How this instrument works
BIPM's SI Brochure lists the day among units accepted for use alongside SI, fixed at 86,400 seconds. It stops there. A month has no such entry, because no fixed number of days defines one — the Gregorian calendar hands out 28, 29, 30 or 31 depending on which month and whether February falls in a leap year. This converter has to pick something, so it uses the mean Julian month: a 365.25-day year divided evenly by twelve, giving 30.4375 days, the constant behind the 0.0328542094456 multiplier above.
Twelve months is itself an old settlement. Rome's earliest calendar ran ten months and 304 days, leaving roughly two winter months uncounted; tradition credits king Numa Pompilius, around 713 BCE, with adding January and February to make twelve. Julius Caesar's reform of 46 BCE then fixed each month's length and introduced a leap day every fourth year, producing that 365.25-day average — the same figure this page's month length descends from, unchanged in count since, even after Pope Gregory XIII trimmed the year's decimal tail in 1582.
A different 30-day month lives in finance, and it is worth not confusing with this one. Bond markets, municipal debt and many fixed-rate mortgages price interest under the 30/360 day-count convention, which treats every month as exactly 30 days and every year as 360 — a simplifying fiction chosen for tidy arithmetic, not calendar accuracy. This converter's 30.4375 days serves a different purpose, estimating real elapsed time, and the two constants should never be swapped into each other's formulas.
- Enter your span into the Days (day) field; 60 is preloaded as a starting point.
- Read Months (mo) beneath it, recalculated with every keystroke to eleven decimal places.
- Fractional days are accepted, stepping in thousandths, so 45.5 days returns roughly 1.495 months.
- Reversing direction, multiply your Months (mo) figure by 30.4375 to recover a day count.
- Negative entries are refused, since a span of time has length but no direction below zero.
Worked example — a 60-day notice-to-vacate clause
A residential lease requires 60 days' written notice before either party can end the tenancy. Enter 60 into Days (day) and Months (mo) reads 1.97125256674 — just under two months, because 60 days is 0.875 of a day shy of two mean 30.4375-day months.
That shortfall is exactly why careful lease drafting says '60 days' rather than 'two months': a tenant who waits until they believe two months have passed may hand in notice the better part of a day too late for the clause to bind. Multiply 1.97125256674 back by 30.4375 and 60 returns to eleven decimal places, confirming nothing besides that final rounding was lost along the way.
Questions
Why isn't there one fixed number of days per month?
Because the Gregorian calendar deliberately gives months uneven lengths — 31 days in January, March, May, July, August, October and December; 30 in April, June, September and November; 28 or 29 in February. No conversion factor can capture that variation exactly, so tools that need one, this page included, fall back on an average: the mean length a month would have if every year's days were spread evenly across its twelve months.
Where does 30.4375 come from?
It is one twelfth of the Julian year, 365.25 days, the average year length produced by adding a leap day every fourth year: 365.25 ÷ 12 = 30.4375 exactly. This differs slightly from a Gregorian mean month based on 365.2425 days, roughly 30.436875, because the Gregorian calendar drops three of every four centuries' leap days that the plain Julian rule would otherwise add. The gap between the two is under two minutes per month, invisible at everyday scales.
Why do contracts specify '60 days' instead of 'two months'?
Precision. Two calendar months measured from an arbitrary date can run anywhere from 59 days (two Februaries in a non-leap year, unlikely but possible in edge cases) to 62 days (July plus August), while '60 days' means exactly that many 24-hour periods regardless of which months it crosses. Notice periods, cooling-off windows and insurance waiting periods lean on day counts for precisely this reason — they remove any argument about how long a named month actually was.
Is this the same constant finance uses for 30-day months?
No, and mixing them up produces wrong answers. Bond pricing and many mortgage calculations use the 30/360 day-count convention, which simply declares every month 30 days and every year 360 for the sake of clean interest arithmetic, regardless of any real calendar. This page's 30.4375-day month instead approximates true elapsed time using the actual Julian year average. Use 30/360 for interest schedules that specify it; use this page for genuine day-to-month estimates.
How do I convert months back into days?
Multiply by 30.4375. Three months becomes roughly 91.3125 days; six months becomes about 182.625 days. Because that constant is itself an average, expect real calendar spans to land a day or two either side of the result — a genuine six months from 1 March to 1 September, for instance, covers 184 days in a common year, 1.375 days more than this estimate.
How many days does a specific calendar month actually have?
Anywhere from 28 to 31: January, March, May, July, August, October and December run 31 days; April, June, September and November run 30; February runs 28 in a common year and 29 in a leap year, one that divides by 4 but, if it also divides by 100, must divide by 400 too. For a genuine month, count between the two actual dates rather than relying on this page's average, which is built for spans that do not name specific calendar months.