Menstrual Cycle Calculator
The menstrual cycle runs from the first day of one period to the first day of the next, averaging 28 days. Enter the date of your last period and your average cycle length to see your next period, fertile window, and ovulation for the current cycle.
Next period
—
Fill in the fields above to calculate.
- Fertile window
- —
- Ovulation
- —
- Next period
- —
The four phases of the cycle
| Phase | When it happens | What occurs |
|---|---|---|
| Menstrual | the bleeding days — 8 days or fewer is normal | shedding of the uterine lining (period) |
| Follicular | after the period until ovulation; the variable half | follicle matures, estrogen rises |
| Ovulatory | 36 to 44 hours after the LH surge begins | egg release (ovulation) |
| Luteal | from ovulation to the next period, typically about 14 days | progesterone rises; if no pregnancy, it drops and the period starts |
The asymmetry in that table is the point. The follicular phase is what makes one woman’s cycle 26 days and another’s 34; the luteal phase after ovulation stays comparatively fixed, which is why ovulation is estimated by counting backward from the next period rather than forward from this one.
What counts as a normal cycle
Normal frequency is every 24 to 38 days, both in the Office on Women’s Health guidance and in the FIGO criteria StatPearls cites — not the 21-to-35 range that still circulates widely. Cycles shorter than 24 days or longer than 38 are worth raising with a clinician, as is bleeding that lasts more than 8 days.
Regularity is a separate question from length. StatPearls’ definition: cycles are regular when the difference between your shortest and longest is 7 days or less (ages 26 to 41), or 9 days or less at ages 18 to 25 and 42 to 45. Some variation is the norm — in a pooled analysis of 581 U.S. women, the median cycle was 29 days and half of all cycles fell between 27 and 32.
Worked example
Last period started May 1, 28-day cycle:
- Next period: May 29
- Ovulation: May 15
- Fertile window: May 10 to May 16
Tracking your cycle helps you predict your period, understand mood and energy through the month, and plan (or avoid) pregnancy. To predict only your period, see the next period calculator; to focus on the day the egg is released, the ovulation calculator.
This calculator is informational and does not replace medical evaluation. Persistent changes in your cycle deserve a visit to a gynecologist.
Frequently asked questions
How long does a normal menstrual cycle last?
The Office on Women's Health counts periods as regular when they come every 24 to 38 days, the same range StatPearls cites from FIGO. The typical cycle is 28 days, but that's an average, not a requirement.
How do you calculate cycle length?
Count the days from the first day of one period to the day before the next one starts. That number is your cycle length.
What are the phases of the cycle?
Menstrual, follicular, ovulatory, and luteal. The follicular phase is the one that stretches and shrinks; the luteal phase after ovulation is the steadier half.
My cycle length changed: is that normal?
Small shifts are common. Cycles count as regular when your shortest and longest differ by 7 days or less (ages 26 to 41), or 9 days or less at 18 to 25 and 42 to 45.
Sources
- Your menstrual cycle — Office on Women's Health, U.S. Department of Health and Human Services
- Physiology, Menstrual Cycle — StatPearls (NCBI Bookshelf, U.S. National Library of Medicine)
- Najmabadi S et al. Menstrual bleeding, cycle length, and follicular and luteal phase lengths in women without known subfertility. Paediatr Perinat Epidemiol. 2020 (PMC8495765)
Written by Lucas Silva · Reviewed by Lucas Silva · Updated on August 14, 2026