Ovulation Calculator
Free ovulation calculator: enter your last period date and cycle length to see your fertile window, ovulation day and next period on a 3-month calendar.
How the ovulation calculator works
Most women ovulate about 14 days before their next period starts — not 14 days after the last one. So ovulation usually falls on cycle day “cycle length − 14”, counting the first day of your period as day 1: day 14 in a 28-day cycle, day 18 in a 32-day cycle. Your next period is due one cycle length after the last one started.
Your fertile window is the 5 days before ovulation plus ovulation day. Sperm can live up to 5 days inside the body, but an egg lives only about 12–24 hours, so the 2–3 days just before ovulation give the best chance of pregnancy.
Fertile window by cycle length
| Cycle length | Likely ovulation | Fertile window | Best days |
|---|---|---|---|
| 24 days | Day 10 | Days 5–10 | Days 8–10 |
| 26 days | Day 12 | Days 7–12 | Days 10–12 |
| 28 days | Day 14 | Days 9–14 | Days 12–14 |
| 30 days | Day 16 | Days 11–16 | Days 14–16 |
| 32 days | Day 18 | Days 13–18 | Days 16–18 |
| 35 days | Day 21 | Days 16–21 | Days 19–21 |
Day 1 is the first day of your period.
Irregular periods or PCOS?
If your cycle length changes from month to month, tick “My cycles are irregular” and enter your shortest and longest cycle. We then show a wider fertile window (from day “shortest − 18” to day “longest − 11”) so you are less likely to miss it. With PCOS or very irregular cycles, a calendar can only give a rough guide — ovulation predictor kits (LH tests), tracking cervical mucus or basal body temperature, or a scan from your doctor are more reliable.
Signs of ovulation
- Clear, stretchy, slippery cervical mucus (like raw egg white)
- A positive ovulation (LH) test 24–36 hours before ovulation
- A small rise in basal body temperature after ovulation
- Mild one-sided lower tummy pain, breast tenderness or higher sex drive
Tips when trying to conceive
- Have sex every 1–2 days during your fertile window rather than only on ovulation day.
- Start 400 mcg folic acid daily, ideally 3 months before trying.
- Eat well and aim for a healthy weight — see our free fertility diet plan.
- Once pregnant, use our pregnancy diet plan for each trimester.
Important: this calculator is not a method of birth control. Ovulation can shift from cycle to cycle, and predicted “safe days” are not reliable for avoiding pregnancy.
Frequently asked questions
How do I calculate my ovulation day?
Take your cycle length and subtract 14 — that is your likely ovulation day, counting the first day of your period as day 1. For a 28-day cycle that started on 1 October, ovulation is around day 14, which is 14 October.
How many days after my period can I get pregnant?
It depends on your cycle length. In a 28-day cycle the fertile window is about days 9–14 counting from the first day of your period — so roughly 4–9 days after a 5-day period ends. In shorter cycles it can start soon after your period.
Can I get pregnant outside the fertile window?
It is unlikely but possible, because ovulation can come earlier or later than predicted, especially with irregular cycles, stress, illness or after stopping hormonal birth control.
When should I take a pregnancy test?
Take a home pregnancy test from the day your period is due, or at least 14 days after ovulation. Testing earlier can give a false negative.
When should we see a doctor?
See a doctor after 12 months of trying if you are under 35, after 6 months if you are 35 or older, or sooner if your periods are very irregular or absent.
Sources: ACOG: Fertility awareness-based methods · NHS: Trying to get pregnant · Timing of intercourse and conception (NEJM 1995)
Reviewed for accuracy against the sources above. This page is general information, not medical advice — talk to your doctor or a registered dietitian about your own needs.