按末次月经和周期长度预测下次月经、排卵日、易孕期。
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 | Oct 24, 2026 | Day 1 |
| Fertile window starts | Nov 2, 2026 | Day 9 |
| Ovulation (peak fertility) | Nov 7, 2026 | Day 14 |
| Fertile window ends | Nov 8, 2026 | — |
| Next period (predicted) | Nov 21, 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.
平均周期 28 天（± 7 天正常）：
| 阶段 | 28 天周期 | 21 天周期 | 35 天周期 |
|---|---|---|---|
| 月经期 | 第 1-5 天 | 第 1-5 天 | 第 1-5 天 |
| 卵泡期 | 第 6-13 天 | 第 6-6 天 | 第 6-20 天 |
| 排卵日 | 第 14 天 | 第 7 天 | 第 21 天 |
| 黄体期 | 第 15-28 天 | 第 8-21 天 | 第 22-35 天 |
周期变化 ± 7 天正常。黄体期固定 14 天，卵泡期长度决定周期总长。压力 / 旅行 / 疾病会推迟排卵。周期 > 35 或 < 21 天建议就医。
Cross-checked against authoritative formulas (IRS, NIST, ACSM) and updated for 2025/2026 tax years and rates.
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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.
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