# When is my next period?

Predicts the next three periods from the first day of the last period and the usual cycle length, with the likely ovulation day and fertile window.

- Page: https://www.acalculator.org/health/period-calculator
- JSON spec: https://www.acalculator.org/health/period-calculator.json
- Version: 2f006bcb2d06

## Default answer

Example with the default inputs (First day of your last period September 15, 2026, Cycle length (days) 28, Period length (days) 5, Today’s date September 29, 2026) on the example date Tuesday, September 29, 2026: With a 28-day cycle, your next period should start on October 13, 2026.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| lmp | First day of your last period | The first day of bleeding of your most recent period. |
| cycle | Cycle length (days) | Days from the first day of one period to the first day of the next; 28 on average. |
| days | Period length (days) | How many days your bleeding usually lasts. |
| on | Today’s date | The date to count from: the next period is the first predicted period on or after it. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| next | Next period starts | The first day of the next period: the first of last period + k × cycle length (k = 1, 2, …) on or after today. |
| nextEnd | Next period ends | The last day of the next period: its first day + period length − 1. |
| fertileStart | Fertile window starts | Five days before the estimated ovulation day. |
| ovulation | Estimated ovulation | The likely ovulation day and end of the fertile window: 14 days before the next period. |
| second | Period after that | The first day of the period after next: next period + cycle length. |
| third | Third period | The first day of the third predicted period: next period + 2 × cycle length. |

## Method

Next period = the first of last period + k × cycle length (k = 1, 2, 3, …) on or after today; ovulation = next period − 14 days; fertile window = ovulation − 5 days to ovulation; later periods add the cycle length again.

## Assumptions

- Every cycle has the same length as the one you enter. Real cycles vary from month to month.
- If your last period was more than one cycle ago, the dates continue the same rhythm, so the next period shown is never before today.
- Ovulation happens 14 days before the next period (the luteal phase), the average ACOG gives; in real cycles it varies.
- The fertile window is the 6 days that end on the ovulation day (Wilcox and others, 1995).
- The dates are estimates for information. They are not a reliable way to prevent pregnancy and are not medical advice.

## Worked examples

1. lmp = 2026-09-01, cycle = 28, days = 5 gives next = 2026-09-29, nextEnd = 2026-10-03, fertileStart = 2026-09-10, ovulation = 2026-09-15, second = 2026-10-27, third = 2026-11-24. Source: hand calculation in content.mdx: ACOG (28-day cycle, ovulation about 14 days before the next period); Wilcox and others 1995 (6-day fertile window ending on ovulation).
2. lmp = 2026-12-20, cycle = 32, days = 4 gives next = 2027-01-21, nextEnd = 2027-01-24, ovulation = 2027-01-07, fertileStart = 2027-01-02. Source: hand calculation in content.mdx: a 32-day cycle across the new year.
3. lmp = 2028-02-10, cycle = 21, days = 7 gives next = 2028-03-02, nextEnd = 2028-03-08, ovulation = 2028-02-17, fertileStart = 2028-02-12. Source: hand calculation in content.mdx: a 21-day cycle through February 29, 2028 (a leap year).
4. lmp = 2026-06-01, cycle = 28, days = 5, on = 2026-09-28 gives next = 2026-10-19, nextEnd = 2026-10-23, fertileStart = 2026-09-30, ovulation = 2026-10-05, second = 2026-11-16, third = 2026-12-14. Source: hand calculation in content.mdx: 119 days since June 1; 119 ÷ 28 = 4.25, so k = 5 and June 1 + 140 days = October 19.
5. lmp = 2026-08-31, cycle = 28, days = 5, on = 2026-09-28 gives next = 2026-09-28, second = 2026-10-26. Source: hand calculation in content.mdx: 28 days since August 31, so k = 1 and the next period starts today.

## FAQ

### How do I calculate when my next period is due?

Add your cycle length to the first day of your last period. With a 28-day cycle and a last period that started on September 1, the next one should start 28 days later, on September 29. Add the cycle length again for each later period.

### What is a normal cycle length?

The average is about 28 days, but few cycles are exactly 28. In a study of more than 600,000 cycles, the average was 29.3 days (Bull and others, 2019). ACOG advises teens to see a doctor if periods come more often than every 21 days or less often than every 45 days, or last more than 7 days.

### How do I count my cycle length?

Count from the first day of bleeding of one period up to, but not including, the first day of the next. If your cycles vary, use the average of the last few.

### When do I ovulate?

In an average 28-day cycle, ovulation happens about 14 days before the next period starts (ACOG). The calculator counts back 14 days from the predicted period. ACOG notes that the time from ovulation to the next period is the most consistent part of the cycle.

### What is the fertile window?

The days when sex can lead to pregnancy. Wilcox and others (1995) found that nearly all pregnancies came from sex in the 6 days ending on the day of ovulation. ACOG gives the window as 5 days before ovulation to 1 day after.

### Can I use this calculator for birth control?

No. It assumes every cycle is the same length and that ovulation is always 14 days before the period, and neither is true for everyone. Wilcox and others (2000) found that the fertile window is often earlier or later than expected, even in regular cycles.

## Sources

- American College of Obstetricians and Gynecologists, Fertility Awareness-Based Methods of Family Planning (FAQ024, last reviewed February 2025). https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning
- American College of Obstetricians and Gynecologists, Heavy and Abnormal Periods (TFAQ007, last reviewed April 2026). https://www.acog.org/womens-health/faqs/heavy-and-abnormal-periods
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine 1995;333:1517–1521. doi:10.1056/nejm199512073332301
- Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ 2000;321:1259–1262. doi:10.1136/bmj.321.7271.1259
- Bull JR, Rowland SP, Scherwitzl EB, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine 2019;2:83. doi:10.1038/s41746-019-0152-7