How ovulation is estimated
A menstrual cycle has two phases. The first, from the start of a period to ovulation, varies a lot between people and between cycles. The second, from ovulation to the next period (the luteal phase), is more constant and lasts about 14 days on average. That is why calendar methods count back from the expected next period rather than forward from the last one.
Next period = first day of last period + cycle length
Ovulation ≈ next period − 14 days
Fertile window = ovulation − 5 days … ovulation day
Why the fertile window is six days
An egg can be fertilised for only about 12 to 24 hours after ovulation, but sperm can survive in the reproductive tract for up to five days. A landmark study of couples trying to conceive (Wilcox and colleagues, 1995) found that pregnancies came from intercourse in a six-day window ending on the day of ovulation, with the highest chances in the two to three days just before ovulation and on the day itself.
Worked example
Your last period started on 1 March 2026 and your cycle is 28 days.
- Next period: 1 March + 28 days = 29 March.
- Ovulation: 29 March − 14 days = 15 March.
- Fertile window: 10 to 15 March, with the best chances from about 13 to 15 March.
With a 32-day cycle, the next period moves to 2 April and ovulation to about 19 March. Longer cycles mean later ovulation, not a longer second half.
How reliable is the calendar method?
For a typical person the estimate is a reasonable starting point, but real ovulation can easily be several days earlier or later than the calendar suggests, even in regular cycles. Stress, illness, travel, recent hormonal contraception and breastfeeding can all shift it. The calendar is least reliable when cycles vary by more than a week from month to month.
To pinpoint ovulation more precisely, combine the calendar with one or more of these signs:
- Ovulation predictor kits detect the surge of luteinising hormone (LH) in urine, usually 24 to 36 hours before ovulation.
- Cervical mucus becomes clear, slippery and stretchy, like raw egg white, in the most fertile days.
- Basal body temperature rises by about 0.2–0.5 °C after ovulation. It confirms ovulation afterwards rather than predicting it, but over a few cycles it shows your pattern.
Using it to plan or avoid pregnancy
If you are trying to conceive, having intercourse every one to two days during the fertile window covers the most likely days without needing to know the exact day of ovulation. Most couples conceive within a year of trying; the usual advice is to see a doctor after 12 months without success, or after 6 months if you are 35 or older.
The calendar method alone is not a reliable form of contraception. If you want to avoid pregnancy, talk to a healthcare professional about effective methods.
Common mistakes
- Assuming ovulation is always on day 14. That is only true for a 28-day cycle with a 14-day luteal phase.
- Counting the cycle from the last day of bleeding. Count from the first day of one period to the first day of the next.
- Using one unusual cycle. Average your last three to six cycles for a better estimate.
This calculator gives an estimate for information only and is not a method of contraception or a fertility diagnosis.
Ovulation by cycle length
Counting the first day of the period as day 1, and assuming a 14-day luteal phase:
| Cycle length | Estimated ovulation | Fertile window |
|---|---|---|
| 24 days | day 10 | days 5–10 |
| 26 days | day 12 | days 7–12 |
| 28 days | day 14 | days 9–14 |
| 30 days | day 16 | days 11–16 |
| 32 days | day 18 | days 13–18 |
| 35 days | day 21 | days 16–21 |
Individual luteal phases range from about 10 to 16 days, so these days are estimates. Tracking ovulation signs for a few cycles shows your own pattern.
When to see a doctor
Consider medical advice if cycles are regularly shorter than 21 days or longer than 35 days, very irregular, unusually painful, or if periods stop. These can have treatable causes, and they also make calendar estimates unreliable.
Frequently asked questions
When do I ovulate with a 28-day cycle?
Around day 14, counting the first day of your period as day 1. The fertile window is roughly days 9 to 14.
Can I get pregnant outside the fertile window?
It is much less likely, but because ovulation can shift unexpectedly, the calendar cannot guarantee that any day is completely safe.
How long does the fertile window last?
About six days: the five days before ovulation and the day of ovulation, because sperm can live up to five days and the egg about one day.
What if my cycles are irregular?
Use ovulation tests or track cervical mucus and basal body temperature, and talk to a doctor if cycles are very irregular or longer than 35 days.
Does a long cycle mean a longer fertile window?
No. A longer cycle usually means ovulation happens later, but the fertile window is still about six days.
When do I ovulate with a 32-day cycle?
Around day 18, counting the first day of your period as day 1; the fertile window is roughly days 13 to 18.
Can ovulation happen at different times each month?
Yes. Stress, illness and other factors can shift it by several days even in regular cycles.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine, 1995; 333: 1517–1521.
- World Health Organization. Family Planning: A Global Handbook for Providers (fertility awareness methods).
- American College of Obstetricians and Gynecologists. Evaluating infertility (patient guidance on when to seek help).
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.