How to Calculate Discounts (And Why 20% + 10% Off Isn't 30%)

A sign says "50% off, then an additional 20% off." Another store offers "$50 off your purchase over $200." Which is actually better? You're buying a shirt for $60 and you have a 15% off coupon. Does it matter if you use it today or next week? These questions trip people up constantly. The math is simple once you understand it, but retailers rely on most people not understanding it. Let's fix that.

Basic Discount Math

The simplest discount formula: New Price = Original Price × (1 - Discount Rate).

So if something is originally $100 and there's a 25% discount, the new price is $100 × (1 - 0.25) = $100 × 0.75 = $75. You save $25.

Why use (1 - discount rate) instead of just multiplying by the discount? Because the discount rate tells you what percentage you're NOT paying. You're paying 75% of the original. So you multiply the original by 0.75.

What "Percentage Off" Actually Means

"50% off" means you pay 50% of the original price. No wait — that's backwards. "50% off" means you get 50% taken off, so you pay the remaining 50%. If the shirt is $100 and it's 50% off, you pay $50.

This confusion happens constantly. When stores say "30% off," they mean the discount is 30%. You save 30% of the original price. You pay 70% of the original price.

The mental math version: If something is 10% off, you pay 90%. If it's 20% off, you pay 80%. If it's 30% off, you pay 70%. Just subtract the discount percentage from 100 and you know your payment percentage.

Stacked Discounts: Why 20% + 10% Isn't 30%

This is where most people go wrong. When a store advertises "20% off, then an additional 10% off," the second discount applies to the already-discounted price, not the original.

You save 28%, not 30%. The difference is small on a $100 purchase, but scale it up and it matters. On a $500 purchase, you'd expect to save $150 (30% off), but you actually save $140. That's $10 left on the table.

The formula for stacked discounts: Final Price = Original × (1 - Discount 1) × (1 - Discount 2) × (1 - Discount 3)... and so on. Each discount multiplies the previous result, not the original.

Worked Examples: Real Numbers

Scenario 1: Winter coat on sale
Original price: $200
First markdown: 30% off
Price after first discount: $200 × 0.70 = $140

Additional sale today: 15% off
Final price: $140 × 0.85 = $119

Total you save: $81 (which is 40.5% off the original)

Scenario 2: Online shopping with coupon and sale
Item: $80
Store is having 25% off (you pay 75%)
Price after sale: $80 × 0.75 = $60

You have a coupon for $10 off
Final price: $60 - $10 = $50

Total savings: $30 (37.5% off)

Notice the difference: when you have a percentage off sale plus a dollar amount coupon, the math is different. The percentage off applies to the original price, then the dollar amount is subtracted. But when you have percentage off stacked with another percentage off, each one applies to the remaining amount.

When Coupons Are Actually Worth Using

You have a $5 coupon. It takes you 15 minutes to find it, apply it, and deal with any complications. Is the $5 worth 15 minutes of your time? That's you deciding whether something worth $20/hour (15 minutes = 1/4 hour, so $5 = $20/hour) is worth your effort.

The break-even rule: If a coupon takes you 15 minutes to use, it needs to save you at least $5 to be worth it. If it takes 30 minutes, it needs to save $10. If it's a quick one-click coupon code, even $2 off is worth doing.

Where coupons are genuinely valuable: Grocery stores, where you're buying multiple items and small per-item discounts add up. A 50-cent coupon might seem small, but if you have 20 coupons, that's $10 off a $100 grocery trip. Drugstores often do this too. Digital coupons (usually free to apply) beat physical coupons every time.

Where coupons aren't worth it: When you're buying something you wouldn't normally buy just because you have a coupon. That $15 off a $60 kitchen gadget you don't need doesn't save you $15 — it costs you $45. Don't be tempted by the discount; be rational about whether you actually want the thing.

Black Friday Math: Decoding the Real Deals

Black Friday and Cyber Monday flood you with percentages. "70% off!" "Save $300!" Here's how to tell which ones are actually good deals.

Step 1: Find the original price. Stores often show a crossed-out "original" price that's inflated. A TV might be listed as $1,500 original, then 40% off for $900. But a quick search shows that same TV sells for $750 on Amazon year-round. The "original" was fake.

Step 2: Compare to normal market price. Use CamelCamelCamel (for Amazon price history) or Honey (price tracking) or just Google the item. If the Black Friday price matches the regular price, there's no deal.

Step 3: Remember that bigger percentage doesn't always mean bigger savings. 50% off a $100 item saves you $50. 30% off a $500 item saves you $150. The bigger purchase with the smaller percentage can be the better deal.

Real example: Two TVs on sale. TV A: Originally $400, now 50% off = $200 savings. TV B: Originally $1200, now 25% off = $300 savings. TV B saves you more money even though the percentage is lower. But TV B is also more expensive overall, so which is right for your budget depends on your needs.

Psychological Pricing Tricks Stores Use

"Anchor" pricing: They show you a ridiculously high original price so the discount looks amazing. You see "$500 originally, now $99!" and feel like you're getting a steal. But the item is worth $99 — the $500 was never real. By anchoring you to a high number, they make the actual price feel lower.

Percentage vs dollar amount: Which sounds better: 40% off or $50 off? For a $200 item, both save you $80. But 40% off sounds like more. For a $100 item, $50 off (50%) sounds like more. Stores use percentages when it sounds impressive and dollar amounts when that sounds impressive. Don't fall for the framing — do the actual math.

Charm pricing: Ever notice prices end in 99? Like $19.99 instead of $20? Your brain treats $19.99 as being in the "$10-$19" category even though it's nearly $20. A $99.99 item feels like it's under $100. Retailers price everything to exploit this psychological bias. It's legal and universal — just be aware it's happening.

Limited-time pressure: "Sale ends today!" Your brain panics and makes worse decisions. Most retailers have sales constantly. If something you want isn't on sale, wait a week. It probably will be. Don't let urgency force you into a bad decision.

How to Compare Discounts Across Stores

You want a specific item (say, wireless headphones). Store A offers 30% off. Store B has the same headphones for a flat lower price. Which is better?

The answer: calculate the actual final price at each store.

Store A: $200 original, 30% off = $200 × 0.70 = $140
Store B: $150 listed price (no discount)

Store A is cheaper. Store A's 30% off makes the price lower than Store B's "regular" price, even though Store B's regular price looks higher than Store A's before-discount price. This is exactly what catches people: they see 30% off and feel good without checking if the original price was inflated.

Spotting Fake Discounts

Sign 1: The "original" price looks suspicious. If you've never seen that item priced that high before, it probably wasn't. Check price history online or ask yourself: when's the last time I saw this for that price?

Sign 2: The math is hard to verify. Fake discounts hide in complexity. Percentage off stacked with dollars off, with special conditions for loyalty programs, with shipping costs hidden until checkout. If you have to spend 10 minutes calculating the actual price, the retailer might be hoping you won't bother.

Sign 3: The item is exclusive to the sale. If you can't comparison shop because it's only available at this store for this sale, be skeptical. Real deals on popular items are usually available across stores.

Use our discount calculator to quickly compute final prices. The golden rule: if something seems like an unbelievable deal, it probably is. Actually calculate the numbers before you get excited.