How to Calculate a Discount: Formulas, Mental Maths Tricks and Examples

Seeing “30% off” on a price tag is easy. Knowing what you’ll actually pay, in your head, before the checkout, is a skill worth having. Here’s the formula and the shortcuts that make it quick.

The discount formula

Two numbers matter: how much you save and what you pay.

• Amount saved = Original price × (Discount % ÷ 100)
• Sale price = Original price − Amount saved

Or in one step:

Sale price = Original price × (1 − Discount % ÷ 100)

Example: 30% off $80

• Saved: 80 × 0.30 = $24
• You pay: 80 × 0.70 = $56

The discount calculator does this instantly, including stacked discounts and tax, but the shortcuts below make you fast without a phone.

Mental maths shortcuts

1. Start from 10%

Move the decimal point one place left to get 10%. Then scale:

• 10% of $64 = $6.40
• 20% = double it = $12.80
• 5% = half of 10% = $3.20
• 15% = 10% + 5% = $9.60
• 30% = 3 × 10% = $19.20

2. Calculate what you pay, not what you save

For 25% off, you pay 75%, which is three-quarters. $120 at 25% off → ¾ × 120 = $90. For 20% off you pay 80%: multiply by 8 and move the decimal. $45 × 0.8 = $36.

3. Swap the numbers

x% of y equals y% of x. 8% of 50 is the same as 50% of 8 = 4. Handy when one of the numbers is round.

Common discounts at a glance

Discount	You pay	Quick method
10% off	90%	Subtract one-tenth
15% off	85%	Subtract 10%, then half of that again
20% off	80%	Subtract one-fifth
25% off	75%	Pay three-quarters
30% off	70%	Subtract 3 × 10%
33% off	≈ 67%	Pay about two-thirds
50% off	50%	Half price
75% off	25%	Pay one-quarter

Working backwards: what was the original price?

If an item costs $63 after 30% off, the original wasn’t $63 + 30%. That gives the wrong base. Divide instead:

Original price = Sale price ÷ (1 − Discount ÷ 100) → 63 ÷ 0.7 = $90

This is the “reverse percentage” mode in our percentage calculator.

Finding the discount percentage

Know both prices? Discount % = (Original − Sale) ÷ Original × 100.

A jacket reduced from $150 to $105: (150 − 105) ÷ 150 × 100 = 30% off.

Discounts plus tax

In most places tax is charged on the discounted price. $80 at 30% off with 8% sales tax: 56 × 1.08 = $60.48. The order makes no difference mathematically, since multiplication is commutative, but receipts usually show the discount first.

Watch out for these

• “Up to” discounts: only some items get the maximum.
• Inflated original prices: a “was” price the item never actually sold for.
• Multiple discounts don’t add up. 20% + 10% is not 30% off. See Stacked Discounts Explained: Why 20% + 10% Isn't 30% Off.
• Percent vs. fixed amount coupons: $10 off is better than 10% off on anything under $100.

Try a few prices below to build intuition.