Month Calculator
Count months between dates or add and subtract months
What the Month Calculator Does
Many commitments are measured in months rather than days: a 12-month subscription, a 36-month car loan, a 6-month probation period, a 9-month pregnancy, a lease that runs until the end of next quarter. The Month Calculator works with these periods. Enter two dates to see how many months lie between them, or enter a date and a number of months to find the date that many months earlier or later.
Where Month Calculations Come Up
- Loans and finance. Terms are often stated in months, and knowing how many remain helps with planning repayments.
- Subscriptions and memberships. Find the renewal date or see how long you have been subscribed.
- Employment. Probation periods, notice periods and length of service are commonly expressed in months.
- Leases and contracts. Calculate end dates, break clauses and rent review points.
- Health and family. Track a baby's age in months, or a treatment or training schedule.
- Project planning. Convert a roadmap into months to see how much time is realistic.
How to Use the Month Calculator
- Choose whether you want to find the gap between two dates or to add or subtract months.
- Enter the start date, and the end date or the number of months as appropriate.
- Run the calculation.
- Read the result, which may show whole months with any remaining days.
- Check the answer against the wording of your contract or plan if it is important.
For example, adding 6 months to 15 January 2026 gives 15 July 2026, and 36 months from the same date is 15 January 2029.
The Problem With Months of Different Lengths
Unlike days or weeks, months are not a fixed length. They range from 28 to 31 days, which makes month arithmetic more subtle than it looks. The classic trap is the end of the month. What is one month after 31 January? February does not have a 31st. Different systems give different answers: many calculators settle on the last day of the shorter month, so 28 February 2026, while others simply overflow into March. Neither is wrong, but the difference matters if a payment or deadline hangs on it.
The same effect appears when measuring gaps. The period from 31 January to 28 February is a bit less than a calendar month in the strict sense, but it spans the whole of February. A good approach is to treat month results as a convenient summary and, where the exact day count matters, to check it with a days calculation too.
Whole Months and Leftover Days
When two dates do not fall on the same day of the month, the result usually has two parts: a number of complete months and some extra days. From 10 March to 25 August, you have five complete months plus 15 days. If you need a single figure, you can express this as a decimal by dividing the extra days by roughly 30, but remember that this is an approximation. For billing or legal purposes, rely on the actual terms rather than a rounded result.
Tips for Reliable Results
- Use the same convention throughout a project, whether you count from the exact day or from the start of the month.
- Be careful with dates near the end of the month, particularly the 29th, 30th and 31st.
- Write dates in the unambiguous style set out in ISO 8601, such as 2026-10-07, to avoid confusion between day-first and month-first formats.
- For contracts, confirm how the document itself defines a month. Some count calendar months, others use a fixed 30 days.
How It Compares With Other Methods
Spreadsheet functions and programming libraries can add months to dates, and developers often use tools such as the JavaScript Date object, though they must handle month-end behaviour carefully. For an occasional question, a calculator is quicker than building a formula. The Month Calculator on WebTOOLkt is free, needs no sign-up and sits next to related tools for days, hours and age, so you can look at the same period from different angles.
Related WebTOOLkt Tools
- Days Calculator to find the exact number of days in a period.
- Hours Calculator to measure shorter periods in hours and minutes.
- Age Calculator to see age in years, months and days.
- Loan Calculator to turn a term in months into payment figures.
- Time Converter to convert between units of time.
- Percentage Calculator to show how much of a term has elapsed.
Useful External Resources
Frequently Asked Questions
What is one month after 31 January?
There is no 31 February, so many calculators return the last day of February, which is 28 February in 2026. Some systems roll over to early March instead.
Is a month always 30 days?
No. Months have between 28 and 31 days, so the number of days in a month-based period varies.
How many months are in 3 years?
Thirty-six. Multiply the number of years by twelve.
