How Many Seconds Are in a Day?
A day contains exactly 86,400 seconds. This comes from 24 hours × 60 minutes × 60 seconds. The same number applies to any calendar day — Gregorian, Julian, or solar — since the day length unit is defined as 86,400 SI seconds.
Quick Answer: There are 86,400 seconds in a day.
Formula
Breakdown — Seconds per Time Unit
| Time Unit | Seconds |
|---|---|
| 1 minute | 60 |
| 1 hour | 3,600 |
| 6 hours | 21,600 |
| 12 hours (half day) | 43,200 |
| 18 hours | 64,800 |
| 24 hours (1 day) | 86,400 |
| 1 week (7 days) | 604,800 |
| 1 month (30 days) | 2,592,000 |
| 1 year (365 days) | 31,536,000 |
Why Exactly 86,400?
The number 86,400 traces back to the ancient Babylonian base-60 number system, where the hour was divided into 60 minutes and the minute into 60 seconds. A day was then divided into 24 hours (12 daytime + 12 nighttime). Multiplying yields 24 × 60 × 60 = 86,400.
In modern science, the second is defined as an SI base unit (the duration of 9,192,631,770 periods of radiation from the caesium-133 atom). A “day” in most contexts is then simply 86,400 of these SI seconds — even though the actual rotation of the Earth varies slightly (handled by leap seconds).
For a year, the total is 365 × 86,400 = 31,536,000 seconds, or 31,622,400 seconds in a leap year.
Frequently Asked Questions
How many seconds are in a day?
There are exactly 86,400 seconds in a day (24 hours × 60 minutes × 60 seconds).
How many seconds in an hour?
There are 3,600 seconds in an hour (60 minutes × 60 seconds per minute).
How many milliseconds are in a day?
There are 86,400,000 milliseconds in a day (86,400 seconds × 1,000 milliseconds per second).
How many seconds in a week?
There are 604,800 seconds in a week (7 days × 86,400 seconds per day).
Related Time References
- How many minutes in a day? — 1,440
- How many weeks in a year? — 52 weeks + 1 day
- How many hours in a year? — 8,760
- How many minutes in a year? — 525,600
- How many seconds in a year? — 31,536,000
- Time Unit Converter — convert any time value
- All Time & Date Calculators
All values use standard 24-hour days (86,400 SI seconds). For astronomical precision, a sidereal day is ~86,164 seconds and a solar year is ~31,556,925 seconds — but for civil time and most practical calculations, 86,400 and 31,536,000 are exact.
