Months to Days Converter

months to days — quick reference

The formula, worked line by line

The arithmetic is a single multiplication and the constant is where all the argument lives. Twelve months make a year, and a Gregorian year averages 365.2425 days, so the mean month is that divided by twelve. The division comes out exactly: 30.436875, with no recurring tail.

That rule is the legal answer to this page, and it is not a conversion. It says a month is defined by a date rather than a duration, which is why a deadline of one month from 31 January falls on the last day of February and why no day count reproduces it.

days = months x 30.436875
365.2425 / 12 = 30.436875 days, exactly
3 x 30.436875 = 91.31 days - real range 89 to 92
12 x 30.436875 = 365.2425 days
under 30/360: every month is 30 days, every year 360

Which constant to use, and when

Use the mean month to compare durations, to annualise a monthly figure, or to size an estimate. Use the real month lengths whenever the result is a date. Use 30/360 only when an instrument or a rulebook specifies it, never as a general-purpose simplification.

The month-end convention

Finance has its own answer to the 31 January problem. Where an end-of-month convention is specified, the payment dates fall on the last calendar day of each month rather than on a numbered date, which keeps a schedule that starts at month end at month end throughout.

How to use the months to days converter

A month is the one calendar unit with no fixed length. Twelve of them make a year, so the average month is 365.2425 divided by twelve, or 30.436875 days exactly. That figure is arithmetically correct and describes no month in the calendar, which is the whole difficulty in one sentence.

Seven months have 31 days, four have 30, and February has 28 or 29. The average sits between the two commonest lengths and matches neither. So converting months to days always produces an answer that is right on average and wrong for any particular month, and the only question is whether that matters for what you are doing.

It matters enormously in finance, and the industry solved it by inventing a month that behaves. Under a 30/360 convention every month counts as thirty days and every year as 360, so interest accrues identically in February and in July. It is not an approximation of the calendar; it is a deliberate replacement for it.

Law took the opposite route. Rather than fixing a month at thirty days, it refuses to convert at all: a period expressed in months ends on the corresponding date in the later month. One month from the fifteenth is the fifteenth, whether that span was 28 days or 31, and the day count is simply not part of the calculation.

The thirty-day month that finance invented

Day-count conventions exist because interest has to accrue predictably. If February paid less than July for the same nominal month, every instrument would need a calendar attached to its price. The fix was to define the month rather than measure it, and the definitions are set out in the rulebooks rather than left to agreement.

One caution about a figure often quoted alongside this. Dividing by 360 rather than 365.2425 produces about 1.46 percent more, but that describes an actual/360 calculation, where real days are counted over a 360-day year. A full 30/360 year is 360 days over 360 and accrues exactly one year of interest.

Why the calendar month is not a lunar month

The word survives from the moon and the unit no longer follows it. A synodic month, new moon to new moon, averages about 29.53 days. The Gregorian calendar month averages 30.44. The gap is nearly a day, which compounds to roughly eleven days a year and is exactly why lunar and solar calendars drift apart.

This is worth knowing mainly because it explains the irregularity rather than excusing it. The month lengths are not the result of a calculation gone wrong; they are the residue of fitting twelve named periods into a solar year that no whole number of lunar cycles divides.

Every months value, worked out

69 common months figures each get their own page, with the answer at full precision, the arithmetic, what rounding costs, and the nearest real-world reference point on the scale.

Questions people ask