Article — Months to Years Converter
Months to years: convert with the 12-month year
A month is exactly one-twelfth of a year. To convert months to years, divide by 12; to go back, multiply by 12. That makes 18 months equal to 1.5 years (one year and six months), and 360 months equal to 30 years — the term of a standard US mortgage. The math is clean because the calendar year is defined as 12 named months, regardless of the fact that those months themselves run anywhere from 28 to 31 days each.
The calculator above does both directions. The article below covers when the conversion matters in practice — mortgages and leases, baby ages, CV entries, warranty terms, and the difference between a "calendar month" and a "bank month" in loan contracts.
The math: divide by 12
The whole conversion sits on one number. One year = 12 months. Divide months by 12 to get the decimal year value, or multiply years by 12 to get the month count.
years = months ÷ 12 (exact)months = years × 12 (exact)18 mo ÷ 12 = 1.5 yr (1 yr 6 mo)360 mo ÷ 12 = 30 yr (US mortgage)That works because the calendar definition fixes 12 months per year by convention, the same way the inch is fixed at 25.4 mm or the foot at 12 inches. The variable lengths of individual months — 28 days for February, 31 for July, and so on — do not change the count. A "month" in this conversion is always one-twelfth of a year, not a specific 30-day or 31-day stretch.
Years and months: the mixed format
People rarely say "1.25 years." They say "1 year and 3 months." The mixed format is the natural way to talk about durations that fall between whole years.
To split a month count into years and months: take the floor of months ÷ 12 for the whole years, then take the remainder for the leftover months. 100 months becomes 8 years (96 months) plus 4 months. The decimal version (8.33 years) is mathematically identical but harder to picture.
- 6 months = 0.5 years (half a year)
- 12 months = 1 year exactly
- 18 months = 1.5 years (1 yr 6 mo)
- 24 months = 2 years exactly
- 30 months = 2.5 years (2 yr 6 mo)
- 36 months = 3 years exactly
- 48 months = 4 years exactly
- 100 months = 8.33 years (8 yr 4 mo)
- 120 months = 10 years exactly
- 360 months = 30 years exactly
Why the year has 12 months
Babylonian astronomy. Around 2000 BC, Babylonian priests tracked the lunar cycle, which lasts about 29.5 days. Twelve lunar cycles cover roughly 354 days — close to a solar year but 11 days short. To stay synchronised with the seasons, Babylonians inserted an extra month every two or three years, a practice called intercalation.
The Athenian astronomer Meton, working around 432 BC, refined the system by noticing that 235 lunar months fit almost exactly into 19 solar years (accurate to about two hours over the cycle). This "Metonic cycle" let calendar-makers insert seven extra months across each 19-year period with very high precision. Julius Caesar simplified the whole structure in 46 BC by switching to a pure solar year of 12 fixed months with adjusted lengths, which is essentially the calendar we still use 2000 years later.
September means "seventh month" (Latin septem), but it is the ninth. October, November, and December are the eighth-named, ninth-named, and tenth-named months by their roots, but they sit tenth, eleventh, and twelfth on the calendar. The reason: the original Roman calendar attributed to Romulus had only 10 months and started in March. King Numa Pompilius added January and February around 713 BC. When the Romans later shifted the start of the year to January, the older month names kept their positions and lost their numerical meaning.
Loans, leases, and warranties
Almost every consumer financial product states its term in months, even when people think about it in years. Auto loans run 36, 48, 60, 72, or 84 months. Standard mortgages are 180 (15-year) or 360 (30-year). Manufacturer warranties typically run 12, 24, or 36 months. Apartment leases in the US are often "12-month" or "24-month" agreements.
The reason: months map cleanly to payment cycles. A 360-month mortgage has exactly 360 monthly payments. A 24-month car lease has 24 monthly payments. Stating the term in months makes the payment schedule obvious. Stating it in years (30 years) requires translation back to months for any payment calculation.
