Period Calculator
Track your menstrual cycle and predict your next period
Period calculator in 8 quick sections
1) Core idea
Use your last period date, average cycle length, and period length to predict next periods, ovulation, and fertile days.
2) Flow you can trust
- Inputs → last period (day 1), cycle length, period length
- Next periods → add cycle length in days to the last period
- Ovulation → about 14 days before the next period
- Fertile window → ovulation −5 days through ovulation +1 day
3) Sanity checks
- Typical cycles 21–35 days; periods 3–7 days.
- Variability of a few days is normal—even with regular cycles.
- Predictions are estimates, not medical advice or contraception.
4) Shortcuts
- If you don’t track yet, assume 28‑day cycles and adjust later.
- Recompute after 2–3 months of tracking for better accuracy.
5) Pitfalls
- Using spotting instead of day‑1 full flow.
- Assuming ovulation is always cycle day 14—luteal length varies.
6) Micro examples
- Last period Jan 1; 28‑day cycle → next period Jan 29; ovulation Jan 15.
- 30‑day cycle → ovulation ≈ day 16; fertile window days 11–17.
7) Mini‑FAQ
- Irregular cycles? Predictions will be wider; keep tracking.
- Birth control? Hormones can change cycle timing and ovulation.
- Trying to conceive? Also watch LH tests and cervical mucus.
8) Action tip
Log the first day of full flow for 3+ months. Use the averages here, then fine‑tune as your pattern emerges.