# Ovulation and fertile window calculator

> Ovulation date, fertile window and next period for the coming cycles, from the first day of your last period and your cycle length.

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

Ovulation is estimated by counting back a 14-day luteal phase from the next expected period, which is the last period's start plus the usual cycle length. The fertile window runs from 5 days before ovulation to 1 day after: Wilcox and colleagues (1995) found conception only followed intercourse in the six days ending on ovulation day, and one day is added as a margin. The same dates are projected over the coming cycles.

People use it to time intercourse when trying to conceive or to anticipate the next period. With a period starting on 5 January 2026 and a 28-day cycle, ovulation falls around 19 January, the fertile window runs from 14 to 20 January, and the next period is due on 2 February.

Calendar dates assume a regular cycle and a fixed luteal phase. In 612,613 app-recorded cycles (Bull and colleagues, 2019) the luteal phase averaged 12.4 days and varied widely, and the authors recommend tracking signs such as basal body temperature rather than cycle length alone. A calendar estimate is not reliable contraception on its own.

## Inputs

- **First day of last period**
- **Usual cycle length**: Days from the first day of one period to the first day of the next.
- **Cycles to show**
- **Period length**
- **Luteal phase length**: Days from ovulation to the next period; 14 is the textbook value.

## Results

- Estimated ovulation (this cycle) — main result
- Fertile window from
- Fertile window to
- Next period expected
- Next ovulation from today
- Cycle day today

## Formula

$$
\text{ovulation} = \text{LMP} + \text{cycle} - \text{luteal},\qquad \text{fertile window} = [\text{ovulation} - 5,\ \text{ovulation} + 1]
$$

## Worked examples

### 28-day cycle, period started 5 Jan 2026

- First day of last period: 2026-01-05
- Usual cycle length: 28 days
- Cycles to show: 3
- **Estimated ovulation (this cycle): 2026-01-19**
- **Fertile window from: 2026-01-14**
- **Fertile window to: 2026-01-20**
- **Next period expected: 2026-02-02**
- **Cycle day today: 11**
- Checked against: Python 3.8 datetime: next = LMP + 28 d, ovulation = next − 14 d, window −5/+1 d (Wilcox 1995)

### 35-day cycle

- First day of last period: 2026-01-01
- Usual cycle length: 35 days
- Cycles to show: 3
- **Estimated ovulation (this cycle): 2026-01-22**
- **Fertile window from: 2026-01-17**
- **Fertile window to: 2026-01-23**
- **Next period expected: 2026-02-05**
- Checked against: Python 3.8 datetime: 2026-01-01 + 35 − 14 = 2026-01-22

### Short 21-day cycle

- First day of last period: 2026-01-10
- Usual cycle length: 21 days
- Cycles to show: 3
- **Estimated ovulation (this cycle): 2026-01-17**
- **Fertile window from: 2026-01-12**
- **Next period expected: 2026-01-31**
- Checked against: Python 3.8 datetime: 2026-01-10 + 21 − 14 = 2026-01-17

### Across the new year, ovulation already passed

- First day of last period: 2025-12-20
- Usual cycle length: 30 days
- Cycles to show: 3
- **Estimated ovulation (this cycle): 2026-01-05**
- **Next ovulation from today: 2026-02-04**
- **Cycle day today: 27**
- Checked against: Python 3.8 datetime: cycle 1 ovulation 2025-12-20 + 16 d; cycle 2 ovulation 2026-01-19 + 16 d; today 2026-01-15 is day 27

### 12-day luteal phase

- First day of last period: 2026-01-05
- Usual cycle length: 28 days
- Cycles to show: 3
- Luteal phase length: 12 days
- **Estimated ovulation (this cycle): 2026-01-21**
- **Fertile window to: 2026-01-22**
- Checked against: Python 3.8 datetime: 2026-02-02 − 12 d

## Questions

### When do you ovulate in a 28-day cycle?

Around cycle day 15, counting the first day of the period as day 1, if the luteal phase is the textbook 14 days: ovulation falls 14 days before the next period. In a 35-day cycle that moves to about day 22, and in a 21-day cycle to about day 8. Real cycles vary: across 612,613 recorded cycles the average luteal phase was 12.4 days, which puts ovulation later than the textbook count.

### How long is the fertile window?

About six days: the five days before ovulation and the day of ovulation. In Wilcox and colleagues' 1995 study of 221 women and 625 cycles, conception followed intercourse only in that window, and the chance of conception rose from 10 % five days before ovulation to 33 % on ovulation day itself. This calculator adds the day after ovulation as a margin, so the window shown is seven days.

### Can you get pregnant right after your period?

Yes, especially with short cycles. Intercourse up to five days before ovulation can lead to conception (Wilcox 1995), so a 21-day cycle with a 14-day luteal phase puts ovulation around day 8 and the start of the fertile window around day 3, when bleeding may not have finished. With a 28-day cycle the window starts around day 10.

### How accurate is the ovulation and fertile window 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, “28-day cycle, period started 5 Jan 2026” is checked against Python 3.8 datetime: next = LMP + 28 d, ovulation = next − 14 d, window −5/+1 d (Wilcox 1995).

### Where does the method come from?

Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21; ACOG — Fertility awareness-based methods of family planning (FAQ).

## Sources

- [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)
- [ACOG — Fertility awareness-based methods of family planning (FAQ)](https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning)

_Note: medical information, not professional advice._