Age Calculator

How to use this calculator

1. Set the second date for any cut-offSchemes and school admissions ask for age at a specific date, not today. Set it explicitly rather than adjusting in your head afterwards.
2. Use the days total for infantsUnder a year, weeks and days say far more than a fraction of a year.
3. Check the next birthday figureUseful for anything with an age threshold approaching — a policy renewal, a scheme eligibility date, a milestone.
4. Do not divide days by 365It drifts. Over a lifetime the error runs to weeks.

Why exact age is not days divided by 365

Age looks like the simplest calculation anywhere and hides more subtlety than most.

The correct method counts calendar units in order. **Completed years first** — how many birthdays have passed. **Then completed months** since that birthday. **Then the leftover days.**

That is what people mean by age and what forms expect.

The tempting shortcut — total days divided by 365 — gives a different answer, and the gap grows over a lifetime.

The reason is leap years. A year is not 365 days; it averages 365.2425. Roughly one in four has 366, except centuries not divisible by 400 — which is why 1900 was not a leap year and 2000 was.

Over forty years, dividing by 365 overstates age by around ten days. Over eighty, by nearly three weeks. Small, and wrong in a way a calendar count is not.

**Months are the harder part**, because they are not equal and “one month later” has no single definition when the day does not exist in the next month.

What is one month after 31 January? There is no 31 February. The convention used here takes the last valid day — 28 February, or 29 in a leap year. That is a convention rather than arithmetic, and systems differ.

The same question makes **29 February birthdays** genuinely ambiguous. Someone born on 29 February 2000 has a real birthday every fourth year. In other years, do they age on 28 February or 1 March?

Schemes answer that differently, and where an age threshold matters the scheme's own rule governs. This calculator uses the calendar date, so the year completes on 1 March.

One more point. **Age is counted in completed units, never rounded.** Someone eleven months old is zero years old. Every threshold that matters — school entry, licensing, scheme eligibility — runs on completed years, and rounding up is never the convention.

Worked example: born 15 June 1990

A date of birth of 15 June 1990, measured to 5 August 2026.

**Completed years:** the 2026 birthday on 15 June has passed, so 36 years.

**Completed months since then:** 15 June to 15 July is one month; 15 July to 15 August has not completed. So one month.

**Remaining days:** 15 July to 5 August is 21 days.

**Exact age: 36 years, 1 month and 21 days.**

In total days that is 13,199 — or 1,885 weeks and 4 days.

Now the shortcut for comparison. 13,199 ÷ 365 = 36.16 years. But 13,199 ÷ 365.2425 = 36.14. The gap is about nine days, which is exactly the leap days accumulated over that period.

Nine days is nothing in a lifetime and everything in a form with a cut-off.

The next birthday falls on 15 June 2027 — 314 days away.

Now change the measuring date. At 1 September 2026 the age is 36 years, 2 months and 17 days. At 14 June 2026, the day before the birthday, it is **35** years, 11 months and 30 days.

That last one is the whole point of counting completed units. One day earlier gives a different whole number of years, and that is exactly what an age threshold turns on.

For a child the total matters more than the fraction. A baby born on 1 March 2026 is, on 5 August 2026, 5 months and 4 days old — or 157 days, or 22 weeks and 3 days.

And a leap-year case. Someone born on 29 February 2000, measured to 5 August 2026, is 26 years, 5 months and 7 days. Their last actual 29 February birthday was 2024; the next is 2028.

The same birth date, different measuring dates

The first two rows are one day apart and a year apart, which is why age is counted in completed units rather than rounded. The last row shows the month convention: 31 January plus one month has no valid date, so 28 February counts as 28 days rather than a completed month.

Where an exact age actually matters

Most of the time an approximate age is fine. The cases where it is not share a feature: a threshold with a date attached.

**School admission.** Primary 1 registration in Singapore works on age at a specific reference date rather than at registration, so the question is never “how old is the child now” but “how old will the child be on that date”. The second field exists for exactly this.

**Scheme and benefit eligibility.** Age bands in government schemes are assessed at a stated date. Getting that date right matters more than the arithmetic does.

**Employment and CPF.** Contribution rates step down through age bands, and the band applies from a specific point rather than from a birthday party. Which band someone falls into on a given payroll date is a real question with money attached.

**Insurance.** Many policies price on age next birthday rather than age now, so a fortnight can change a premium. Worth checking which basis a quote uses.

**Legal thresholds.** These run on completed years without exception. A person is not twenty-one until the twenty-first birthday, and no rounding applies.

**Infants.** Age in weeks or days is the working unit, and for a premature baby corrected age — measured from the due date rather than the birth — is often what clinicians use. That is a distinction this calculator does not make.

A few practical notes. If a form asks for age “as at” a date, put that date in rather than adjusting mentally — mental adjustment across a month boundary is where errors creep in.

If a scheme has its own rule for 29 February birthdays, use that rule. They differ, and the scheme's is the one that counts.

And if you are reading an age off a document, check the date format first. A date written 06/07/1990 is 6 July in most of the region and 7 June in the United States, and that single ambiguity has produced more real errors than every leap-year edge case combined. The calculator uses YYYY-MM-DD internally for exactly that reason.

Assumptions and limitations

• Counts completed units — no rounding up to the next year
• Month arithmetic uses the last valid day where the date does not exist
• 29 February birthdays are treated by calendar date in non-leap years
• Schemes with their own age rules override this convention
• Does not calculate corrected age for premature infants
• Does not account for time of day or time zones
• Ambiguous date formats are the commonest real-world source of error

Questions people ask about this in Singapore