How to Calculate a Discount (Including Stacked Discounts)

You see a “30% off” tag on a pair of shoes. The original price is $89.99. Quick — what's the actual price? Most people can't do this in their head, and honestly, stores kind of rely on that. If you can't quickly figure out what you're saving, you're more likely to buy something you don't need because the deal feels good.

Our discount calculatorhandles the math instantly, but understanding the formulas yourself means you'll never get fooled by a fake deal again. Here's how it all works.

The Basic Discount Formula

The formula is dead simple. Two steps:

Find the savings: Original Price × Discount% = Savings
Subtract: Original Price − Savings = Final Price

Let's walk through that $89.99 example:

Savings: $89.99 × 0.30 = $27.00 (well, $26.997, but we round to the nearest cent)
Final Price: $89.99 − $27.00 = $62.99

You could also use the shortcut version — multiply by the percentage you're paying, not the percentage you're saving:

Final Price = Original Price × (1 − Discount%)
Final Price = $89.99 × 0.70 = $62.99

Same answer, one fewer step. I use this version most of the time because it's faster to punch into a calculator.

The Mental Math Shortcut

You don't always have your phone out. Here's the trick: find 10% first, then build from there. Finding 10% of any number is just moving the decimal point one spot to the left.

10% of $85.00 = $8.50
10% of $143.00 = $14.30
10% of $47.50 = $4.75

Once you have 10%, everything else is just multiplication:

20% off: Double the 10% number. (10% of $85 = $8.50, so 20% = $17.00)
30% off: Triple it. ($8.50 × 3 = $25.50)
25% off: Find 10%, then add half of that. ($8.50 + $4.25 = $12.75)
15% off: Find 10%, then add half of that. ($8.50 + $4.25 = $12.75... wait, that's 25%. For 15%, find 10% and add half of the 10%. So $8.50 + $4.25 = $12.75. No, let me redo that. Half of 10% is 5%. So 10% + 5% = $8.50 + $4.25 = $12.75. Yes, 15% of $85 is $12.75.)
5% off: Find 10% and cut it in half. ($8.50 ÷ 2 = $4.25)

For ugly numbers like $47.50 at 30% off: 10% is $4.75, times 3 is $14.25. Your price is $47.50 − $14.25 = $33.25. Not that hard once you practice a few times.

Stacked Discounts: The Trap Almost Everyone Falls For

Here's where stores really get you. You see “20% off everything, plus an extra 15% off at checkout!” and your brain says “that's 35% off.”

It's not. It's never 35% off. Stacked discounts are applied sequentially, not additively.

Let's use a $100 jacket as the example:

First discount (20% off): $100 × 0.80 = $80.00
Second discount (15% off the already discounted price): $80 × 0.85 = $68.00

You paid $68. That's a $32 savings, which is 32% off — not 35%. The second discount is applied to a smaller number, so it saves you less money than it looks like.

The actual combined discount formula is:

Combined Discount = 1 − (1 − D1) × (1 − D2)

Plugging in our numbers: 1 − (1 − 0.20) × (1 − 0.15) = 1 − (0.80 × 0.85) = 1 − 0.68 = 0.32, or 32%.

The gap gets bigger with larger discounts. “50% off + 30% off” sounds like 80% off, but it's actually 65% off. On a $200 item, that's the difference between paying $40 and paying $70. That's a $30 surprise most people don't see coming until they're at the register.

If you're dealing with stacked discounts, our discount calculatorhandles the chaining automatically so you don't have to think about it.

“Buy One Get One” vs. Percentage Off: Which Is Better?

Stores love BOGO deals because they move twice as much product while sounding like a great deal. But “buy one get one 50% off” is mathematically equivalent to 25% off your total purchase. Here's why:

Say you buy two shirts, each originally $40:

Shirt 1: $40.00 (full price)
Shirt 2: $20.00 (50% off)
Total: $60.00 out of $80.00 = 25% savings

Now compare that to a flat 30% off sale on both shirts:

Shirt 1: $28.00
Shirt 2: $28.00
Total: $56.00 out of $80.00 = 30% savings

The 30% off deal saves you $4 more, and you have the freedom to buy just one shirt instead of being pressured into two. A straight “buy one get one FREE” (not 50% off) is equivalent to 50% off if you were going to buy two items anyway. But if you only needed one? You just spent money on something you didn't want.

