Period Calculator
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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.
Frequently Asked Questions
How do I calculate my next period date?
Add your average cycle length to the first day of your last period. A period that started July 1 with a 28-day cycle predicts the next one on July 29. The calculator repeats the addition for the two periods after that and marks the expected end of each.
Why is my period late if the calculator said today?
The prediction assumes your next cycle matches your average, and normal cycles genuinely vary by a few days — stress, illness, travel, and sleep changes all nudge ovulation, which moves the period after it. If you’re sexually active and more than a few days late, a home pregnancy test is the reliable first step; tests work best from the first missed day onward.
What’s a normal cycle length?
Anything from about 21 to 35 days is common in adults (a bit wider in teens), and the same person can vary cycle to cycle. The 28-day figure is an average, not a target — enter your own average for meaningful predictions.
Can I get pregnant right after my period?
With a short cycle, yes. In a 21-day cycle the fertile window spans roughly days 2–7 — overlapping the tail end of a period — and sperm survive up to 5 days. The “period = safe” rule of thumb fails exactly where cycles are short or irregular.
How accurate are period calculators?
They’re calendar arithmetic on your average cycle, so they’re as regular as you are: reliable within a day or two for steady cycles, rough guidance for variable ones. They can’t detect pregnancy, ovulation, or health changes — they only project your own past pattern forward.
Does this work for irregular periods?
Partially. Use your last three cycles’ average and read results as a window of several days. If cycles swing widely (more than 7–9 days between shortest and longest), calendar prediction loses meaning — symptom-based tracking or a provider conversation is more useful.
Sources & References
- [1]Your menstrual cycle — Office on Women's Health, U.S. Department of Health & Human Services
- [2]Fertility in the menstrual cycle — NHS (UK National Health Service)
- [3]Menstrual health fact sheet — World Health Organization (WHO)
Methodology. This calculator uses formulas and health categories recognized by the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC). It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 2026
Disclaimer. This tool provides estimates for general informational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider before making decisions about your health.