- Decimal years:
years = months / 12. - Calendar breakdown: Converts whole months into calendar years and remaining months with
years = floor(|months| / 12),months = |months| mod 12, preserving sign. - Fractional months: Estimated as days using
30.436875average days per month. - Date math: When a start date is provided, whole months roll over using true calendar rules; fractional months become ~days added/subtracted.
- Rounding: Choose nearest, up, down, or none and set decimal places.
| Months | Years (decimal) | Years + Months |
|---|---|---|
| 1 | 0.0833 | 0 years 1 months |
| 2 | 0.1667 | 0 years 2 months |
| 3 | 0.2500 | 0 years 3 months |
| 4 | 0.3333 | 0 years 4 months |
| 5 | 0.4167 | 0 years 5 months |
| 6 | 0.5000 | 0 years 6 months |
| 7 | 0.5833 | 0 years 7 months |
| 8 | 0.6667 | 0 years 8 months |
| 9 | 0.7500 | 0 years 9 months |
| 10 | 0.8333 | 0 years 10 months |
| 11 | 0.9167 | 0 years 11 months |
| 12 | 1.0000 | 1 years 0 months |
| 13 | 1.0833 | 1 years 1 months |
| 14 | 1.1667 | 1 years 2 months |
| 15 | 1.2500 | 1 years 3 months |
| 16 | 1.3333 | 1 years 4 months |
| 17 | 1.4167 | 1 years 5 months |
| 18 | 1.5000 | 1 years 6 months |
| 19 | 1.5833 | 1 years 7 months |
| 20 | 1.6667 | 1 years 8 months |
| 21 | 1.7500 | 1 years 9 months |
| 22 | 1.8333 | 1 years 10 months |
| 23 | 1.9167 | 1 years 11 months |
| 24 | 2.0000 | 2 years 0 months |
FAQs
1) What is the difference between decimal years and calendar years?
Decimal years express months as a fraction of a year using months ÷ 12. Calendar years show whole years and remaining months for clearer scheduling and planning.
2) How are fractional months handled?
Fractional months are converted to an approximate number of days using the astronomical average of 365.2425 days per year divided by 12. This is useful when a start date is given.
3) Why do my results differ from a 30-day assumption?
Some fields assume 30 or 30.4 days per month. This tool uses 365.2425 ÷ 12 ≈ 30.4369 days for better long‑term accuracy across leap years.
4) Can I use negative months?
Yes. Negative values move backward in time for date math. The sign is preserved in the summarized calendar breakdown.
5) What rounding option should I choose?
Use nearest for general reporting. Round up for conservative planning buffers and round down for minimum guaranteed durations. You can also switch off rounding.
6) Does this account for leap years?
Yes for whole months when using the optional start date. Fractional months are approximated as days using an average year length that already accounts for leap cycles.
7) How should I cite the formulas?
Use years = months ÷ 12 for decimal conversion and integer division and remainder for the calendar breakdown. Note the fractional month day estimate uses 365.2425 ÷ 12.