The most common percentage mistake: order matters in Type 3
Percentage change is not symmetrical. A 50% increase followed by a 50% decrease does not bring you back to where you started — you end up 25% lower. This is because the percentages apply to different base values. The calculator uses the original (first) value as the base, which is the standard mathematical convention.
Quick mental math cross-check
For 10%: move the decimal point one place left. For 1%: move it two places left. For 5%: find 10% and halve it. For 25%: divide by 4. For 20%: find 10% and double it. These shortcuts let you sanity-check the calculator's output instantly.
Reverse percentages
A common mistake: a shop marks an item up by 40%, then marks it down by 40% in a sale. The final price is not the original price. If original is $100, mark-up gives $140, mark-down of 40% gives $84 — you've lost $16. Use the calculator to work backwards: if the sale price is $84 and the mark-down was 40%, what was the pre-sale price? Enter 84 as the "part" and solve for the whole where the percentage is 60% (100% − 40%).
Percentage points vs percentage change
If interest rates rise from 2% to 3%, they have risen by 1 percentage point but by 50% in relative terms. These are completely different things. Percentage points describe an absolute change in a percentage value. Percentage change describes a relative change in any quantity. The calculator handles percentage change; percentage point differences are just subtraction.
Use for VAT and tax calculations
To find the VAT amount on a price: use Type 1 (what is 20% of £249?) To find the pre-VAT price from the total: the total is 120% of the pre-VAT price, so use Type 2 (£249 is what % of the pre-VAT price? — no, reverse it: the pre-VAT price is £249 ÷ 1.20).