Ovulation Calculator
Pregnancy is only possible in a short window each cycle, in the days leading up to and just after ovulation, when an egg is released. This calculator estimates that window from the first day of your last period and the length of your cycle. Ovulation typically occurs about 14 days before the next period, so in a regular 28-day cycle it falls on day 14, while in a 35-day cycle it falls around day 21. Because sperm can survive for up to five days and the egg for roughly one, the fertile window runs from five days before ovulation to the day after. The estimate assumes a regular cycle; stress, illness, travel and conditions such as PCOS can move ovulation, so combine it with other signs if you are trying to conceive.
How to use the ovulation calculator
- Enter the first day of your most recent period.
- Enter your average cycle length, counting from day 1 of one period to day 1 of the next.
- Leave the luteal phase at 14 days unless you know yours is different.
- Read your estimated ovulation day, the fertile window and the next period date — and look at the next two cycles to plan ahead.
Formula
Ovulation day ≈ first day of last period + (cycle length − luteal phase). With a 28-day cycle and a 14-day luteal phase, ovulation is on day 14. Fertile window = ovulation − 5 days to ovulation + 1 day. Next period = first day of last period + cycle length. The luteal phase is more consistent between people than the first half of the cycle, which is why the calculation counts back from the next period.
Worked examples
A 28-day cycle
With a period starting on 1 March 2026 and a 28-day cycle, ovulation is estimated on Sunday, March 15, 2026, with a fertile window of Tuesday, March 10, 2026 – Monday, March 16, 2026, and the next period due Sunday, March 29, 2026.
A longer cycle
With a 35-day cycle ovulation moves later: Sunday, March 22, 2026. Assuming ovulation on day 14 is the most common error for people with longer cycles.
A shorter luteal phase
With a 28-day cycle but a 12-day luteal phase, ovulation shifts to Tuesday, March 17, 2026.
Understanding the fertile window
Ovulation day by cycle length (14-day luteal phase)
| Cycle length | Ovulation on cycle day |
|---|---|
| 24 days | Day 10 |
| 26 days | Day 12 |
| 28 days | Day 14 |
| 30 days | Day 16 |
| 32 days | Day 18 |
| 35 days | Day 21 |
Signs to watch for
- Clear, stretchy cervical mucus
- A positive ovulation (LH) test, which detects the hormone surge before ovulation
- A sustained small rise in basal body temperature after ovulation
If you have been trying for a year (six months if over 35) without success, speak to a doctor. Once pregnant, see the due date calculator.
Practice problems
Try these yourself first, then check your answer. The answers come from the calculator above.
- The last period began 5 May 2026 and the cycle is 30 days. When is ovulation?
Show answer
Estimated ovulation day: Thursday, May 21, 2026 · Fertile window: Saturday, May 16, 2026 – Friday, May 22, 2026
Common mistakes to avoid
- Assuming everyone ovulates on day 14. That is true only for a 28-day cycle with a 14-day luteal phase.
- Treating the estimate as exact for irregular cycles. If your cycle varies by more than a few days, use ovulation tests or temperature tracking alongside it.
- Ignoring that the fertile window starts before ovulation. Intercourse in the five days before ovulation can lead to pregnancy.
- Using this as contraception. It is not a reliable way to avoid pregnancy.
- Entering the wrong date. Use the first day of full flow, not spotting.
Frequently asked questions
How do I calculate my ovulation day?
Subtract your luteal phase (usually 14 days) from your cycle length and count that many days from the first day of your last period. In a 28-day cycle, that is day 14.
How long is the fertile window?
About six days: the five days before ovulation and the day of ovulation, plus the day after, since sperm survive up to five days and an egg about a day.
What are the signs of ovulation?
Clearer, stretchy cervical mucus, a slight rise in basal body temperature after ovulation, mild one-sided pelvic pain and a positive ovulation (LH) test. See the ACOG guidance linked below.
Can I get pregnant outside the fertile window?
It is unlikely but cycles vary, so no estimate is a guarantee. This calculator should not be used as birth control.
What if my cycle is irregular?
Calculated dates are less reliable. Ovulation predictor kits or a clinician’s advice will give a better idea.
Is this medical advice?
No. It is an educational estimate; talk to a healthcare provider about fertility concerns.
Sources and further reading
Reviewed September 26, 2026 by the SolveCalcPro editorial team. Found a mistake? Tell us; see our editorial policy.