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Ovulation Calculator: Estimate Your Fertile Window
Enter the first day of your last period, your usual cycle length and luteal phase length to estimate the day of ovulation, the six-day fertile window and the dates for the next three cycles.
How ovulation is estimated
Ovulation is usually placed a fixed number of days before the next period, because the luteal phase (ovulation to period) is steadier than the first half of the cycle. The calculator uses ovulation day = cycle length − luteal phase. With a 28-day cycle and a 14-day luteal phase that is cycle day 14. With a 35-day cycle it is day 21.
The fertile window covers the five days before ovulation and the day of ovulation itself. Sperm may survive in the body for several days, while the egg survives only about a day.
Choosing the cycle and luteal inputs
Use an average from several recent cycles, not a single one. Count from the first day of full flow to the day before the next period begins. If you have measured the luteal phase with temperature charts or test strips, enter that figure; otherwise 14 days is the common default. Luteal phases between 10 and 16 days are accepted here.
- A shorter luteal phase moves the ovulation day later in the cycle.
- Cycles that vary by more than a few days make any calendar estimate rough.
- Stress, illness, travel, breastfeeding and hormonal medicines can all shift ovulation.
Calendar estimates compared with body signs
A calendar method predicts; it does not detect. Urine tests for luteinising hormone, basal body temperature charts and changes in cervical fluid detect what is happening in the current cycle. Many people combine a calendar estimate with one of these signs. If you have been trying for a year, or six months over the age of 35, a clinician can arrange fertility checks.
The conception calculator and pregnancy calculator can be used once a pregnancy is confirmed.
Important limits
Calendar methods have a considerable failure rate when used to avoid pregnancy, because ovulation can come early or late in any cycle. People who need reliable contraception should use a method recommended by a clinician. The tool also does not diagnose conditions such as polycystic ovary syndrome, thyroid disease or early menopause, which can make cycles unpredictable.
Signs that can refine the estimate
Calendar estimates assume a regular cycle. Many people add other signals to refine timing: cervical mucus that becomes clear and stretchy, a positive urine test that detects the luteinising hormone surge, and a small sustained rise in basal body temperature, which confirms that ovulation has already happened rather than predicting it. If your cycles are irregular, or you have been trying to conceive for a year (six months if you are over 35) without success, speak to a clinician.
Worked example
A cycle starts on 1 March and normally lasts 30 days, with a 14-day luteal phase.
Inputs
| First day of last period | 2026-03-01 |
|---|---|
| Average cycle length | 30 days |
| Luteal phase length | 14 days |
Result
Cycle day 16. Fertile window: 12 March 2026 to 17 March 2026.
This is an estimate of timing only. It is not a form of contraception and should not be used to avoid pregnancy.
| Estimated ovulation day | Tuesday 17 March 2026 (cycle day 16) |
|---|---|
| Fertile window | 12 March 2026 to 17 March 2026 |
| Next period expected | 31 March 2026 |
| Earliest reliable pregnancy test | 31 March 2026 |
This cycle and the next two
| Cycle starts | Fertile window | Ovulation | Next period |
|---|---|---|---|
| 1 March 2026 | 12 March 2026 to 17 March 2026 | 17 March 2026 | 31 March 2026 |
| 31 March 2026 | 11 April 2026 to 16 April 2026 | 16 April 2026 | 30 April 2026 |
| 30 April 2026 | 11 May 2026 to 16 May 2026 | 16 May 2026 | 30 May 2026 |
Medical disclaimer: This tool is for education and general planning. It is not medical advice and does not diagnose any condition. If you are pregnant, under 18, have an eating-disorder history or a medical condition, speak to a doctor or registered dietitian before changing your diet or exercise.
Frequently asked questions
How do I calculate my ovulation day?
Subtract your luteal phase, usually 14 days, from your cycle length. In a 28-day cycle that gives day 14, counting the first day of your period as day 1.
How long is the fertile window?
About six days: the five days before ovulation and the day of ovulation. The days just before ovulation carry the highest chance because sperm are already waiting.
Can I use this as birth control?
No. Ovulation can shift in any cycle, and calendar estimates are unreliable for preventing pregnancy.
What if my cycle is irregular?
The estimate will be rough. Ovulation tests or a clinician's advice give more dependable timing than a calendar.
What is the luteal phase?
It is the time between ovulation and the start of the next period. It is typically about 12 to 14 days and is relatively consistent for each person.
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References and sources
- Wilcox AJ, Weinberg CR, Baird DD (1995). Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine 333:1517-1521.
- American College of Obstetricians and Gynecologists: patient education on fertility awareness and the menstrual cycle. Link
- World Health Organization: guidance on fertility awareness and family planning. Link