Find your most fertile days with an ovulation calculator
This ovulation calculator estimates when you will ovulate, your fertile window, when your next period is due and the earliest day a home pregnancy test is likely to be accurate. It projects up to six cycles ahead and marks every period, fertile day and ovulation day on a month calendar.
Unlike calculators that assume ovulation always falls on day 14, this one uses your own cycle length and luteal phase, so the estimate stays realistic for 25-, 32- or 35-day cycles. It also works as a safe period calculator, showing the calendar method’s fertile and lower-fertility days — with a clear warning that this is not reliable contraception. Nothing you type leaves your device.
How to use the ovulation calculator
- Enter the first day of your last period (real bleeding, not spotting).
- Type your average cycle length — the days between the start dates of your periods.
- Leave the luteal phase at 14 days unless you know yours, and set your usual period length.
- Read your ovulation date and fertile window, then browse the calendar and the table of upcoming cycles.
- For the safe period, enter your shortest and longest cycles and read the calendar-method table — remembering it is not reliable contraception.
Ovulation Calculator features
Personal cycle maths
Ovulation is counted back from your next period with your own cycle and luteal phase, not a fixed day 14.
Six-day fertile window
The five days before ovulation plus ovulation day, with the three most fertile days pointed out.
Colour calendar
Period, predicted period, fertile and ovulation days are colour-coded; today is outlined.
Test-day guidance
Your next period date — the earliest day most home pregnancy tests are reliable.
Late-period check
If more days have passed than your cycle length, the calculator says so and suggests next steps.
Safe period (calendar method)
Enter your shortest and longest cycles to see the calendar rhythm method’s fertile days and the Standard Days Method, with dates for this cycle.
How ovulation and the fertile window are calculated
A menstrual cycle has two halves. The follicular phase, from your period to ovulation, varies in length and is the reason cycles differ. The luteal phase, from ovulation to the next period, is more stable — usually 12 to 14 days. So the calculator works backwards: ovulation = next period − luteal phase, and next period = last period + cycle length.
The fertile window is the five days before ovulation plus ovulation day. Sperm can survive up to five days, but an egg lives only about 12–24 hours, so the days before ovulation matter most: Wilcox and colleagues (1995) found the highest chance of conception in the two days before ovulation and on the day itself.
Worked example: a period starting 1 March 2024 with a 28-day cycle and 14-day luteal phase gives ovulation on 15 March, a fertile window of 10–15 March and the next period on 29 March. With a 32-day cycle and 12-day luteal phase, ovulation moves to 21 March and the next period to 2 April.
Signs that confirm ovulation
A calendar estimate is a starting point. These signs pin down ovulation in a real cycle:
- Ovulation (LH) tests: a positive result means the LH surge has begun; ovulation typically follows within about 24–36 hours.
- Cervical mucus: near ovulation it becomes clear, slippery and stretchy, like raw egg white.
- Basal body temperature: resting temperature rises by roughly 0.2–0.5 °C (0.4–1 °F) after ovulation and stays up until your period — it confirms ovulation afterwards rather than predicting it.
- Other clues: one-sided pelvic twinges (mittelschmerz), breast tenderness or higher libido.
Safe period calculator: how the calendar method works
The calendar rhythm method in the WHO Family Planning Handbook (2022 edition, ch. 18) uses at least six months of cycle records: the first fertile day is the shortest cycle minus 18, the last fertile day the longest cycle minus 11. For a regular 28-day cycle that makes days 10–17 fertile and days 1–9 and 18–28 the so-called safe period; for a 40-day cycle, days 22–29. The Standard Days Method treats days 8–19 as fertile and is only for cycles of 26–32 days.
The handbook reports about 12 pregnancies per 100 women in a year of typical use of the Standard Days Method (5 with perfect use) and about 15 per 100 for periodic abstinence, mostly calendar rhythm. Ovulation can shift in any cycle: the safe period is an estimate, not protection.
Irregular cycles and when to get advice
Calendar predictions work best when cycles vary by only a few days. PCOS, thyroid problems, breastfeeding, stopping hormonal contraception, stress, intense training and perimenopause can all make ovulation unpredictable; then LH tests or temperature charting are more useful.
Doctors usually suggest a fertility check after 12 months of regular unprotected sex without pregnancy if you are under 35, or 6 months if 35 or older — sooner if periods are very irregular or absent. This tool gives estimates, not medical advice.
After a positive test, the pregnancy due date calculator gives your due date; to learn your true cycle length, log periods with the period calculator.
Ovulation Calculator FAQ
How accurate is an ovulation calculator?
With regular cycles it gives a good estimate, but ovulation can shift by several days from month to month. Use the dates as a guide and confirm with ovulation tests or body signs.
How many days after my period do I ovulate?
Subtract your luteal phase (usually 14 days) from your cycle length: around day 14–15 of a 28-day cycle, day 21 of a 35-day cycle. Day 1 is the first day of your period.
What are my most fertile days?
The two days before ovulation and ovulation day. The full fertile window is about six days because sperm survive up to five days.
Can I use the safe period calculator as birth control?
Not on its own. Ovulation can come earlier or later than predicted and sperm survive for days, so calendar methods fail for about 12–15 in 100 women a year with typical use. Speak to a doctor or family-planning service about contraception.
When should I take a pregnancy test?
Most home tests are reliable from the first day of your missed period (the “next period” date). Testing earlier can give a false negative.
What is a luteal phase and why does it matter?
The time between ovulation and your next period — fairly constant for each person, usually 12–14 days, so the best way to count back to ovulation. One shorter than about 10 days is worth mentioning to a doctor if you are trying to conceive.