How this calculator works
Compatible numbers are nearby values chosen to make mental arithmetic easier. This tool uses one reproducible choice: round every input to the same place value, then apply the operation from left to right. A coarser place is often easier but can give a less accurate estimate, especially in division.
Formula
rounded value = nearest multiple of 10^place, with halves away from zero. Estimate = the selected operation on the rounded values.
Worked example
198 + 403 rounds to 200 + 400 at the hundreds place, giving 600 instead of 601. For 49 × 21 at the tens place, 50 × 20 = 1000 instead of 1029.
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Frequently asked questions
Are compatible numbers unique?
No. Mental estimation may choose convenient factor pairs instead. This tool uses place-value rounding so its choice is explicit.
How are several divisions evaluated?
From left to right: 100 / 5 / 2 means (100 / 5) / 2.
Why can division fail after rounding?
A small non-zero divisor may round to zero. Use a finer rounding place rather than dividing by zero.