Predict your next period, ovulation day, and fertile window from your last period start date and average cycle length.
These cycle events assume a 28-day cycle beginning with an LMP of September 4, 2026.
| Event | Date | Days away (from 2026-09-04) |
|---|---|---|
| Next period | 2026-10-02 | 28 days |
| Ovulation | 2026-09-17 | 13 days |
| Fertile window start | 2026-09-15 | 11 days |
| Fertile window end | 2026-09-20 | 16 days |
| Next period after next | 2026-10-30 | 56 days |
| PMS expected start | 2026-09-26 | 22 days |
Assumes 28-day cycle. Individual cycles vary 21–35 days. ACOG.
A period calculator predicts your next menstrual period, ovulation date, and fertile window based on the rhythm method. Given the first day of your last period and your average cycle length, it computes when to expect your next period (last period start + cycle length), your ovulation day (next period - luteal phase, typically 14 days), and the fertile window (ovulation ± 5 days). It also shows which cycle phase you are in today (menstrual, follicular, ovulation, or luteal).
Average is 28; ranges 21–35 are typical
Population average: 14 days
Average is 4–5 days
Day 15
14 days
Follicular — egg matures. Low-to-moderate fertility leading up to ovulation.
| Event | Date | Cycle days |
|---|---|---|
| Last period start | Sep 2, 2026 | Day 1 |
| Fertile window starts | Sep 11, 2026 | Day 9 |
| Ovulation (peak fertility) | Sep 16, 2026 | Day 14 |
| Fertile window ends | Sep 17, 2026 | — |
| Next period (predicted) | Sep 30, 2026 | Day 29 |
Predictions are estimates based on the rhythm method. Actual ovulation can vary by ±2-3 days from cycle to cycle. For pregnancy planning, combine with ovulation test kits or basal body temperature tracking. For birth control, use a proven contraceptive method — this tool is not a contraceptive.
Cross-checked against authoritative formulas (IRS, NIST, ACSM) and updated for 2025/2026 tax years and rates.
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Estimates only. Calculator results are for educational and planning purposes — not professional advice. For binding decisions on taxes, investments, health, or legal matters, consult a qualified professional.
FAQ
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A period calculator is a free menstrual cycle tracker that predicts your next period, ovulation date, and fertile window from your last period start date and average cycle length. Our period calculator also identifies which cycle phase you are in today (menstrual, follicular, ovulation, or luteal). The period calculator uses the rhythm method: next period = last period + cycle length; ovulation = next period − luteal phase (typically 14 days); fertile window = ovulation ± 5 days.
How it works
Our period calculator estimates your next period date, ovulation day, and fertile window from your last period and average cycle length.
Pick the date your most recent period started (the first day of bleeding, not spotting). The period calculator uses this as the reference point.
Type the number of days from the start of one period to the start of the next. Average is 28; ranges 21–35 are normal. If you do not know, 28 is a reasonable default.
Most women have a 12–14 day luteal phase and a 3–7 day period. Leave at 14/5 unless you track precisely. The period calculator uses these to refine the ovulation and fertile window predictions.
The period calculator shows today's cycle day, current phase (menstrual / follicular / ovulation / luteal), days until your next period, and a calendar of upcoming events: fertile window start, ovulation, fertile window end, and predicted next period.