What is ovulation?
Ovulation is the moment your ovary releases an egg, usually once in each menstrual cycle. The egg survives for only about 12–24 hours, but sperm can live in the body for up to 5 days. That makes a fertile window of about six days: the five days before ovulation and the day of ovulation itself. Knowing when it falls is the simplest way to improve your chances of getting pregnant naturally.
How the ovulation calculator works
After ovulation, the second half of the cycle (the luteal phase) is fairly constant at about 14 days for most women. The calculator therefore works backwards from your next expected period:
- Next period = first day of last period + cycle length
- Ovulation day = next period − 14 days
- Fertile window = 5 days before ovulation to 1 day after
Example: last period started 1 January 2026 with a 28-day cycle → next period 29 January, ovulation around 15 January, fertile window 10–16 January. With a 32-day cycle, ovulation moves about 4 days later.
Cycle length and ovulation: low, normal and irregular
| Cycle length | Level | Likely ovulation |
|---|---|---|
| Under 21 days | Short | Very early; may signal hormone imbalance — see a doctor |
| 21 – 35 days | Normal | About day 7 (21-day cycle) to day 21 (35-day cycle) |
| Over 35 days or varies a lot | Long / irregular | Hard to predict; ovulation may not happen every cycle (common in PCOS) |
Signs you may be ovulating
- Clear, stretchy, “egg-white” cervical mucus
- A small rise (0.3–0.5 °C) in basal body temperature after ovulation
- Positive LH ovulation test
- Mild one-sided pelvic twinge (mittelschmerz) or breast tenderness
Benefits of tracking ovulation
- Better timing when trying to conceive.
- Understand your cycle and spot changes early.
- Useful information for your doctor if pregnancy is taking longer than expected.
- Plan your nutrition: start folic acid and improve your diet before conceiving — see our fertility nutrition service.
Health warnings
- Do not use this calculator as contraception. Ovulation can shift from cycle to cycle, and sperm survive up to 5 days.
- Very irregular, absent or very painful periods need a medical check (for example for PCOS, thyroid disease or endometriosis).
- See a doctor if you are under 35 and not pregnant after 12 months of trying, or over 35 after 6 months.
- Calculators estimate; ovulation tests and ultrasound tracking are more precise.
Nutrition for fertility
A healthy weight, steady blood sugar and enough folate, iron, iodine, vitamin D and omega-3s all support ovulation and early pregnancy. For PCOS, small weight loss (5–10%) can restore ovulation in many women. Read our PCOS and diet guide and the 7-day PCOS meal plan, and check your BMI.
Frequently asked questions
When do I ovulate?
Most women ovulate about 14 days before their next period. In a 28-day cycle that is around day 14, but in a 32-day cycle it is closer to day 18.
How many days am I fertile each month?
About six days: the five days before ovulation plus ovulation day. The two days before ovulation and the day itself give the highest chance of pregnancy.
Can I get pregnant outside the fertile window?
It is unlikely but possible, because ovulation can come earlier or later than expected. That is why calendar methods are not reliable contraception.
How accurate is an ovulation calculator?
It is a good guide for women with regular cycles. Ovulation predictor kits (LH tests), cervical mucus and temperature tracking make timing more accurate.
Does PCOS affect ovulation?
Yes. PCOS often causes irregular or absent ovulation. Healthy eating, regular activity and modest weight loss can help many women ovulate more regularly. See a doctor for testing and treatment.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–1521.
- American College of Obstetricians and Gynecologists. Fertility awareness-based methods of family planning.
- NICE. Fertility problems: assessment and treatment (CG156).