Percentage is the one calculation almost everyone gets wrong at least once a month, usually at the worst possible moment. Here are the three patterns that keep showing up, and the traps hiding inside each one.
1. Percentage change
Change = (new - old) / old x 100
If a price goes from 400 to 460, that is (460 - 400) / 400 = 15%. Simple enough. The trap is direction: a 50% discount followed by a 50% price rise does not bring you back to the original price. It leaves you 25% below, because the second percentage applies to the smaller number.
2. The discount that is not a discount
A "20% off" tag on a 500 item sounds like 100 saved. But if the shop first raised the price from 450, the real saving is 50, not 100. Always ask what the price was before the sale. Percentage off is meaningless without a baseline.
3. Margin versus markup
This one catches freelancers constantly.
- Markup is applied to your cost: cost + 30% markup
- Margin is applied to the selling price: profit as a share of what the customer pays
A 30% markup is only about a 23% margin. Quote the wrong one and you quietly shrink your own profit on every single job.
4. Reverse percentage
If something now costs 690 after a 15% rise, the original was 690 / 1.15 = 600. You divide by 1 plus the rate. Multiplying by 0.85 here gives the wrong answer, and that mistake shows up in invoices, rent revisions and tax filings all the time.
Doing this without a calculator app full of ads
These are all one-line formulas, but when you are standing in a shop or on a call with a landlord you want the answer in two seconds. I keep a set of free, no-sign-up calculators at https://calcpromaster.com for exactly this. The ones I use most are the percentage tools, the EMI calculator for loan maths, and the unit converters when a spec arrives in the wrong units.
Get the baseline right, and the percentage stops being a trap.
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