Everyday Math Calculators

The four mistakes these tools exist to prevent

Percent and percentage points are different things. A rate rising from 4% to 6% is two percentage points, and also a 50 percent increase. Both descriptions are correct and they differ by a factor of twenty-five, which is why news reporting about interest rates is so frequently confusing.

Reversing a percentage is a division, not a subtraction. An item costing $80 after 20% off was not $96 originally — it was $80 divided by 0.8, which is $100. The same applies to stripping tax from a total, and the error grows with the percentage involved.

Stacked percentages compound rather than add. A 20% discount followed by a 10% discount is not 30% off. The second applies to the already-reduced price, so $100 becomes $80 and then $72 — an effective 28%.

Increases and decreases are not symmetric. Recovering from a 50% fall needs a 100% gain, not a 50% one, because the second percentage applies to a smaller base. From an 80% fall it takes 400%.

Why these are worth a calculator at all

Each of these is arithmetic anyone can do, and each is arithmetic people routinely get wrong under time pressure — standing in a shop, splitting a bill, or checking whether a sale price is really what the sign claims. The value is not in the multiplication; it is in the tool applying the correct method rather than the intuitive one.

Every page shows its working rather than just its answer, so the method is visible and you can carry it away. Everything runs in your browser and nothing you enter is transmitted anywhere.

Frequently asked questions