- 36 months = 3 years (standard US auto loan, extended electronics warranty)
- 60 months = 5 years (max common auto loan, certificate of deposit)
- 72 months = 6 years (extended auto loan, sometimes seen on larger vehicles)
- 84 months = 7 years (long auto loan, increasingly common)
- 180 months = 15 years (short fixed-rate US mortgage)
- 240 months = 20 years (common Polish and EU mortgage)
- 300 months = 25 years (UK mortgage standard)
- 360 months = 30 years (US mortgage standard since the 1950s)
The EU consumer-goods directive (2019/771) sets the minimum legal warranty at 24 months for new goods sold to consumers across all member states. That is why so many manufacturer warranties on consumer electronics in Europe land at exactly two years.
Why pediatricians count in months
Pediatricians give a baby's age in months until 24 to 36 months, then switch to years. The reason is developmental: in the first 24 months of life, the difference between a 12-month-old and an 18-month-old is enormous. One is just starting to stand and say first words; the other is walking, running, and using short phrases.
Calling both "one-year-olds" loses critical information. Calling them "12 months" and "18 months" preserves it. The American Academy of Pediatrics and the CDC publish developmental milestone checklists at 2, 4, 6, 9, 12, 15, 18, 24, and 30 months — months are the unit pediatric practice uses, because the developmental clock moves that fast.
If a doctor asks how old your baby is, give the age in months until at least 24 months. "Eighteen months" is more useful clinically than "a year and a half" because the milestone tables are indexed by month, not by half-year. After age 2, the year-based answer becomes standard again.
Calendar month vs. bank month
The standard calendar month is 28 to 31 days. The "bank month" used in many loan and bond contracts is exactly 30 days, with a "bank year" of 360 days. This 30/360 convention dates to the pre-calculator era when financial math needed to be fast and clean. Every month has 30 days, every year has 12 months of 30 days each, and interest calculations come out in whole-cent amounts more often than under the actual calendar.
The catch: a 30/360 year (360 days) is 1.4% shorter than a real 365-day year. So when interest is quoted on a 30/360 basis but actually accrues over a real calendar year, the lender collects 1.4% more interest than the headline rate suggests. The Consumer Financial Protection Bureau requires US mortgage lenders to disclose the actual annual percentage rate (APR), which corrects for this difference.
Read the contract. Many corporate loans, bond agreements, and credit-card terms use the 30/360 convention, treating every month as exactly 30 days for interest calculations. If your loan agreement says "1 month = 30 days," that's the 30/360 rule. Personal mortgages and consumer credit usually use actual calendar months (28-31 days) but the APR disclosure normalizes the figure either way.
CVs, jobs, and "months of experience"
Job applications, recruiting platforms, and HR systems usually state experience in years, but actual tenure runs in months. Someone who started in March and left in November has 9 months of experience — call it "9 months" on the CV. Pretending it is "1 year" inflates the figure and is technically misleading.
Most professional recruiters and HR systems round month-level tenure to one decimal in years. 18 months becomes "1.5 years." 30 months becomes "2.5 years." 8 months stays "8 months" because rounding to "less than 1 year" loses useful information. The conversion is simple — divide by 12 — but the convention is to keep months when the figure is under a year and switch to years (with a decimal) above that.
LinkedIn calculates tenure automatically by reading the start and end dates of each role. Internal HR systems do the same. The figure you see in years-of-experience filters is always derived from month-level data on the back end. Saying "5 years 3 months" and "5.25 years" mean exactly the same thing to the algorithm.
Common conversion mistakes
Thinking 18 months is 1 year 8 months. The most common mistake. 18 ÷ 12 = 1.5, which is 1 year 6 months — not 1 year 8 months. The system runs base-12, not base-10. The decimal 0.5 represents 6 months, not 8.
Confusing 6 months with half a year of days. Six months is exactly half a year by month count (0.5 years), but in days it depends on which six months. January through June: 181 days. July through December: 184 days. The half-year ratio is exact only when the year is taken as a 12-month abstraction.
Treating "12 months" and "1 year" as different in contracts. Some contracts use both phrases interchangeably and others mean specific things by each. A "12-month lease" usually starts and ends on calendar dates 12 months apart (e.g., January 15 to next January 14), regardless of how many days that totals. A "1-year lease" usually means the same. But a "365-day lease" is literally 365 days, which in a leap year ends a day earlier than "1 year." Read the wording carefully.
Rounding partial years wrong on CVs. 11 months of experience is not "1 year." Round down, not up. The honest figure is "11 months." Recruiters notice when stated experience implies a longer tenure than the dates support.