ÷ (1 − Discount / 100)
         = 24.99 ÷ 0.4
         = $62.48

Whenever you reverse-calculate a discount, you’re dividing the sale price by what the customer pays (1 minus the discount). At 60% off, the customer pays 40% — so divide by 0.4.

Quick reverse-discount reference:

Tag says	Divide sale price by
10% off	0.9
20% off	0.8
25% off	0.75
33% off	0.67
50% off	0.5
60% off	0.4
70% off	0.3
75% off	0.25
80% off	0.2

This is useful when you’re trying to tell whether a “60% off” tag is actually a deal or whether the store just inflated the “original” price.

The stacked-discount trap

This is where most people get the math wrong: “30% off” plus “an extra 20% off” is not 50% off. The second discount applies to the already-reduced price, not the original.

A $200 item at 30% off:

• Sale price after first discount: $200 × 0.7 = $140
• Now apply 20% off to $140: $140 × 0.8 = $112
• Total savings: $200 − $112 = $88 (44% off the original)

If it had really been “50% off,” you would have paid $100, not $112. The stacked version saves you $44 less than the headline math implies. Retailers know this. Coupon stacks always look bigger than they are.

The general rule for stacked discounts:

Effective discount = 1 − [(1 − d1) × (1 − d2) × … × (1 − dn)]

So 30% off + 20% off + 10% off = 1 − (0.7 × 0.8 × 0.9) = 1 − 0.504 = 49.6% off, not 60% off. Three stacked discounts of 20% each = 1 − (0.8³) = 48.8% off, not 60% off.

When the math is ugly — three coupons, weird percentages, or BOGO-plus-percentage — punching it into the discount calculator is faster than working it out by hand. It handles stacked discounts correctly out of the box.

Percent off with sales tax

Sales tax is applied after the discount, not before. So a $100 item at 20% off in a state with 8% sales tax:

Discounted price: $100 × 0.8 = $80
Tax:              $80  × 0.08 = $6.40
Final price:      $80  + $6.40 = $86.40

Some stores quote prices including tax (most of Europe), some don’t (most of the US). When comparing across regions, make sure you’re comparing tax-inclusive to tax-inclusive, otherwise a “lower” US price plus 9% sales tax can easily end up more expensive than a “higher” European price that already includes 20% VAT.

When the discount is a fixed dollar amount

If the deal is “$25 off” rather than a percentage, the math is simpler but the comparison is harder. To figure out the equivalent percentage:

Equivalent % = (Discount / Original price) × 100

So “$25 off a $80 item” is the same as 25 ÷ 80 = 31.25% off. The reason this matters: if you have a choice between “20% off” and “$25 off” on the same item, you want to pick the one that translates to the higher percentage at the price you’re paying. On expensive items, percent off usually wins. On cheap items, dollar off usually wins.

When to just use a calculator

Mental math is great until it isn’t. Use the discount calculator when:

• The percentages aren’t round (37.5% off, 18% off, etc.)
• You’re stacking three or more discounts
• You need to factor in sales tax
• You’re comparing several deals side-by-side
• You’re a retailer modeling promotion impact on margin

It’s free, no signup, and gives you the final price plus savings in one step — including the correct stacked-discount math that most people get wrong by hand.

The five-second checklist before you buy

Before any “X% off” purchase, run this in your head:

1. What’s 10% of the original price? (move the decimal)
2. What’s the discount in $? (scale the 10% up or down)
3. What’s the final price? (subtract)
4. Is this actually better than competitors right now? (the discount is meaningless if everyone else sells it cheaper)
5. Would I buy this if it weren’t on sale? (the most expensive thing is a discounted item you didn’t need)

That last one is the one that actually saves money. The math is just to make sure the store isn’t lying.