Discount Calculator
The discount calculator helps you quickly solve 3 common discount problems:
Problem 1 — Find the sale price: 20% off 1.000.000 → save 200.000, pay only 800.000.
Problem 2 — Find the discount percentage: Original price 1.000.000, sale price 750.000 → 25% off, saving 250.000.
Problem 3 — Stacked discounts: Applying multiple discounts sequentially. For example: 20% off then 10% off ≠ 30% off. In reality: 1.000.000 × (1−20%) × (1−10%) = 720.000 (effective discount: 27.999999999999993%).
Discounts are the most common marketing tool in e-commerce and retail. Understanding how they work helps you shop smarter and avoid misleading "extra discount" promotions.
Discount calculation formulas
1. Find the sale price
Sale price = Original price × (1 − discount% / 100)
Example: 1.000.000 at 20% off = 1.000.000 × (1 − 20/100) = 800.000
Savings = 1.000.000 − 800.000 = 200.000
2. Find the discount percentage from prices
Discount % = ((Original − Sale) / Original) × 100
Example: (1.000.000 − 750.000) / 1.000.000 × 100 = 25%
3. Stacked discounts (sequential discounts)
When a store applies multiple discounts one after another, each discount is applied to the already-discounted price:
Final price = Original × (1 − d₁/100) × (1 − d₂/100) × ...
Example: 20% off then 10% off:
- After 20% off: 1.000.000 × 0.8 = 800.000
- After additional 10% off: 800.000 × 0.9 = 720.000
- Total discount: 27.999999999999993% (not 30%!)
Important note
Stacked discounts do not add up. 50% off + 50% off ≠ free. In reality: price × 0.5 × 0.5 = price × 0.25 (75% off, not 100%). This is a common mistake when shoppers see "extra 50% off".
Worked example: Buying a discounted phone
| Parameter | Value |
|---|---|
| List price | 20.000.000 |
| Discount round 1 (Black Friday) | 20% |
| Additional discount (VIP member) | 10% |
| Additional discount (app payment) | 5% |
Step-by-step calculation
- After 20% off: 20.000.000 × 0.80 = 16.000.000
- After additional 10% off: 16.000.000 × 0.90 = 14.400.000
- After additional 5% off: 14.400.000 × 0.95 = 13.680.000
Summary
- Total savings: 6.320.000
- Effective discount %: 31.6%
- Compared to simple addition: 20% + 10% + 5% = 35% → 13.000.000 (wrong)
- Reality: Stacked discount = 31.6% → 13.680.000 (correct)
Frequently asked questions
Does 50% off + 50% off mean the item is free?
No. Stacked discounts do not add up. 50% off then another 50% off = price × 0.5 × 0.5 = price × 0.25 (75% off, not 100%). This is a common marketing trick that confuses many shoppers.
How much do I save with 30% off a $100 item?
Savings = $100 × 30% = $30. Sale price = $100 − $30 = $70. The same formula works for any currency: VND, USD, EUR, etc.
How to compare a percentage discount vs a flat dollar discount?
Convert both to the same unit. Example: $10 off a $50 item = 20% off. If the other offer is 25% off → saves $12.50 → the 25% deal is better. Always compare the final price, not just the discount type.
Is tax applied before or after the discount?
It depends on the country. In most places, sales tax (VAT/GST) is calculated on the post-discount price. So a $100 item at 20% off = $80, then 10% tax = $8, total = $88.
Is a 15% coupon + free shipping better than a 20% coupon?
It depends on the shipping cost. If shipping is $5 on a $50 order: 15% coupon saves $7.50 + free shipping saves $5 = total $12.50. 20% coupon saves $10. So 15% + free shipping is better in this case.
Are flash sale discounts always cheaper than regular prices?
Not always. Some stores inflate the original price before applying a discount. Always compare the actual final price (ignoring the discount percentage) with prices at other stores. This calculator helps you find the real final price for an objective comparison.