The rule of thumb: BOGO deals are only better than percentage discounts when you were already planning to buy multiple items and the percentage discount is less than what the BOGO effectively gives you. Do the math before you grab the second item.

Discounts and Taxes: Which Comes First?

In the US, discounts are almost always applied before sales tax. So the sequence is:

Apply the discount to get the sale price
Calculate tax on the sale price
Add tax to the sale price for your total

Example: $100 jacket, 25% off, 8% sales tax:

Sale price: $100 × 0.75 = $75.00
Tax: $75.00 × 0.08 = $6.00
Total: $75.00 + $6.00 = $81.00

If a store applied tax first and then the discount, you'd pay slightly less ($100 × 1.08 = $108, then 25% off = $81.00... actually the same in this case because multiplication is commutative). But some states have specific rules, and the rounding can create a cent or two of difference. In practice, every POS system I've seen applies discount first, then tax, and that's what you should expect.

If you're shopping online and the checkout page looks weird, check whether the discount was applied to the pre-tax or post-tax amount. It should be pre-tax. If it's not, that might be worth a chat with customer service.

The “Was/Now” Pricing Trick

One more thing to watch for: some retailers inflate the “original” price to make the discount look bigger. That “was $150, now $75” TV might have never actually sold for $150. Amazon does this constantly — the “list price” is often set by the seller and has no relationship to what the item normally costs.

The best way to check is to look up the price history. CamelCamelCamel (for Amazon) or Keepa will show you the actual price over time. If the item has been $75 for the last three months and suddenly the “original” is listed as $150, you're not getting a 50% discount. You're paying the normal price while being told it's a deal.

A real discount is compared against what you'd normally pay, not against a fictional number a retailer made up.

Frequently Asked Questions

Is 20% off the same as $20 off?

Only if the original price is $100. “20% off” scales with the price — on a $50 item, it's $10. On a $200 item, it's $40. “$20 off” is always $20 regardless of price. For expensive items, percentage discounts are usually better. For cheap items, a flat dollar amount might save you more. Always calculate both if you have the option.

How do I calculate 25% off in my head?

Find 10% by moving the decimal one spot left, then figure out 5% by cutting that number in half. Add them together. Example: 25% off $64. 10% is $6.40, 5% is $3.20, so 25% is $6.40 + $3.20 = $9.60. Your price is $64 − $9.60 = $54.40. Alternatively, just divide by 4, since 25% is one quarter. $64 ÷ 4 = $16 off, so $48. Wait — that's a cleaner way to think about it. 25% off means you pay 75%, and 75% of $64 is $48. Both methods work; pick whichever clicks for you.

What's a “good” discount percentage?

It depends on the category and the time of year. For clothing, 30-50% off is common during end-of-season clearance and actually represents a decent deal. For electronics, 10-20% off is typical for a genuine sale; anything above 20% on a current-model TV or laptop is pretty good. For furniture and mattresses, 40-60% “sales” are common, but those are often inflated baseline prices. The percentage matters less than whether you're paying less than the item's actual market value.

Are stacked discounts better than a single discount?

Usually worse, actually. Two stacked discounts of 20% and 15% give you 32% off total, not 35%. A single 35% discount would save you more. Stores use stacked discounts because the numbers sound bigger than they are, which drives more purchases. Always convert stacked discounts to their actual combined percentage before comparing deals.

How do I know if a sale price is actually good?

Check the price history. For Amazon, use CamelCamelCamel or Keepa to see what the item has actually sold for over the past few months. For other retailers, Google Shopping often shows recent price trends. If the “sale price” is the same price it's been for weeks, it's not really on sale — the store just slapped a “was/now” label on it.

Related Calculators

Discount Calculator — Calculate sale prices, savings, and stacked discounts
Percentage Calculator — Calculate any percentage of any number
Percentage Change Calculator — Calculate percentage increase or decrease between two values