Period Calculator
Predict upcoming period start dates from the first day of your last period and your usual cycle length.
Cycle timing can vary naturally. This is a calendar estimate only and should not be used to diagnose pregnancy, fertility, abnormal bleeding or a menstrual disorder.
How Does a Period Calculator Work?
A period calculator predicts your upcoming cycles using two inputs: the first day of your last period and your usual cycle length. It adds your cycle length to that start date to estimate your next period, then repeats the math to project several cycles ahead. It can also estimate your fertile window — the days you are most likely to ovulate — since ovulation typically happens about 14 days before your next period starts.
Understanding Your Menstrual Cycle
A cycle is counted from the first day of one period to the first day of the next. Cycles between 21 and 35 days are considered normal for adults, with 28 days the most common average. Here is what happens across a typical 28-day cycle:
| Phase | Typical Days | What Happens |
|---|---|---|
| Menstrual | 1–5 | Bleeding as the uterine lining sheds. |
| Follicular | 1–13 | Estrogen rises; an egg matures in the ovary. |
| Ovulation | ≈ 14 | The egg is released; fertility peaks. |
| Luteal | 15–28 | Progesterone rises; if no pregnancy, the cycle restarts. |
Want to pinpoint ovulation more precisely? Try our ovulation calculator, or check your safe days if you are tracking fertility.
How Accurate Are Period Predictions?
Calendar predictions are estimates, not guarantees. Stress, illness, travel, intense training, and hormonal shifts can move ovulation — and therefore your next period — by several days. Predictions get noticeably better once you have tracked 3 or more cycles, because the calculator learns your average rather than assuming 28 days. Treat the dates as a planning guide, and see a doctor if your cycles are consistently shorter than 21 days, longer than 35 days, or suddenly change.
Related Calculators
- Ovulation Calculator — estimate your most fertile days
- Safe Days Calculator — understand fertile vs. infertile windows
- Pregnancy Week Calculator — track pregnancy by week
- Women & Kids Health Calculators — browse the full collection
Frequently Asked Questions (FAQ)
Disclaimer: This calculator provides calendar estimates for educational purposes only and is not a substitute for professional medical advice. Consult your doctor about cycle concerns, fertility, or contraception.