Maths
Five percentage mistakes that cost real money
Percentages are simple until they are applied twice, in reverse, or to a different base. These five errors are worth a minute each.
By the CALQEVA editorial team1 min read
1. Adding stacked discounts together
An offer of 50% off plus an extra 20% is not 70% off. The second discount applies to the already-reduced price, giving 60% off in total. On a ₹1,000 item you pay ₹400, not ₹300.
2. Confusing percent with percentage points
A rate moving from 4% to 5% has risen by one percentage point and by 25%. Both are correct and they describe very different things. When a headline says rates rose by 1%, it almost always means one percentage point.
3. Assuming a fall and a rise cancel out
A 20% fall followed by a 20% rise leaves you 4% down, because the rise applies to a smaller base. Recovering a 20% fall takes a 25% rise; recovering a 50% fall takes 100%.
4. Taking a percentage off an inclusive figure
To find the value before an 18% addition, divide by 1.18 rather than subtracting 18%. This applies to GST, to service charges and to any price that already includes a markup.
5. Comparing returns over different periods
A 60% return over five years and a 60% return over two years are not comparable. Convert both to a compound annual growth rate first: the five-year figure is about 9.9% a year, the two-year figure about 26.5%.
Try it with your own numbers
- Percentage CalculatorPercentage of a number, what percent one number is of another, and percentage change.
- Discount CalculatorSale price and savings, including stacked discounts.
- GST CalculatorAdd or remove GST and split it into CGST, SGST or IGST.
- CAGR CalculatorCompound annual growth rate between a starting and ending value.
Keep reading
- GST basics for a small businessWhat CGST, SGST and IGST mean, how inclusive pricing works, and the arithmetic mistakes that show up most often on invoices.
- Compound interest, explained without the hypeHow compound interest works, why frequency matters, and what the rule of 72 actually tells you, with worked examples.