What the rounding calculator does
The rounding calculator rounds any number to the precision you choose: decimal places, the nearest whole, the nearest 10 / 100 / 1000, or any custom multiple. Pick from six rounding modes — half-up (the standard), half-even (banker's), half-down, ceiling, floor and truncation — and the tool also shows every mode at once so you can compare them side by side.
The six rounding modes
- Half up — the textbook default. 2.5 → 3, 2.49 → 2, 2.51 → 3.
- Half even (banker's) — used by most banks and statistics packages. 2.5 → 2, 3.5 → 4, 2.51 → 3. Avoids bias when many numbers are rounded.
- Half down — the opposite of half-up at the tie: 2.5 → 2, 3.5 → 3.
- Ceiling — always rounds upward (toward positive infinity). 2.1 → 3, −2.9 → −2.
- Floor — always rounds downward (toward negative infinity). 2.9 → 2, −2.1 → −3.
- Truncate — chops the digits, always toward zero. 2.9 → 2, −2.9 → −2.
Banker's rounding, in one sentence
When the next digit is exactly 5 and there is nothing after it, round to the even neighbour. So 2.5 rounds to 2 (because 2 is even), and 3.5 rounds to 4. Over many transactions this keeps the average rounding error near zero, while half-up adds a tiny positive bias because every tie goes the same way.
Rounding to a multiple
"Nearest 10" means rounding 47 to 50, 43 to 40, and 45 to 50 (half-up) or 40 (half-even). The same idea generalises: nearest 0.25 turns 1.13 into 1.25 and 0.06 into 0.0; nearest 0.05 is great for prices. The calculator's "Nearest multiple of…" mode lets you set any positive value.
The float problem
Computers store numbers in binary, and many decimal values (like 0.1) don't have an exact binary representation. So 0.1 + 0.2 evaluates to 0.30000000000000004 in nearly every programming language. Rounding usually hides this, but edge cases (like rounding 1.005 to two decimals) can surprise you because the underlying value is actually 1.00499999…. For currency, the safest approach is to do the maths in integer cents and only divide by 100 at the end.
When not to round
Don't round in the middle of a multi-step calculation if you can avoid it — small errors compound. Carry full precision through every step and only round the final value you display. For the same reason, store and transmit numbers at full precision; rounding is a presentation choice, not a storage choice.
Use with the other tools
For working with parts of 100, the percentage calculator handles the four common percent questions. For statistics with rounded summaries, the average calculator and standard deviation calculator are quick companions.