Menstrual Cycle Calculator
Predict your next period, ovulation date, and fertile window.
This is an estimate based on average cycle patterns. Cycles vary and this tool is not a substitute for medical advice.
About Menstrual Cycle Calculator
This free menstrual cycle calculator helps you predict your next period, ovulation date, and fertile window from your last period date and average cycle length. Cycle lengths typically range from 21 to 35 days, with ovulation occurring approximately 14 days before the next period. The fertile window spans roughly five days before ovulation through one day after. Use this tool to plan ahead and better understand your body's natural rhythms — all calculations happen entirely in your browser and nothing is sent to any server.
Built and maintained by Meet Shah · Last updated
What this tool is used for
- Estimating the next period from a cycle length.
- Getting an approximate fertile window from an average cycle length.
- Tracking cycle regularity over several months.
- Planning travel or an event around an expected date.
- Getting dates to discuss at an appointment.
Frequently Asked Questions
- How is the next period predicted?
- By adding your typical cycle length to the first day of the last one. That assumes a regular cycle, which many people do not have — cycle length varies by a few days for most and much more around puberty, breastfeeding and perimenopause.
- How is the fertile window estimated?
- Ovulation is placed about 14 days before the *next* period, because the luteal phase is the more consistent half of the cycle. The window covers the five days before and the day of ovulation, since sperm can survive that long in the reproductive tract.
- Is this reliable as contraception?
- No. Calendar-based methods have a typical-use failure rate around 12–24% per year, because ovulation timing shifts with stress, illness, travel and sleep. Use a real contraceptive method; this is for planning and awareness only.
- What counts as a normal cycle?
- Roughly 21 to 35 days in adults, with bleeding for 2 to 7 days. Outside that range, or a sudden change in your own pattern, is worth raising with a clinician — as is bleeding between periods or a cycle that has become markedly heavier.
- Is my data stored anywhere?
- No. Everything is computed in your browser and nothing is transmitted or saved on a server — which for reproductive-health data is not a small detail, given how often such data has been subpoenaed or sold by app operators.
- How much does cycle length actually vary?
- Considerably, and normal variation is wider than the textbook 28 days suggests — cycles from 21 to 35 days are typical, and the same person's cycles differ month to month. A prediction from an average is a rough guide.
Common errors and gotchas
- Treating the estimate as reliable contraception, which it is not.
- Assuming a 28-day cycle when yours differs, which shifts everything.
- Using an average when cycles are irregular, where the estimate has little meaning.
- Ignoring that ovulation timing varies within the same person month to month.
- Reading the calculation as medical advice.