Percentage Formulas for Everyday Math: Discounts, Tips, and Change

Three questions, three formulas

Almost every percentage problem people search for collapses into one of three questions: What is X% of Y? What percent is X of Y? And what is the percentage change from an old value to a new value? Learn those three and discounts, tips, marks, and growth rates become mechanical.

Formula 1: X% of Y

Value = (X ÷ 100) × Y. Example: 18% GST on a ₹2,000 item is (18 ÷ 100) × 2000 = ₹360. A 15% tip on a ₹1,240 bill is 0.15 × 1240 = ₹186. In the calculator, use the “What is X% of Y” mode.

Formula 2: X is what percent of Y

Percent = (X ÷ Y) × 100. Example: a score of 42 out of 60 is (42 ÷ 60) × 100 = 70%. A channel that retains 1,800 of 2,400 trial users retains 75%. Watch for Y = 0; the percent is undefined.

Formula 3: percentage change

Change % = ((new − old) ÷ old) × 100. From 80 to 100 is +25%. From 100 to 80 is −20%. Note that those percentages are not symmetric: a 25% rise followed by a 20% fall returns to the start only in this special pair of numbers, not in general.

Discounts are often easier as Formula 1 (percent of original), while “sales grew from A to B” is Formula 3. If a retailer says “was ₹999, now ₹749,” percentage decrease = ((749 − 999) ÷ 999) × 100 ≈ −25.03%.

Worked discount path

• Original price ₹2,499; discount 20% → savings = 0.20 × 2499 = ₹499.80.
• Sale price ≈ 2499 − 499.80 = ₹1,999.20.
• If an extra 5% coupon applies to the sale price, compute 5% of 1,999.20—not 5% of the original—unless the terms say otherwise.

Tips for clean results

Money usually rounds to two decimals; grades may round differently. Keep full precision until the final display. For taxes stacked on taxes, confirm whether each rate applies to the base or the running total. Percentages are unit-free: the same math works for kg, users, or rupees.

Using ShishwaLab

Pick the mode that matches your question, enter the two values, and read both the result and the plain-language equation. Copy the result into a spreadsheet note if you are comparing multiple scenarios. Pair with the currency converter when you need FX plus a percent markup.

Bottom line

If you can identify which of the three questions you are asking, the arithmetic is short. The calculator exists so you can move faster—and so you can see the substitution written out when a number looks surprising.