How Rounding Works
Rounding replaces a number with a simpler nearby value. You round when exact precision doesn’t matter — estimating a bill, reporting a measurement, or cleaning up a long decimal — and the key skill is choosing how much precision to keep.
Decimal Places vs. Significant Figures
Decimal places count digits after the point: 3.14159 to 2 decimals is 3.14. Significant figures count meaningful digits from the first non-zero digit: 0.004567 to 3 sig figs is 0.00457. Scientists prefer sig figs because they reflect real measurement precision.
Common Rounding Modes
- Half up: 2.5 → 3 (the school standard, used by this calculator).
- Floor / ceiling: always down or always up — used in pricing, pagination, and capacity planning.
- Nearest 10/100/1000: for big-picture estimates — 4,782 to the nearest hundred is 4,800.
Crunching the numbers before rounding? Try our four-function calculator or the scientific calculator.