Find the approximate number of days in a given number of months.
Multiply the number of months by 30, since real months vary from 28 to 31 days.
Days (estimate): Months × 30
1 mo = 30 d
1 d = 0.033333 mo
1 mo = 30 d
2 mo = 60 d
3 mo = 90 d
4 mo = 120 d
6 mo = 180 d
9 mo = 270 d
12 mo = 360 d
18 mo = 540 d
24 mo = 720 d
36 mo = 1080 d
60 mo = 1800 d
This calculator gives a quick estimate of how many days lie in a given number of months. It multiplies the months by 30, a round figure that sits close to the true average month length. The result is labelled as a 30-day estimate because real months run from 28 to 31 days.
The estimate is useful whenever you need a rough size for a period and do not yet know the exact dates: an 11-month rent agreement, a 6-month probation, an 18-month project or a 3-month course. For anything where a single day matters, such as interest calculations, visa validity or legal deadlines, you should count the real calendar days instead. Knowing how far the estimate can be off helps you decide which one you need.
1. Write down the number of months, including any fraction.
2. Multiply by 30 to get the estimate: Days ≈ Months × 30.
3. For a better long-run average, multiply by 30.44 instead, since a year of 365.25 days divided by 12 gives 30.4375 days per month.
4. For an exact answer, list the actual months involved and add their lengths: 31 for January, March, May, July, August, October and December; 30 for April, June, September and November; 28 or 29 for February.
5. Compare the estimate with the exact count to see how large the gap is for your period.
Months come from the old lunar cycle of about 29.5 days, but the Gregorian calendar stretched and squeezed them so that twelve months fill a solar year of 365 or 366 days. The result is an uneven pattern of 31, 30 and 28 or 29 days. Because of this, no single multiplier is exactly right for every month. Any months-to-days conversion that does not know the actual dates is an estimate by nature.
The true average month is about 30.44 days, so using 30 undercounts by roughly 0.44 days per month. Over 12 months the estimate gives 360 days against a real 365 or 366, a gap of five or six days. Over short spans the error depends on which months you hit: February alone is two days short of 30, while a run of July and August is two days over. For two or three months the estimate can be off by a day or two in either direction.
Some fields use a 30-day month on purpose, because it keeps sums simple. The 30/360 day-count convention used for certain bonds treats every month as 30 days and every year as 360. Some payroll systems work out a day's pay as monthly salary divided by 30 regardless of the month. When you use this calculator, you are applying the same simplification, so it matches those conventions but not calendar-based ones.
Kavya's startup project is planned to run for 18 months, and she wants a rough day count for a budgeting sheet before the start date is fixed.
Answer: Days (30-day estimate) 540
Using the 30-day estimate for bank interest, loan tenure or legal deadlines, where the exact number of calendar days decides the amount or the date.
Treating 12 months as 360 days in a yearly calculation and then being surprised by a five-day gap.
Assuming every month has 4 weeks, which gives 28 days and undercounts even more than the 30-day rule.
Mixing the estimate with exact counts in the same spreadsheet, which makes totals inconsistent.
Getting a quick sense of how long an 11-month rental agreement or a 6-month probation really is.
Rough project and budget planning before dates are fixed.
Estimating daily costs when a subscription or fee is quoted per month.
Matching payroll or accounting rules that already use a 30-day month.
Why 30 days and not the real length of each month?
Without knowing exactly which months are involved, 30 days is a commonly used estimate. For an exact answer, use the Date Difference calculator with real calendar dates instead.