# Period calculator: next period dates

> Next period date and the following 5 predicted periods, with each cycle's fertile window and ovulation day, from your last period and average cycle length.

Interactive version: https://www.calcopenly.com/health/period-calculator
Subject: Health and fitness calculators

The next period is expected one average cycle length after the first day of the last one, and each later period another cycle length on. A period of 5 days that started on 5 January 2026, in a 28-day cycle, points to periods starting on 2 February, 2 March, 30 March and so on. Ovulation in each cycle is placed 14 days before the period that ends it, with a fertile window from 5 days before ovulation to 1 day after.

People use it to plan around the next few periods and to see which phase of the cycle they are in today. The calendar layout is built for periods; the ovulation calculator on this site covers fertile days in more detail for anyone trying to conceive.

Predictions assume every cycle is the same length. In 612,613 app-recorded cycles (Bull and colleagues, 2019) the standard deviation of one person's cycle lengths averaged 2.6 days, so each later prediction is less certain than the one before.

## Inputs

- **First day of last period**
- **Average cycle length**: From the first day of one period to the first day of the next.
- **Period length**
- **Periods to predict**
- **Luteal phase length**: Days from ovulation to the next period; 14 is the textbook value.

## Results

- Next period starts — main result
- Next period ends
- Days until the next period
- Days late
- Cycle day today
- Cycle phase today
- Next estimated ovulation
- Next fertile window, from
- Next fertile window, to

## Formula

$$
\text{period}_k = \text{LMP} + k \times \text{cycle},\qquad \text{ovulation}_k = \text{period}_k - \text{luteal},\qquad \text{fertile}_k = [\text{ovulation}_k - 5,\ \text{ovulation}_k + 1]
$$

## Worked examples

### Period on 5 Jan 2026, 28-day cycle, 5 days

- First day of last period: 2026-01-05
- Average cycle length: 28 days
- Period length: 5 days
- Periods to predict: 6
- **Next period starts: 2026-02-02**
- **Next period ends: 2026-02-06**
- **Days until the next period: 18**
- **Days late: not applicable**
- **Cycle day today: 11**
- **Cycle phase today: Follicular phase, fertile window**
- **Next estimated ovulation: 2026-01-19**
- **Next fertile window, from: 2026-01-14**
- **Next fertile window, to: 2026-01-20**
- Checked against: Python 3.8 datetime with today 2026-01-15: LMP + 28 d; period end + 4 d; ovulation 2026-02-02 − 14 d; window −5/+1 d (Wilcox 1995)

### 30-day cycle, this cycle's ovulation already past

- First day of last period: 2025-12-20
- Average cycle length: 30 days
- Period length: 5 days
- Periods to predict: 6
- **Next period starts: 2026-01-19**
- **Days until the next period: 4**
- **Cycle day today: 27**
- **Cycle phase today: Luteal phase**
- **Next estimated ovulation: 2026-02-04**
- **Next fertile window, from: 2026-01-30**
- **Next fertile window, to: 2026-02-05**
- Checked against: Python 3.8 datetime: 2025-12-20 + 30 d; ovulation 2026-01-05 has passed, so the next is 2026-02-18 − 14 d = 2026-02-04

### Late period

- First day of last period: 2025-12-10
- Average cycle length: 28 days
- Period length: 5 days
- Periods to predict: 6
- **Next period starts: 2026-01-07**
- **Days late: 8**
- **Days until the next period: not applicable**
- **Cycle day today: 37**
- **Cycle phase today: Period overdue**
- Checked against: Python 3.8 datetime: 2025-12-10 + 28 d = 2026-01-07, 8 days before 2026-01-15

### Edge: period started today

- First day of last period: 2026-01-15
- Average cycle length: 28 days
- Period length: 5 days
- Periods to predict: 6
- **Next period starts: 2026-02-12**
- **Days until the next period: 28**
- **Cycle day today: 1**
- **Cycle phase today: Period**
- **Next estimated ovulation: 2026-01-29**
- Checked against: Python 3.8 datetime: 2026-01-15 + 28 d; ovulation 2026-02-12 − 14 d

### Short 21-day cycle: fertile days start right after the period

- First day of last period: 2026-01-10
- Average cycle length: 21 days
- Period length: 5 days
- Periods to predict: 6
- **Next period starts: 2026-01-31**
- **Days until the next period: 16**
- **Cycle day today: 6**
- **Cycle phase today: Follicular phase, fertile window**
- **Next estimated ovulation: 2026-01-17**
- **Next fertile window, from: 2026-01-12**
- Checked against: Python 3.8 datetime: ovulation 2026-01-31 − 14 d = 2026-01-17 (cycle day 8); window from 2026-01-12 (day 3)

## Questions

### How do I calculate my next period date?

Add your average cycle length to the first day of your last period. Cycle length is counted from the first day of one period to the first day of the next, so a period that started on 5 January with a 28-day cycle points to 2 February. For an average, add up the lengths of your last few cycles and divide by the number of cycles.

### What is a normal menstrual cycle length?

21 to 35 days in adults, according to ACOG, with bleeding that usually lasts up to 7 days. Only a minority of cycles are exactly 28 days: in 612,613 cycles logged in a fertility app, 13 % were 28 days long and the mean was 29.3 days. Cycles regularly shorter than 21 days or longer than 35 are worth mentioning to a clinician.

### How many days late can a period be?

A few days either way is within the usual variation. Across 612,613 cycles from 124,648 app users, Bull and colleagues (2019) found the standard deviation of each person's cycle lengths averaged 2.6 days, rising to 3.1 days at age 45. A period 2 or 3 days after the predicted date is therefore common. If a period is late and pregnancy is possible, a pregnancy test is the usual next step.

### When am I most fertile in my cycle?

In the 6 days ending on ovulation day, which in a 28-day cycle is around cycle day 15. Wilcox and colleagues (1995) found pregnancies only followed intercourse in that window. The ovulation calculator on this site gives those dates for the next few cycles and lets you change the luteal phase length.

### How accurate is the period calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “Period on 5 Jan 2026, 28-day cycle, 5 days” is checked against Python 3.8 datetime with today 2026-01-15: LMP + 28 d; period end + 4 d; ovulation 2026-02-02 − 14 d; window −5/+1 d (Wilcox 1995).

### Where does the method come from?

ACOG — Abnormal uterine bleeding (FAQ): normal cycle 21–35 days, bleeding up to 7 days; Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med 2019;2:83; Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21.

## Sources

- [ACOG — Abnormal uterine bleeding (FAQ): normal cycle 21–35 days, bleeding up to 7 days](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding)
- [Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med 2019;2:83](https://doi.org/10.1038/s41746-019-0152-7)
- [Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21](https://doi.org/10.1056/NEJM199512073332301)

_Note: medical information, not professional advice._