Percentage Decrease Calculator — Calculate Percent Off & Discount

Percentage decrease = ((old − new) / old) × 100, returned as a positive percentage when there's an actual drop. Worked example: old = 80, new = 60 → (80 − 60) / 80 × 100 = 20 / 80 × 100 = 25% decrease. Note the convention: the result is positive (25%) even though the change is mathematically negative (−25). The UI displays "−25%" with the minus sign for direction visibility, but the underlying magnitude is positive — matching the everyday phrasing "a 25% reduction" rather than "a negative 25% change". Foundational from ISO 80000-1:2022 percentage definition.

They're complementary expressions of the same multiplicative relationship: new_price = old_price × (1 − discount_fraction). For a 75% discount: new_price = old_price × (1 − 0.75) = old_price × 0.25 = pay 25% of original. For a 60% discount: new_price = old_price × 0.40 = pay 40%. The mental rule "X% off = pay (100 − X)% of original" works for any single discount. For stacked discounts the rule extends multiplicatively: 30% off + 20% extra = pay 0.70 × 0.80 = 56% of original = 44% effective off. This is why retailers favour "% off" framing — the headline number sounds bigger than "pay 25%" even though the math is identical.

The FTC Guides Against Deceptive Pricing (16 CFR Part 233), enforced under FTC Act § 5 (15 U.S.C. § 45), require retailers to compare against an actual "regular price" previously charged in the recent past — not a fictitious MSRP that was never charged. The historical FTC Picture Tube Rule (16 CFR Part 410) governing TV-size advertising was repealed October 2018, but FTC Act § 5 continues to apply broadly to discount framing. California Business and Professions Code § 17501 strengthens this at the state level (three-month "prevailing market price" requirement). The EU Price Indication Directive 98/6/EC was amended by the Omnibus Directive 2019/2161 (which added Article 6a) to require disclosure of the lowest price applied during the 30 days before the announced "% off" claim — the "lowest-price-30-days" rule effective since 28 May 2022 across EU member states.

The US IRS Modified Accelerated Cost Recovery System (MACRS, established by the Tax Reform Act of 1986; current schedule in IRS Publication 946) prescribes depreciation as percentage decreases per period, varying by asset class. 5-year property (vehicles, computers, office equipment) under the half-year convention: Year 1 20.00%, Year 2 32.00%, Year 3 19.20%, Year 4 11.52%, Year 5 11.52%, Year 6 5.76% — front-loaded for tax-deferral benefit. 7-year property (office furniture, agricultural equipment): 14.29 / 24.49 / 17.49 / 12.49 / 8.93 / 8.92 / 8.93 / 4.46% over 8 years. Commercial real estate uses 39-year straight-line (~2.564% per year). Each period's depreciation reduces the asset's book value, and the new book value becomes next period's base — equivalent to repeated percentage decreases.

To match everyday human phrasing. We say "a 25% reduction" or "the price dropped 25%", not "a negative 25% change". The calculator returns the positive magnitude (25) and uses the UI label "−25%" or "down 25%" to indicate direction. This matches the convention of dedicated decrease tools (vs the percentage-change tool which preserves the mathematical sign for direction-agnostic comparisons). When the inputs imply an increase (new > old), the page surfaces a hint suggesting the percentage-increase calculator instead, rather than returning a negative "decrease". Reduces user confusion at the cost of slightly less mathematical purity.