RunToolRunPercentage Calculator
Calculate percentages easily.
How to Calculate Percentages
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"What is X% of Y", "X is what percent of Y", or percentage change between two values.
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The answer appears as you type, with no rounding surprises.
Percentages and percentage points are different things
If an interest rate rises from 2% to 3%, it has risen by one percentage point — and by fifty percent. Both statements are true and they describe the same change. Confusing them is the most common percentage error, and it appears constantly in news reporting and internal reports alike.
The rule is simple once stated: percentage points measure the arithmetic difference between two percentages; percent measures the relative change. When someone says a figure 'went up 50%', it is worth asking which of the two they mean, because the difference can be enormous.
Increases and decreases do not cancel
Add 10% to 100 and you get 110. Take 10% off 110 and you get 99, not 100. This is not a rounding artefact — the two percentages are calculated from different bases, so they cannot cancel.
The consequence shows up everywhere. A share that falls 50% must rise 100% to recover. A salary cut of 20% followed by a rise of 20% leaves you 4% worse off. A price increased by 25% and then discounted by 25% is 6.25% below where it started.
Working backwards from a total
Finding the original price from a price that includes tax is not a matter of subtracting the tax rate. If 20% tax has been added, the total is 120% of the original, so the original is the total divided by 1.2 — not the total minus 20%, which gives a different and wrong answer.
The same reasoning applies to any figure that already contains a percentage: a sale price after a discount, a gross amount including a commission, a total including a service charge. Divide by one plus the rate; do not subtract the rate.
Averaging percentages is usually wrong
Two shops each report a 50% conversion rate, so the combined rate is 50%? Only if they had the same number of visitors. If one had 10 visitors and the other 1,000, the true combined rate is dominated by the larger shop, and the simple average is meaningless.
Percentages must be recombined from their underlying counts, not averaged. This error is common in dashboards that aggregate rates across regions or periods, and it can point a decision in exactly the wrong direction.
Where the arithmetic bites in practice
Compound growth is the case with the largest consequences: 7% a year is not 70% over a decade but roughly 97%, because each year's growth applies to the previous year's total. The same compounding works against you on debt.
The calculation runs in your browser, so salary figures, margins and financial numbers typed here are not transmitted anywhere — worth noting, since those are exactly the numbers people paste into unfamiliar web calculators without thinking.