How the calculator estimates ovulation
A menstrual cycle runs from the first day of one period to the first day of the next. Ovulation, when an ovary releases an egg, happens partway through. The part of the cycle after ovulation, called the luteal phase, is fairly steady from cycle to cycle. The NHS says ovulation happens around 10 to 16 days before the next period in most women, and 14 days is the usual average.
So the calculator works backwards from your next expected period:
Ovulation ≈ next period − luteal phase (14 days by default)
Fertile window = 5 days before ovulation to ovulation day
Why the fertile window is six days
An egg can only be fertilised for a short time after it is released, usually less than a day. Sperm last much longer: the NHS notes they can survive in the fallopian tubes for up to 7 days. That is why sex in the days before ovulation can lead to pregnancy.
A well-known 1995 study in the New England Journal of Medicine by Allen Wilcox and colleagues followed 221 women trying to conceive. Pregnancies came only from sex during a six-day window that ended on the day of ovulation. The chance was lowest at the start of that window and highest in the last three days: the two days before ovulation and the day itself. The calculator marks those as the highest-chance days.
Worked examples
A regular 28-day cycle. Your last period started on 1 January 2026.
- Next period: 1 January + 28 days = 29 January.
- Ovulation: 29 January − 14 days = 15 January (cycle day 15).
- Fertile window: 10 to 15 January, with the highest chances on 13, 14 and 15 January.
A longer 35-day cycle. Same start date. The next period is due on 5 February, so ovulation is estimated on 22 January and the fertile window is 17 to 22 January. Longer cycles move ovulation later, not the luteal phase.
A shorter luteal phase. If you know from tracking that your luteal phase is 12 days, a 28-day cycle puts ovulation on 17 January instead.
| Cycle length | Ovulation (cycle day) | Fertile window (cycle days) |
|---|---|---|
| 21 days | Day 8 | Days 3–8 |
| 28 days | Day 15 | Days 10–15 |
| 32 days | Day 19 | Days 14–19 |
| 35 days | Day 22 | Days 17–22 |
These assume a 14-day luteal phase. Day 1 is the first day of bleeding.
How reliable is a calendar estimate?
It is a reasonable starting point if your cycles are regular, but it is still an average. Ovulation can come earlier or later than expected in any cycle, because of stress, illness, travel, breastfeeding or recently stopping hormonal contraception. Ways to get a more personal picture include:
- Ovulation predictor kits, which detect the surge of luteinising hormone (LH) that comes shortly before ovulation.
- Cervical mucus, which often becomes clear, wet and stretchy, like raw egg white, around the fertile days.
- Basal body temperature, which rises slightly after ovulation. It confirms ovulation has happened rather than predicting it.
This is not birth control
Do not use this calculator to avoid pregnancy. Fertility awareness methods taught properly combine several daily signs, not just a calendar, and the NHS says they are 91% to 99% effective with perfect use but only about 76% effective with typical use. If you want to avoid pregnancy, talk to a doctor, nurse or pharmacist about contraception.
When to see a doctor
- Your cycles are often shorter than 21 days or longer than 35 days, or very irregular.
- Your periods have stopped, are very heavy or very painful.
- You have been trying for a year without getting pregnant, or 6 months if you are 35 or older.
After a positive test
Most home pregnancy tests work from the first day of a missed period. If you get a positive result, the pregnancy due date calculator estimates your due date and shows the key dates ahead. To count days between dates for any other reason, use the date calculator.
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