Ovulation Calculator
Ovulation calculator: enter your last period and usual cycle length to map your fertile window, likely ovulation day and next period for 3 months.
Written by HealthSensibly Editorial TeamReviewed by the HealthSensibly editorial teamUpdated
How we estimate your ovulation date
Ovulation is best timed from the end of a cycle rather than the start: for most women it happens roughly 14 days ahead of the next bleed. If the first day of bleeding is day 1, your likely ovulation day is simply your cycle length minus 14 — day 14 when cycles run 28 days, and day 18 when they run 32. The next period is then expected one full cycle length after the previous one began.
The fertile window covers six days: ovulation day itself and the five days leading up to it. Sperm may survive for as long as 5 days in the reproductive tract, whereas an egg stays viable for only around 12–24 hours — which is why the 2–3 days right before ovulation carry the highest chance of conceiving.
Fertile days for common cycle lengths
| Length of cycle | Expected ovulation | Fertile days | Peak 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 |
Counting starts on day 1, the day your bleeding begins.
Cycles that vary, including PCOS
When your cycle is a different length each month, select “My cycle length varies” and add the shortest and longest cycles you have had. The calculator then stretches the fertile window so it runs from day “shortest − 18” through to day “longest − 11”, making it harder to miss. For PCOS or highly unpredictable cycles, any calendar estimate is only approximate; LH ovulation test kits, charting cervical mucus or basal body temperature, or an ultrasound arranged by your doctor will give a more dependable picture.
Clues that you may be ovulating
- Cervical mucus that turns clear, slippery and stretchy, much like raw egg white
- An LH (ovulation) test that turns positive, typically 24–36 hours ahead of ovulation
- A slight increase in basal body temperature once ovulation has passed
- A mild ache on one side of the lower abdomen, tender breasts or a stronger libido
Helpful steps if you are trying for a baby
- Aim for intercourse every 1–2 days across the whole fertile window instead of saving it for ovulation day alone.
- Begin a daily 400 mcg folic acid supplement, ideally around 3 months before you start trying.
- Keep meals balanced and work towards a healthy weight — our fertility diet plan (free) can help.
- After a positive test, switch to our trimester-by-trimester pregnancy diet plan.
Please note: do not rely on this tool as contraception. The timing of ovulation can vary between cycles, so any predicted “safe days” cannot be trusted to prevent pregnancy.
Your questions answered
Does ovulation happen on the same day every month?
Not always. Even when your cycle is fairly regular, ovulation can arrive earlier or later than expected — stress, illness, irregular cycles and coming off hormonal contraception can all move it. That is why the calculator shows a window of days rather than one fixed date.
Why is ovulation counted back from the next period?
The part of the cycle after ovulation is usually close to 14 days long, while the part before it varies far more from woman to woman. Subtracting 14 from your cycle length therefore gives a better estimate. For example, if a 28-day cycle began on 1 October, ovulation would be expected near day 14 — around 14 October.
What if my cycle is shorter than 28 days?
Your fertile days simply come sooner. In a 24-day cycle the window runs from about day 5 to day 10, so it can begin very soon after bleeding stops. In a 28-day cycle it sits nearer days 9–14 — some 4–9 days once a 5-day period has finished.
How long after ovulation should I wait to test for pregnancy?
Hold off until at least 14 days after ovulation, or until the day your next period is expected. A home test used sooner than that may show a false negative even if you have conceived.
How long is it normal to try before getting checked?
If you are under 35, book a doctor’s appointment after 12 months of trying without success; from age 35 onwards, after 6 months. Seek advice earlier if your periods are very irregular or have stopped altogether.
References: ACOG: Fertility awareness-based methods · NHS: Trying to get pregnant · Timing of intercourse and conception (NEJM 1995)
Checked against the sources listed above. This is general health information and not medical advice — please ask your doctor or a registered dietitian what suits you.
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Our editors write every tool explainer and guide in plain language from current guidelines and research, and list the sources on the page.
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