Discount & Percentage Calculator

Every result on this page comes from an arithmetic identity. There are no rates or brackets that change with policy — the formulas have held since 17th-century commercial arithmetic. The calculator just wires them up to a typed input and shows the cross-check so you can trust the answer at a glance.

1. Discount & sale price

Given an original price P and a discount percentage d, the sale price is P × (1 − d/100) and the savings are P × d/100. The reverse calculation, given two prices, divides the absolute difference by the original. If the "sale" price is higher than the original — a markup, not a discount — the calculator labels the result accordingly so percentage increases on price tags are easy to spot.

2. Tip & split

Tip uses the same identity: tip = bill × pct/100. When you split a bill between n people, the per-head share is (bill + tip) ÷ n. Many Sri Lankan restaurants levy a 10% service charge automatically; treat that as a pre-loaded tip and use this calculator only for any additional gratuity. Most international defaults — 15%, 18%, 20% — are one-tap presets above the bill field.

3. Percentage (three sub-modes)

The percentage tab handles the three questions a search for "percentage calculator" almost always means:

• X% of Y — multiplicative use: X × Y ÷ 100.
• X is what % of Y — ratio use: 100 × X ÷ Y. The result is the mathematical definition of percent (per centum). Undefined when Y is zero.
• Percent change — comparison use: 100 × (new − old) ÷ |old|. We use the absolute value of the old number so a swing from a negative base reads with the correct sign.

4. Tax-inclusive ↔ tax-exclusive

To add tax to a net (tax-exclusive) price: gross = net × (1 + rate/100). To strip tax from a gross (tax-inclusive) price: net = gross ÷ (1 + rate/100). Each result is verified by a round-trip: stripping then re-adding should reproduce the gross to the rupee. Sri Lanka's current standard VAT rate is 18% (effective 1 January 2024, IRD), used as the calculator's default — but any non-negative rate is accepted.

• IRD Sri Lanka — Value Added Tax (VAT) rate page
• NIST Handbook 44 — rounding and percent conventions
• Khan Academy — percent word problems
• Value Added Tax (Amendment) Act, No. 32 of 2023 — 18% rate

Formulas and default rates were last cross-checked on 2026-05-11. The page is reviewed annually and whenever the Sri Lanka VAT rate changes.