Exact units, and units with no fixed length
Seconds through weeks convert by whole, unchanging numbers: an hour is always 60 minutes, a day is always 24 hours, and a week is always 7 days. The BIPM SI Brochure (the document that defines the modern metric system) lists minute, hour, and day among the non-SI units "accepted for use with the SI," fixed at exactly those ratios. There is nothing to average or estimate here.
Months and years are different: a calendar month runs anywhere from 28 to 31 days, and a calendar year is 365 days most of the time but 366 in a leap year, so "1 month" or "1 year" is not a single number of seconds the way "1 week" is. This converter uses the mean Gregorian month and year, not one calendar year picked arbitrarily. The Gregorian calendar's own leap-year rule (a year divisible by 4 is a leap year, except century years, which are leap years only if divisible by 400) produces exactly 97 leap years in every 400-year span, per the US Naval Observatory's leap-year explainer, so 400 years always contain exactly 146,097 days. Dividing that cycle evenly gives a mean year of 365.2425 days and a mean month (one-twelfth of that) of 30.436875 days. Both are exact arithmetic consequences of the calendar's own rule, not estimates or rounded guesses.
The seconds-per-unit behind this converter
1 minute = 60 s, 1 hour = 3,600 s, 1 day = 86,400 s, and 1 week = 604,800 s, all exact. The mean month is 2,629,746 s (30.436875 days) and the mean year is 31,556,952 s (365.2425 days), both derived from the 146,097-day, 400-year Gregorian cycle. Because months and years don't divide evenly into whole days, converting "1 year" to days gives 365.2425 rather than a round 365. That 0.2425-day remainder is exactly what leap days exist to correct for, spread out as one extra day roughly every four years, not applied all at once.
Where an exact conversion isn't possible (there is no whole number of weeks in a month, for instance), this tool reports the mean-calendar figure to six significant figures rather than rounding to a convenient but less accurate number like "4 weeks."
One asterisk on "exact": leap seconds
Every ratio this converter uses, 60 seconds to a minute, 24 hours to a day, and the rest, is exact by definition. What isn't perfectly steady is the second itself against how fast Earth actually turns. Since 1972, the world's civil clock (UTC) has absorbed 27 one-second nudges, called leap seconds, to stay within 0.9 seconds of the planet's slightly irregular rotation; the most recent was added on December 31, 2016. In November 2022, the General Conference on Weights and Measures, the same BIPM body behind the SI second used throughout this converter, voted to retire the leap second no later than 2035, letting civil time and astronomical time drift apart by more than a second for the first time since the practice began. None of the unit ratios above change because of this. It only means a small number of days since 1972 ran 86,401 seconds long instead of 86,400.
Worked example: a project timeline
A project is quoted as taking "8 months." Converting: 8 × 4.348125 = 34.785 weeks, or roughly 34 weeks and 5-6 days depending on which actual calendar months the project spans. Because real calendar months vary in length, a project that starts on a long-month boundary (like starting in a 31-day month) will run slightly longer in calendar days than one measured purely by the mean-month figure. Treat it as a rough estimate, not a substitute for counting the actual calendar dates involved.
Worked example: someone's age in seconds
A person turning 30 today has been alive for roughly 30 × 31,556,952 = 946,708,560 seconds, or 30 × 365.2425 = 10,957.275 days. The extra 0.2425 days per year is the accumulated effect of the roughly 7 or 8 leap days they lived through. This is why "how many seconds old am I" calculators that just multiply by a plain 365 will drift off by about 7.3 days worth of seconds (around 628,560 seconds) by age 30, versus the mean-year figure used here.