Guides/Percentage decrease & discounts

Percentage decrease and discount math explained

Percentage decrease measures how much a value fell relative to its start. Discount math is the same idea applied to prices: you find how much you save, then the sale price.

Percentage decrease formula

Percentage decrease = ((old − new) ÷ old) × 100

Equivalently, use the change formula ((new − old) ÷ old) × 100 and take the absolute value when the result is negative. The old value cannot be zero.

Discount (sale price) formulas

Amount saved = original × (discount% ÷ 100)
Sale price = original × (1 − discount% ÷ 100)
Sale price = original − amount saved

Worked examples

Inventory drop: 500 units → 425. (500 − 425) ÷ 500 × 100 = 15% decrease.
20% off $49.99: saved = 0.20 × 49.99 = $10.00; sale = $39.99.
35% off $80: saved = $28; sale = $52.
Price cut: $120 → $90. (120 − 90) ÷ 120 × 100 = 25% decrease.
Stacked discounts (careful): 20% off then 10% off $100 → first $80, then $72 — not a flat 30% off ($70). Order and base matter.

Common mistakes

Dividing by the new price instead of the original.
Adding discount percentages when they are applied sequentially to the reduced price.
Assuming undoing a 20% discount needs a 20% increase (100 → 80 needs +25% to return).

When to use which tool mode

Discount tab — original price + off-% → sale price and savings.
Increase / Decrease — any old vs new values (not only money).
Percentage increase guide — growth in the other direction.
% of a number — “what is 20% of 50?” without a before/after pair.
Original price before discount — reverse a sale price back to the sticker.
Percent off vs percent of — keep discount language straight.

Try it free

Use thePercentage CalculatorDiscount or Increase / Decrease tabs. All math stays in your browser.

Stacked sales:successive discounts·reverse percentage.