A "30% off" tag sounds simple until you need the exact checkout price. These patterns cover most daily percent questions without opening Excel.
Three formulas that cover most cases
| Task | Formula | When to use |
|---|---|---|
| Sale price | Original × (1 − discount% ÷ 100) | Reading a sale tag |
| Reverse discount | (Original − Paid) ÷ Original × 100 | "How much was taken off?" |
| Part of whole | Part ÷ Whole × 100 | Scores, completion rates |
| Percent change | (New − Old) ÷ Old × 100 | Growth or decline |
Example: $80 with 30% off
Sale price = 80 × 0.70 = $56
You save = $24
Example: Paid $84 on a $120 item
(120 − 84) ÷ 120 × 100 = 30% off
Tax-inclusive vs pre-tax base price
Stores may show "$100 including tax" or "$90.91 + tax" on the same shelf. Always apply the discount to the same base the receipt uses. Check whether the strikethrough "was" price includes sales tax before you reverse-engineer the percent off.
Coupon stacking order
Many carts apply item sale → coupon → card cashback in sequence. Thirty percent off, then ten percent off a coupon, is not forty percent total — it lands around thirty-seven percent off the original. Use the cart's estimated total as your ground truth.
Shopping mistakes to avoid
- "Up to 50% off" — May apply to select SKUs only. Confirm the cart total at checkout.
- Tax included or not — The base price for the percent may be pre- or post-tax.
- BOGO vs 50% off — Buy-one-get-one matches 50% off only when you buy two; a single item does not qualify.
- Loans vs quick percent — Mortgages, compound savings, and BMI belong on the Finance Calculator; everyday discounts use a dedicated percent tool.
Promotion types compared ($20 original price)
| Promo | Condition | Effective unit price | Felt discount |
|---|---|---|---|
| 30% off | Buy 1 | $14 | 30% |
| BOGO | Buy 2 for $40 | $20 each | 0% per unit |
| Buy 2 get 1 free | Pay $40 for 3 | ~$13.33 each | ~33% |
| 30% then 10% coupon | Sequential | $12.60 | 37% (not 40%) |
Pre-checkout reverse-discount checklist
- Compare subtotal vs estimated payment — same basis?
- Confirm shipping and fees are excluded from the percent (usually separate line items)
- For loyalty points, treat cash-back value separately from instant shelf discounts
- "Up to 50% off" banners may not apply to your SKU — recalculate per item
Frequently asked questions (FAQ)
Q1. Is 30% off plus 20% off the same as 50% off?
A1. No. Sequential discounts multiply: 30% then 20% on $100 leaves $56 (44% total off), not $50.
Q2. Should I use 0.3 or 30% in formulas?
A2. Do not mix decimals and percents. Use =A1*0.7 or =A1*(1-30/100) consistently.
Q3. How do I handle sales tax?
A3. Apply the discount to the same base the store uses (pre-tax or post-tax) as shown on the receipt or product page.
Q4. The banner says 50% off but my item shows 20% — which is correct?
A4. Both can be true. The banner is a category maximum; your SKU may qualify for less. Reverse the percent from your list and sale prices only.
Q5. Does 5% loyalty points count as a discount?
A5. It is deferred value, not cash off today. Five percent back on $100 is $5 later — useful, but not the same as an immediate thirty percent markdown.
Try the percent calculator in your browser
Enter original price and discount percent — or original and sale price — in the Percent Calculator. All math runs locally with no upload.
Related guides
- Compound interest and savings guide: Long-term growth vs quick percent math
