Health Hub · Free tool
Ovulation Calculator
Enter the first day of your last period and your usual cycle length to see your estimated ovulation date and fertile window — plus what the estimate can and can't tell you.
Important: this is an estimate for people trying to conceive. It is NOT reliable enough to use as contraception — sperm survive up to 5 days and ovulation shifts cycle to cycle.
How this ovulation calculator works
Ovulation happens roughly 14 days before your next period starts — not 14 days after the last one, which is the common misreading. So the calculator takes your cycle length, subtracts 14, and counts that many days forward from the first day of your last period. On a classic 28-day cycle that lands on day 14; on a 35-day cycle it's day 21. The egg then lives for only about 24 hours — but because sperm survive up to five days, the fertile window spans roughly six days, ending the day after ovulation.
When do you ovulate — and how would you know?
Calendars estimate; bodies confirm. The signs many people notice around ovulation:
- Cervical mucus turns clear, slippery and stretchy — like raw egg white. This is the most reliable free signal.
- One-sided low belly ache (mittelschmerz) lasting minutes to hours.
- Breast tenderness, a libido rise, and for temperature-trackers a small rise of ~0.3°C after ovulation has happened.
For real precision, ovulation tests detect the LH hormone surge 24–36 hours before the egg is released — find them in our home health testing range. Pairing the calendar (to know roughly when to start testing) with the tests (to confirm) is the approach fertility clinics actually recommend.
Trying to conceive: making the window count
Aim for sex every 2–3 days across the fertile window rather than saving everything for ovulation day — the two days before ovulation carry the highest chance of all. And start a daily 400mcg folic acid supplement now, not at the positive test: it matters most in the earliest weeks, before most people know. Our pregnancy and conception range covers folic acid and preconception multivitamins.
When the calendar method struggles
Irregular cycles make the maths unreliable — if your cycle varies by more than a few days, lean on ovulation tests and mucus signs instead. Recently stopped hormonal contraception? Give cycles two or three months to settle before reading much into dates. And if you've been trying for over a year (or six months over 35), that's the moment for a GP conversation for both partners — it's routine, and earlier is better.
Frequently asked questions
Can I get pregnant outside the fertile window?
It's unlikely but not impossible — ovulation timing shifts, which is exactly why this tool must never be used as contraception. For actual contraception, talk to our pharmacist.
How accurate is an ovulation calculator?
For regular cycles it lands within a day or two most months. Ovulation tests beat it comfortably; the calculator's best job is telling you when to start testing.
What if my cycles are all different lengths?
Use your shortest and longest recent cycles to bracket the earliest and latest likely fertile days — or skip the calendar and use LH tests from around day 10.