Percentage calculator
Percentages trip up even numerate people. "A 50% increase followed by a 50% decrease" doesn't return you to the start — it leaves you at 75% of the original. And "prices rose 100%" means they doubled, not that they went up by one percent. The word "percent" literally means "per hundred," but that simplicity masks common cognitive errors.
For developers: API access
Same result via GET request (use current inputs above):
curl -s "https://howdeedo.com/api/calc/percentage?operation=percentOf&percent=18&value=47.5"fetch("https://howdeedo.com/api/calc/percentage?operation=percentOf&percent=18&value=47.5").then(r => r.json())Get an API key for higher limits and stable access.
Good to know
Percentage points vs. percentages. If interest rates rise from 5% to 6%, that's a 1 percentage point increase — but a 20% increase. The rate went up by 1 point; it also went up by 20% of its previous value. News reports often conflate these, leading to confusion. When precision matters, specify which you mean.
Percentage change is not symmetric. A 50% drop followed by a 50% gain doesn't break even — you end up at 75% of where you started. ($100 → $50 → $75). To recover from a 50% loss, you need a 100% gain. This asymmetry explains why avoiding losses matters more than capturing gains in investing.
"Up to 50% off" is marketing, not math. One item might be 50% off; most are 10-20% off. Vague percentage claims exploit the gap between what's technically true and what's implied. Calculate the actual discount on items you're considering, not the headline.
Methodology & assumptions
Assumptions
- Standard percentage formulas; no rounding in core.
- Compound changes are applied multiplicatively in sequence.
- CAGR assumes values are positive; doubling time uses Rule of 72.
- Markup is based on cost; margin is based on selling price.
References
Disclaimers & sources
Standard percentage formulas for reference and comparison.