Quick guide
How to use this calculator
- Choose an arithmetic operation.
- Enter two non-negative whole numbers.
- Compare the one- and two-significant-digit suggestions with the exact result.
Calculation method
Compatible numbers are estimation choices
The calculator rounds each operand independently to one and two significant digits, then performs the selected operation on those rounded values.
Compatible-number choices are not unique. The suggestions are transparent heuristics, and another nearby pair may be easier or more accurate for a particular mental calculation.
Worked example
Estimate 407 divided by 5
Using 400 and 5 gives a quick estimate of 80; using 410 and 5 gives 82. The exact result is 81.4.
407 ÷ 5 = 81.4; estimates: 400 ÷ 5 = 80 and 410 ÷ 5 = 82
Supported inputs
Precision and limits
Whole numbers
Inputs must be non-negative integers containing at most 12 digits.
Suggestions, not facts
The proposed compatible numbers are deterministic estimates, not the only valid choices.
Exact comparison
Every suggestion includes its exact absolute error from the original calculation.
Nonzero divisor
Division suggestions require a divisor greater than zero.