Period Calculator
Estimates your next three period start dates, your most probable ovulation day and fertile window based on the first day of your last period, how long bleeding lasts and your average cycle length.
TheCalculators — Period Calculator
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Next Period Expected
December 6, 2026
in 28 days · 28-day cycle, 5-day period
- Cycle length
- 28 days
- Period length
- 5 days
- Next ovulation (est.)
- November 22, 2026
- Fertile window (est.)
- Nov 17, 2026 – Nov 23, 2026
- Cycle 2 start
- Jan 3, 2027
- Cycle 3 start
- Jan 31, 2027
Phase lengths in one cycle (days)
- Menstrual5.00
- Follicular3.00
- Fertile window6.00
- Luteal14.00
Next three predicted cycles
Ovulation is estimated at 14 days before the next period; the fertile window covers the ~5 days sperm survive plus the egg day.
Cycles naturally vary by a few days — stress, travel, illness and contraception can shift dates. Irregular cycles (varying by more than 7–9 days) are hard to predict this way. Estimates for informational purposes only — not medical advice.
Most useful next
Formula
Next period = LMP + cycle length · Ovulation ≈ next period − 14 days · Fertile window = ovulation − 5 to +1 days
- • Count forward one average cycle length from the first day of your last period.
- • Subtract 14 days from the predicted start to estimate ovulation (a 28-day cycle ovulates ~day 14).
- • Sperm survive up to 5 days and the egg ~12–24 hours, so conception is possible from ~5 days before ovulation to ~1 day after.
Pro tips
- • Track 3–6 cycles to find your true average length before trusting long-range predictions.
- • The luteal phase (ovulation → period) is the most stable part of the cycle — that is why it works backwards from 14 days.
- • For pregnancy prevention or achievement, use ovulation tests — calendar estimates are averages, not guarantees.
FAQ
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