A common year holds 31,536,000 seconds — about 31.5 million. A leap year holds 31,622,400, exactly 86,400 more.
Which figure you want depends on the job. For a specific calendar year, use 365 or 366 days. For long-range calculations, the average Gregorian year of 365.2425 days gives 31,556,952 seconds, and astronomy uses the Julian year of exactly 365.25 days — 31,557,600 seconds — because it is the constant baked into the definition of a light-year.
| Type of year | Days | Seconds | Used for |
|---|---|---|---|
| Common year | 365 | 31,536,000 | Most calendar years |
| Leap year | 366 | 31,622,400 | Years divisible by 4 (with exceptions) |
| Average Gregorian year | 365.2425 | 31,556,952 | Long-range date arithmetic |
| Julian year | 365.25 | 31,557,600 | Astronomy, the light-year |
| Tropical year | 365.24219 | 31,556,925 | Equinox to equinox |
| Unit | Common year (365 d) | Leap year (366 d) |
|---|---|---|
| Seconds | 31,536,000 | 31,622,400 |
| Minutes | 525,600 | 527,040 |
| Hours | 8,760 | 8,784 |
| Days | 365 | 366 |
| Weeks | 52 weeks 1 day | 52 weeks 2 days |
| Months | 12 | 12 |
The π × 10⁷ shortcut
There is a well-known approximation among physicists and engineers: a year is about π × 10⁷ seconds.
π × 10⁷ = 31,415,927, against the true 31,536,000 for a common year — low by 0.38%, or about 120,000 seconds (roughly a day and a half). Against the average Gregorian year it is low by 0.45%.
It is a coincidence, not a relationship, but it is a useful one for order-of-magnitude work: if you can remember π you can remember the number of seconds in a year to within half a percent.
The same coincidence gives the "nanocentury" joke — a nanosecond of a century. A century is about 3.156 × 10⁹ seconds, so a nanocentury is about 3.16 seconds, which is π again to within half a percent.
Why there are several different years
A "year" is not one quantity, because the things you might measure do not come out the same length:
- The tropical year — equinox to equinox, which is what governs the seasons — is about 365.24219 days
- The Gregorian calendar approximates it with an average of 365.2425 days, achieved by the leap rule: every 4th year is a leap year, except century years, except again those divisible by 400
- The Julian year is exactly 365.25 days by definition. It is not meant to match the seasons; it is a fixed unit used in astronomy
- The sidereal year, one orbit relative to the fixed stars, is about 365.256 days — slightly longer than the tropical year because the Earth’s axis precesses
The Gregorian approximation is remarkably good: it drifts from the tropical year by only about one day in 3,200 years, which is why the calendar has needed no adjustment since 1582.
Leap seconds, and why they are ending
The second is no longer defined by the Earth. Since 1967 it has been defined by a specific number of oscillations of a caesium-133 atom, which makes atomic time far more stable than the planet’s rotation.
The Earth’s rotation, meanwhile, is irregular and slowly slowing. To stop civil time drifting away from solar time, leap seconds were introduced in 1972 — an extra second inserted at the end of a June or December, 27 times so far.
They cause real problems. A minute containing 61 seconds breaks software that assumes minutes are fixed, and several notable outages have been traced to leap-second handling. In November 2022 the General Conference on Weights and Measures voted to stop using them by 2035, allowing atomic and solar time to diverge instead and dealing with the gap later in some larger unit.
So a calendar year is not always exactly 31,536,000 seconds: a common year containing a leap second has 31,536,001. After 2035, that irregularity is scheduled to disappear.
The light-year connection
The light-year is defined using the Julian year of exactly 31,557,600 seconds, multiplied by the speed of light in a vacuum — itself exactly 299,792,458 metres per second since 1983.
That gives a light-year as exactly 9,460,730,472,580,800 metres, or about 9.46 trillion kilometres. Both inputs are exact definitions, so the light-year is an exact number rather than a measurement.
This is why astronomy uses the Julian year rather than the Gregorian one. A unit of distance cannot depend on the leap-year rules of a particular calendar, so the fixed 365.25-day year is used instead.
Frequently Asked Questions
How many seconds are in a year?
31,536,000 seconds in a common year of 365 days — that is 365 × 86,400. A leap year has 31,622,400 seconds.
How many seconds are in a leap year?
31,622,400 seconds. A leap year has 366 days, and one extra day adds exactly 86,400 seconds.
Is a year really π × 10⁷ seconds?
Almost. π × 10⁷ is 31,415,927 seconds against the true 31,536,000, so the approximation is low by 0.38% — about a day and a half. It is a coincidence, but a handy one for rough calculations.
How many seconds are in a month?
It depends on the month: 2,419,200 for a 28-day February, 2,505,600 for 29 days, 2,592,000 for 30 days, and 2,678,400 for 31. The average month of 30.44 days is about 2,629,746 seconds.
How many minutes and hours are in a year?
525,600 minutes and 8,760 hours in a common year. A leap year has 527,040 minutes and 8,784 hours.
Do leap seconds change the number of seconds in a year?
Yes — a year containing a leap second has one extra, so 31,536,001 instead of 31,536,000. Twenty-seven have been added since 1972, and in 2022 the General Conference on Weights and Measures voted to phase them out by 2035.