Ovulation calculator
Know when your body is most likely to conceive.
Track the pattern, not just one result
This free tool gives you an educational starting point. Use Flow & Glow to keep period dates, symptoms, and cycle notes together so changes are easier to discuss with a qualified healthcare professional.
What is the fertile window?
Your fertile window is the six days each cycle when conception is possible. It runs from five days before ovulation through ovulation itself, because sperm can survive inside the body for up to five days, and the egg lives for about 24 hours after release.
This calculator estimates those days from the first day of your last period and your average cycle length — a fine starting point when your cycle is fairly regular.
Signals that confirm ovulation
Calendar predictions are estimates. These three signals confirm ovulation in the moment:
- Cervical mucus — stretchy, clear, egg-white consistency shows up in the days before ovulation.
- Basal body temperature (BBT) — a sustained rise of about 0.3 °C the morning after ovulation confirms it has happened.
- LH tests — a positive ovulation strip predicts ovulation in the next 12 to 36 hours.
If you're trying to conceive
Most guidance suggests intercourse every 1 to 2 days across the fertile window, not just on ovulation day. Fertility does decline with age, but if you're under 35 and have been trying for less than a year, there's usually no need to rush to a specialist.
If you're 35 or older and trying for six months, or you have irregular cycles, it's reasonable to talk to a clinician sooner rather than later.
Related clinician-reviewed guides
Try another free cycle tool
- Period and menstrual cycle calculator — Estimate your next period, ovulation, and fertile window in one view.
- Endo symptom checker — Understand whether endometriosis is worth ruling out.
- Implantation calculator — Estimate your implantation window and the best day to test.
- PCOS self-screen — See if your symptoms fit the PCOS pattern.
Frequently asked questions
Can I ovulate on a different day than predicted?
Yes, ovulation can shift by several days in any given cycle. Stress, illness, travel, and intense exercise are all common causes. That's why combining the calendar method with one physiological signal — like cervical mucus or LH tests — gives a far better read.
Does this work for irregular cycles?
The calendar method is less accurate when cycle length varies by more than a week. Our fertile window guide covers what to pair it with.
I'm on hormonal birth control — should I track this?
Hormonal contraception typically suppresses ovulation. The calendar method won't meaningfully predict fertility while you're on it.
Is my data private?
Yes. The calculation runs entirely in your browser. Nothing is sent to our servers, and nothing is saved when you leave.
Trust and medical boundaries
This tool is educational and doesn't replace medical advice. If something feels off, talk to a qualified clinician.
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