About the Period Calculator
Knowing roughly when your period will arrive helps with travel plans, event scheduling, spotting a late period early, and simply understanding your own pattern. A menstrual cycle is counted from the first day of one period to the first day of the next; the often-quoted 28 days is only an average — cycles between about 21 and 35 days are common in adults, and your own number is what matters here.
Enter the first day of your last period, your average cycle length, and how long your period usually lasts. You’ll get your next three expected period dates, the expected end of each period, and — because the same math powers both — your estimated ovulation date and fertile window. If you don’t know your averages, track two or three cycles and count the days between first days.
Trying to conceive and want the fertile-window detail? Use the dedicated Ovulation Calculator.
How Period Prediction Works
The calendar method behind every period tracker is two additions and one subtraction:
Next period = last period start + average cycle length Period end = period start + period length − 1 Ovulation ≈ next period − 14 days Fertile days ≈ the 5 days before ovulation + ovulation day
Worked example: last period started July 1 with a 28-day cycle and 5-day period — the next period is expected Wednesday, July 29 (running through August 2), then August 26 and September 23. Estimated ovulation lands July 15, with a fertile window of July 10–15. Every date shifts as one block if your cycle runs early or late.
Cycle Length Cheat Sheet
Where the key days fall for common cycle lengths, counted in days after the first day of your last period (day 1 = the day your period started):
| Cycle length | Next period starts | Estimated ovulation | Fertile window (approx.) |
|---|---|---|---|
| 21 days | day 22 | ~ day 7 | days 2–7 |
| 24 days | day 25 | ~ day 10 | days 5–10 |
| 26 days | day 27 | ~ day 12 | days 7–12 |
| 28 days | day 29 | ~ day 14 | days 9–14 |
| 30 days | day 31 | ~ day 16 | days 11–16 |
| 32 days | day 33 | ~ day 18 | days 13–18 |
| 35 days | day 36 | ~ day 21 | days 16–21 |
Notice that short cycles put the fertile window right after — sometimes overlapping — the period itself, which is why “can’t get pregnant on your period” is not a safe assumption for short cycles.
Irregular Cycles & When to Talk to a Provider
Calendar prediction is only as steady as your cycle. If your cycle length varies by several days month to month, use the average of your last three cycles and treat every date here as a window rather than a promise. Stress, illness, travel, significant weight change, intense training, breastfeeding, and perimenopause can all shift or skip cycles — and hormonal contraception controls bleeding timing itself, so this calculator doesn’t apply while you’re on it.
Worth a conversation with a clinician: periods that stop for three months or more (and pregnancy is ruled out), cycles consistently shorter than 21 or longer than 35 days, bleeding between periods, soaking through protection hourly, or periods painful enough to disrupt life. A missed period with a possibility of pregnancy is a home-test moment — they’re most reliable from the first day of the missed period.