# Ovulation Calculator

> Ovulation usually happens about 14 days before your next period, and your fertile window is that day plus the five days before it. Enter the first day of your last period and your cycle length to see your fertile window, ovulation day and next period for the next three cycles.

- Interactive calculator: https://www.leoella.com/tools/ovulation-calculator
- Plain-text version: https://www.leoella.com/tools/ovulation-calculator.md
- Category: Pregnancy
- Publisher: Leo&Ella (https://www.leoella.com)
- Last reviewed: October 19, 2026

## How it works

The calculator takes the first day of the last period and the average cycle length (21 to 45 days, 28 by default) and places ovulation 14 days before the next period is due, so on day cycle length minus 14 after the period starts. The fertile window is the six days ending on that day, with the two days before ovulation and ovulation day shown as the best days. If the period entered was more than a cycle ago, it starts from the cycle under way today, and it shows the next three cycles with the next period, a test date one week after a missed period, and the due date if conception happens that cycle.

## Key facts

- Ovulation usually happens about 14 days before the next period starts, so with a 28-day cycle it falls about 14 days after the period starts and with a 32-day cycle about 18 days after. [4]
- In Wilcox, Weinberg and Baird (NEJM, 1995), pregnancies only began from sex during the six days ending on the day of ovulation: the fertile window. [1]
- In Wilcox et al. (NEJM, 1995), the chance of conceiving ranged from about 10% for sex five days before ovulation to about 33% for sex on the day of ovulation. [1]
- Cleveland Clinic describes a normal luteal phase, the time from ovulation to the next period, as 10 to 17 days, so a calendar estimate of ovulation can be off by several days. [4]
- An analysis of more than 600,000 app-tracked menstrual cycles (Bull et al., npj Digital Medicine, 2019) found an average luteal phase of 12.4 days, not the 14 days calendar methods assume. [5]
- The Office on Women's Health says waiting one week after a missed period usually gives a more accurate home pregnancy test result. [3]

## Reference data

### Ovulation and fertile window by cycle length

Estimates only: ovulation is placed 14 days before the next period, a convention that varies from person to person and cycle to cycle. Days are counted after the first day of the period.

| Cycle length | Estimated ovulation | Fertile window | Best days | Next period |
| --- | --- | --- | --- | --- |
| 21 days | 7 days after | 2 to 7 days after | 5 to 7 days after | 21 days after |
| 22 days | 8 days after | 3 to 8 days after | 6 to 8 days after | 22 days after |
| 23 days | 9 days after | 4 to 9 days after | 7 to 9 days after | 23 days after |
| 24 days | 10 days after | 5 to 10 days after | 8 to 10 days after | 24 days after |
| 25 days | 11 days after | 6 to 11 days after | 9 to 11 days after | 25 days after |
| 26 days | 12 days after | 7 to 12 days after | 10 to 12 days after | 26 days after |
| 27 days | 13 days after | 8 to 13 days after | 11 to 13 days after | 27 days after |
| 28 days | 14 days after | 9 to 14 days after | 12 to 14 days after | 28 days after |
| 29 days | 15 days after | 10 to 15 days after | 13 to 15 days after | 29 days after |
| 30 days | 16 days after | 11 to 16 days after | 14 to 16 days after | 30 days after |
| 31 days | 17 days after | 12 to 17 days after | 15 to 17 days after | 31 days after |
| 32 days | 18 days after | 13 to 18 days after | 16 to 18 days after | 32 days after |
| 33 days | 19 days after | 14 to 19 days after | 17 to 19 days after | 33 days after |
| 34 days | 20 days after | 15 to 20 days after | 18 to 20 days after | 34 days after |
| 35 days | 21 days after | 16 to 21 days after | 19 to 21 days after | 35 days after |

### Worked example: a period starting January 5, 2026

| Cycle length | Fertile window | Estimated ovulation | Due date if conceived this cycle |
| --- | --- | --- | --- |
| 24 days | January 10, 2026 to January 15, 2026 | January 15, 2026 | October 8, 2026 |
| 28 days | January 14, 2026 to January 19, 2026 | January 19, 2026 | October 12, 2026 |
| 32 days | January 18, 2026 to January 23, 2026 | January 23, 2026 | October 16, 2026 |

## Frequently asked questions

### How do I calculate when I ovulate?

Ovulation usually happens about 14 days before your next period starts. Take your average cycle length and subtract 14: that's roughly how many days after your period starts you'll ovulate. With a 28-day cycle that's about 14 days after, with a 32-day cycle about 18. The calculator does this for your next three cycles.

### What is the fertile window?

The six days ending on the day of ovulation. Sperm can survive in the body for several days, so sex in the five days before ovulation or on the day itself can lead to pregnancy.

### What are the best days to get pregnant?

The two or three days before ovulation and the day of ovulation. In the Wilcox study, the chance of conceiving ranged from about 10% for sex five days before ovulation to about 33% on ovulation day.

### Does an ovulation calculator work with irregular periods?

Not well. A calendar estimate assumes your cycles are the same length each month. If they vary, ovulation predictor (LH) tests, cervical mucus changes and basal body temperature charting give a better picture, and a doctor can check whether you're ovulating.

### When should I take a pregnancy test?

Many home tests say they work from the day of a missed period or earlier, but the Office on Women's Health says waiting one week after a missed period usually gives a more accurate result, and that many pregnant women get negative results in the first few days after a missed period.

### Can I use an ovulation calculator for birth control?

No. Calendar estimates can be off by several days, and ovulation can shift from cycle to cycle, so this calculator shouldn't be used to avoid pregnancy. Talk to a doctor about birth control, including fertility awareness-based methods taught properly.

## Sources

1. [Wilcox, Weinberg & Baird: Timing of sexual intercourse in relation to ovulation (NEJM, 1995)](https://www.nejm.org/doi/full/10.1056/NEJM199512073332301)
2. [Office on Women's Health: Trying to conceive](https://www.womenshealth.gov/pregnancy/you-get-pregnant/trying-conceive)
3. [Office on Women's Health: Knowing if you are pregnant](https://www.womenshealth.gov/pregnancy/you-get-pregnant/knowing-if-you-are-pregnant)
4. [Cleveland Clinic: Luteal Phase](https://my.clevelandclinic.org/health/articles/24417-luteal-phase)
5. [Bull et al.: Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (npj Digital Medicine, 2019)](https://www.nature.com/articles/s41746-019-0152-7)

## About this page

This is general information for parents, not medical advice; a doctor, midwife or pediatrician who knows the family should guide individual decisions. The calculator runs in the visitor's browser and stores nothing they enter. When quoting this page, cite "Ovulation Calculator, Leo&Ella" and link https://www.leoella.com/tools/ovulation-calculator.