What this calculator does
Time units are the one measurement system almost everyone already carries around in their head, everyone knows there are 60 seconds in a minute and 24 hours in a day, but the ladder gets less familiar further out, and months and years in particular are not fixed lengths at all.
This calculator uses an average month (2,629,800 seconds, or about 30.44 days) and a Julian year (365.25 days) rather than any specific calendar month or year, since actual months range from 28 to 31 days and calendar years vary with leap years.
The formula
Every unit converts through the second as a base. The smaller units, milliseconds, minutes, hours, days, weeks, are exact multiples of a second. Months and years are not naturally fixed lengths, so this calculator uses standard average values: a year of 365.25 days accounts for the leap-year cycle, and a month is simply that year divided by twelve.
| Term | Meaning |
|---|---|
| Base unit | The second, which every other unit converts through. |
| Average month | 2,629,800 seconds (≈30.44 days): a year divided by twelve, not any specific calendar month. |
| Julian year | 365.25 days: the average calendar year once leap years are accounted for. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The value to convert. |
| From | The unit you are converting from. |
| To | The unit you want the answer in. |
When to use it
Converting a duration between everyday units
Turning a duration in minutes into hours and days, or the reverse, is one of the most common small calculations: this handles it precisely rather than by mental rounding.
Estimating a long timeframe in years or decades
Project timelines, loan terms and long-range planning often need a duration expressed in years even when the underlying figure started in days or months.
Working with very short durations
Milliseconds cover the fast end, reaction times, computing and lab measurements, using the same base-second method as the longer units.
Checking how a "month" figure was actually calculated
Because a calendar month varies from 28 to 31 days, any average-month conversion, like this one, will differ slightly from a specific real month, which matters when precision to the day is required.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
One of each unit, converted to hours
A single unit of time from milliseconds to decades, each converted to hours to compare true scale.
| From | In hours |
|---|---|
| Millisecond | 0.00000028 h |
| Second | 0.00027778 h |
| Minute | 0.01666667 h |
| Day | 24 h |
| Week | 168 h |
| Month (average) | 730.5 h |
| Year (365.25 d) | 8,766 h |
| Decade | 87,660 h |
Converting 90 minutes to other units
A common meeting or session length of 90 minutes expressed across the other units.
Questions
Why is a "month" not a fixed length in this calculator?
Because calendar months range from 28 to 31 days. Using an exact real month would make the conversion depend on which month is meant, so this calculator uses the average, a year divided by twelve, or about 30.44 days, as a standard reference instead.
Why is a year 365.25 days here rather than 365?
To account for leap years. Over four years there are three 365-day years and one 366-day year, averaging to 365.25 days per year: the same figure the Julian calendar was built around.
How many seconds are in a day?
86,400: 24 hours × 60 minutes × 60 seconds. This is one of the few time conversions that is exact rather than an average, since a day, hour, minute and second are all fixed relative to each other.
How do I convert a duration in days to weeks?
Divide by 7. This calculator does it directly, but it is also one of the simplest conversions in the whole ladder since a week is an exact 7-day unit with no averaging involved.
Is a decade always exactly 10 years?
Yes, by definition: a decade here is exactly ten of the Julian years used throughout this calculator, so it inherits the same 365.25-day-per-year averaging rather than tracking any specific real ten-year span.
For elapsed time between two actual calendar dates, see the days between dates calculator. For adding or subtracting a duration from a specific date, use the date arithmetic calculator.