About the Ovulation Calculator
This ovulation calculator estimates when you ovulate and which days you are most likely to get pregnant (your fertile window). The time between ovulation and your next period (the luteal phase) is fairly constant at about 14 days, so ovulation is estimated as 14 days before your next expected period.
Sperm can survive up to 5 days in the body and an egg lives about a day, so the fertile window runs from 5 days before ovulation to the day after. Your inputs are saved only on this device.
How to use
- Pick the first day of your most recent period.
- Enter your average cycle length and how many days your period lasts. If you’re not sure, use 28 and 5.
- See your estimated ovulation day, fertile window and next period date.
- Check the 3-month calendar: period days are pink, fertile days green and ovulation purple.
How ovulation is calculated
- Next period = first day of last period + cycle length
- Ovulation day = next period − 14 days
- Fertile window = 5 days before ovulation to 1 day after (7 days)
Example: if your period started on October 1 and your cycle is 28 days, your next period is due October 29, ovulation is around October 15 and your fertile window is October 10–16.
Ways to pinpoint ovulation more accurately
Calendar predictions assume a regular cycle. For a more precise picture, combine them with:
- Ovulation tests (OPKs): detect the surge in luteinizing hormone (LH) in urine 24–36 hours before ovulation.
- Basal body temperature: rises by about 0.5–1 °F (0.3–0.5 °C) after ovulation and stays up until your period.
- Cervical mucus: becomes clear, slippery and stretchy, like raw egg white, as ovulation approaches.
- Follicle ultrasound: a fertility clinic can track the follicle directly – the most accurate method.
FAQ
Can I use this to avoid pregnancy?
It’s not recommended. Ovulation can easily shift because of stress, illness or poor sleep, and sperm can occasionally live longer than 5 days, so calendar-based methods have a high failure rate. Use a reliable form of contraception instead.
What if my cycle is irregular?
Entering your average cycle length over the last six months gives a rough guide. If your cycles vary by 7 days or more, ovulation tests or a visit to your doctor will give you a clearer answer.
Which days are best for getting pregnant?
Chances are highest in the 2 days before ovulation and on the day of ovulation. If you’re trying to conceive, having sex every 2–3 days throughout the fertile window is usually recommended.
Is my period data stored?
Only in this browser, so the calculator remembers it next time. Nothing is sent to a server.