Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
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What does this calculator estimate?
Look one digit past the target place: 5 or more rounds up, 4 or less stays. 3.147 to the nearest tenth = 3.1 (next digit 4); to the nearest hundredth = 3.15 (next digit 7). Negative places go left: 1,247 to the nearest ten = 1,250.
- 3.147 → 3.1 (tenth), 3.15 (hundredth), 3 (ones).
- 1,247 → 1,250 (nearest ten) — negative places round left of the decimal.
- Round half up: 2.5 rounds to 3, the standard school convention.
The one-digit-past rule, step by step
3.147 to the nearest tenth: the tenths digit is 1, the next digit is 4, so it stays — 3.1. Only the single next digit matters; everything further right is irrelevant.
- Pick the target place (tenths, say)
- Look at the digit immediately to its right
- 5–9: add one to the target digit, drop the rest
- 0–4: keep the target digit, drop the rest
Rounding left of the decimal point
The same rule runs in reverse for whole-number places. 1,247 to the nearest ten: the tens digit is 4, the next digit is 7, so round the 4 up to 5 — 1,250. To the nearest hundred: the hundreds digit is 2, next digit 4 stays — 1,200. Estimation, crowd counts, and headlines all live in this direction.
Money rounding: where the half-cent goes
Currency rounds to the cent: $19.995 becomes $20.00 under round half up. Sales-tax calculations hit this constantly — a 7% tax on $9.99 is $0.6993, which rounds to $0.70. Different jurisdictions mandate different rounding rules for tax, which is why two checkout systems can disagree by a penny on the same basket.
How this calculator works
Formula
look at the digit one place past the target: 5 or more rounds up, 4 or less stays; negative places round left of the decimal point
Worked example
3.147 to the nearest tenth is 3.1 (next digit 4 is less than 5); to the nearest hundredth it is 3.15 (next digit 7 rounds up). 1,247 to the nearest ten is 1,250.
Sources
Sources support the method, terminology, or planning context. Editable assumptions remain your responsibility.
Assumptions to verify
- Round half up (school convention): .5 always rounds away from zero
- Negative places round left of the decimal: -1 = tens, -2 = hundreds
- Floating-point display may hide that some decimals are stored approximately
Frequently asked questions
How do I round to the nearest ten or hundred?
Use a negative place: -1 for tens, -2 for hundreds. 1,247 to the nearest ten is 1,250 (the digit after the tens place is 7, so round up); to the nearest hundred it is 1,200.
What happens exactly at .5?
This tool uses round half up — 2.5 becomes 3, the convention taught in schools and used in most everyday contexts. Statistics and banking sometimes use round half to even (2.5 → 2, 3.5 → 4) to avoid upward bias over many roundings.
Why does my code round 2.675 to 2.67 instead of 2.68?
Floating-point storage: 2.675 is stored as approximately 2.67499999…, so round-half-up sees a 4 and rounds down. Money code avoids this by working in integer cents.
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