How the menstrual cycle works
A cycle starts on the first day of your period and lasts until the day before your next one. The average is 28 days, but anything from about 21 to 35 days is normal. A cycle has two phases:
- The follicular phase: an egg matures in the ovary. The length of this phase varies between people and from month to month.
- The luteal phase: after ovulation the body prepares for a possible pregnancy. This phase is fairly constant, usually 12 to 16 days, 14 on average.
Because the second phase is so stable, ovulation is best estimated by counting back from your next period:
ovulation = first day of last period + cycle length − 14 days
Your fertile window
An egg survives only 12 to 24 hours after ovulation, but sperm can live in the body for up to five days. That gives a fertile window of about six days per cycle: the five days before ovulation plus ovulation day. The highest chance of pregnancy comes from intercourse in the two days before ovulation and on the day itself. The calculator also shows the day after ovulation as a margin, because ovulation can never be predicted to the exact day.
Example
Your last period started on Sunday 20 September 2026 and your cycle is 28 days.
- ovulation: 20 September + 28 − 14 days = Sunday 4 October 2026
- fertile window: 29 September to 5 October
- most fertile: 2 to 4 October
- next period: Sunday 18 October 2026
With a 32-day cycle everything shifts by four days, and ovulation falls around 8 October. With a 25-day cycle it moves three days earlier, to around 1 October, which is why knowing your own average cycle length matters more than the textbook 28 days.
Signs of ovulation
Besides counting days you can watch for signals from your body:
- Cervical mucus: around ovulation it becomes clear, slippery and stretchy, like raw egg white.
- Basal body temperature: after ovulation your waking temperature rises by about 0.2–0.5 °C (0.4–1 °F). Daily tracking shows afterwards when ovulation happened.
- Ovulation test (OPK): a home test detects the LH surge in urine 24 to 36 hours before ovulation.
- Physical signs: some people feel a slight one-sided pelvic twinge (mittelschmerz) or breast tenderness.
Combining the calendar with one or more of these signs gives a much better picture of your fertile days.
Chances of conceiving
Even with perfect timing, the chance of pregnancy in a single cycle is limited: around 20 to 25% for healthy couples under 35. About eight in ten couples conceive within a year. If you are under 35 and haven't conceived after a year of trying, or over 35 after six months, talk to your doctor. Already pregnant? The due date calculator shows your estimated due date. A healthy weight supports fertility too; the BMI calculator gives a first indication.
Important
- The calculation assumes a regular cycle. With irregular cycles the result is less reliable.
- Stress, illness, travel, breastfeeding and recently stopping hormonal birth control can shift ovulation.
- A fertility calendar is not reliable contraception. If you want to avoid pregnancy, use a contraceptive method.
Frequently asked questions
When do I ovulate?
Usually 14 days before your next period. In a 28-day cycle that is around day 14; in a 32-day cycle around day 18.
How many days a month am I fertile?
About six: the five days before ovulation and ovulation day itself. Outside that window the chance of pregnancy is very small.
Can I get pregnant on my period?
It is unlikely but possible. With a short cycle ovulation can come early, so sperm from intercourse near the end of your period may still be alive.
How accurate is an ovulation calculator?
With a regular cycle it gives a good estimate, but ovulation can shift by a few days from month to month. Ovulation tests or temperature tracking make it more precise.
What if my cycle is irregular?
Use the average of your last six cycles and allow a wider window. With very irregular cycles, a doctor can help find the cause.
When can I take a pregnancy test?
From the day your period is due, a standard test gives a reliable result. Early tests work a few days sooner but are less certain.
Last reviewed: 2026-10-06. Results are estimates for information only.