Calculators
Everyday maths, done properly. Every DO101 calculator shows the formula next to the answer, so you can check the working instead of trusting a black box.
Work out an exact age in years, months and days.
Every percentage question in one place, with the formula shown.
See the sale price, the amount saved and the real discount.
Body Mass Index in metric or imperial, with the range explained.
Why show the formula?
Most online calculators give you a number and nothing else. That is fine until the number looks wrong — and then you have no way to tell whether you typed something incorrectly or the tool made an assumption you did not expect. DO101 prints the exact expression it evaluated underneath every answer.
It matters more often than you would think. Stacked discounts multiply rather than add, so 20% off followed by 10% off is 28% off, not 30%. Age calculations that approximate a month as 30 days drift by days over a lifetime. Seeing the working makes those differences visible.