Percentage calculator

You can increase by any percentage (200%, 500%, 10,000%), but you can only decrease by a maximum of 100% (going to zero). Decreasing by more than 100% is mathematically undefined—you can't remove more than everything. Headlines saying "crime dropped 120%" are wrong; they mean "crime is now 20% of what it was" (an 80% drop).

"50% more" depends on the base. 50% more than $10 is $15. 50% more than $1,000 is $1,500. Without context, percentages are meaningless. Always ask: percent of what? News headlines love to say "X increased 50%!" without telling you the baseline. Small numbers can have huge percentage changes that sound scarier than they are.

In finance, a basis point (bps) is 1/100th of a percentage point. 100 bps = 1%. If interest rates go from 3.0% to 3.5%, that's a 50 bps increase (or a 16.7% relative increase). Basis points avoid confusion between percentage points and percent change. "Rates increased 0.5 percentage points" vs "rates increased 50 bps" mean the same thing.

10% is easy—move the decimal left one place ($47.50 → $4.75). Double it for 20%. For 15%, take 10% and add half of that. For 18%, double 10% and subtract a bit. Mental math beats pulling out a calculator.

"Percent of" finds a portion: 25% of 80 is 20. "Percent change" compares two numbers: going from 80 to 100 is a 25% increase. They use different formulas. "Percent of" is static; "percent change" is dynamic.

"Up to" means one item somewhere in the store might be 50% off, but most aren't. It's marketing language. "Up to X%" is almost always less impressive than it sounds. Real discounts say "50% off everything" or "20% off storewide."