Site search (/) takes the same phrases — try 15% of 84 or 84 plus 15%. For bills and gratuities see the Tip Calculator; for a percentage that repeats every year, the Compound Interest tool.
Percentage changes do not add. Down 50% and then up 50% does not get you back — it leaves you with 75%, because the 50% rise is taken from the smaller number. Change the four boxes and watch the whole chain collapse to one number.
Per cent is Latin for per hundred: 15% is 15/100. Increases multiply by 1 + p, decreases by 1 − p.
Reversing either means dividing by the same factor, not applying the opposite percentage. Subtracting feels like it undoes adding, which is where most real mistakes happen.
Markup is profit over cost; margin the same profit over price. A 50% markup is a 33.3% margin.
20% off then 10% off is 0.8 × 0.9 = 28% off, not 30%.
A savings rate moving from 4% to 5% has risen one percentage point but by a 25% increase in the rate, because 1 is a quarter of 4. Both are true and they sound wildly different, which is why careful reporting says “up one point, from 4% to 5%”. Treat any headline percentage about another percentage with suspicion until you know which is meant.
Add 10% to 100 and you get 110; take 10% off and you get 99, not 100: the decrease is measured against the bigger number. Reversing the order changes nothing. Two equal and opposite moves of X% leave you short by X²/100 percent: ±10% costs 1%, ±50% costs a quarter of your money. And a 20% fall needs a 25% rise to recover: a fund that loses a fifth and gains a fifth is still down.
A £120 total including 20% VAT does not contain £24 of VAT: it is 120% of the net, so the net is £100 and the VAT £20. The same trap in reverse: “what was the price before the 30% discount?” divide by 0.7. The Strip an included % mode does it properly and shows what subtracting would wrongly have given.
A 50% markup on a £100 cost gives a £150 price, a 33.3% margin. Aiming for a 50% margin needs a £200 price, a 100% markup. Quoting one while meaning the other reliably undercharges, and the error grows with the numbers.
20% off then a further 10% off is 28% off, not 30%. The gap widens: three 30% codes stacked come to 65.7% off, not 90%. The same multiplication runs the other way for markups, which is how a 25% margin and a 25% supplier increase never quite cancel.
“Doubles your risk” means nothing without the starting number: 2 in 10,000 to 4 in 10,000 is +100% and also +0.02 percentage points. The relative framing is used to alarm, the absolute to reassure. Always ask for the base rate.
Divide, do not subtract. A total of 120 including 20% tax is 120% of the net, so net = 120 ÷ 1.2 = 100 and the tax is 20. Subtracting 20% gives 96 and overstates the tax by a fifth. The Strip an included % mode does exactly this, and shows what subtracting would have given.
A rate moving from 4% to 5% has risen one percentage point — but that is a 25% increase in the rate, because 1 is a quarter of 4. Both are true and they sound wildly different, which is why careful reporting says "up one point, from 4% to 5%".
The second discount applies to the already-reduced price: 0.8 × 0.9 = 0.72, so 28% off. The gap widens with bigger numbers — three stacked 30% codes give 65.7% off, not 90%.
Markup measures profit against cost; margin measures the same profit against the selling price. A 50% markup on a cost of 100 gives a price of 150 — which is a 33.3% margin. They are never equal, and mixing them up systematically undercharges.
No — 100 becomes 110, then loses 11, finishing at 99. The fall is measured against the bigger number. Two equal and opposite moves of X% always leave you short by X²/100 percent: ±10% costs 1%, ±50% costs a quarter.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.