Sperm can survive up to five days in the reproductive tract, while an egg lives for about 12–24 hours, so the most fertile days are the five days before ovulation and ovulation day itself. Having sex every 1–2 days in this window gives the best chance of pregnancy.
How to use the Ovulation Calculator
- First day of your last period
- Average cycle length
- Luteal phase length
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the first day of your last period and your average cycle length. If you track your luteal phase (from ovulation tests or temperature charts), enter it for a more accurate estimate.
Inputs explained
| Field | What to enter |
|---|---|
| First day of your last period | A date |
| Average cycle length | In days |
| Luteal phase length | In days |
What each result means
- Next ovulation = [First day of your last period] + [Average cycle length] – [Luteal phase length]
- Fertile window starts = [Next ovulation] – 5
- Fertile window ends = [Next ovulation] + 1
- Next period = [First day of your last period] + [Average cycle length]
- Earliest reliable pregnancy test = [First day of your last period] + [Average cycle length]
- Ovulation: following cycle = [Next ovulation] + [Average cycle length]
- Ovulation: third cycle = [Next ovulation] + 2×[Average cycle length]
- Due date if you conceive this cycle = [First day of your last period] + 280 + ([Average cycle length] – 28)
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Next period = LMP + cycle length
Ovulation ≈ next period − luteal phase (usually 14 days)
Fertile window = ovulation − 5 days to ovulation + 1 day
Worked example
Inputs: First day of your last period: 2026-10-18; Average cycle length: 28 days; Luteal phase length: 14 days.
Results: Next ovulation = Sun, November 1, 2026; Fertile window starts = Tue, October 27, 2026; Fertile window ends = Mon, November 2, 2026; Next period = Sun, November 15, 2026; Earliest reliable pregnancy test = Sun, November 15, 2026; Ovulation – following cycle = Sun, November 29, 2026; Ovulation – third cycle = Sun, December 27, 2026; Due date if you conceive this cycle = Sun, July 25, 2027.
Key terms
- Luteal phase
- Days between ovulation and the next period, usually 12–14.
- LH surge
- Hormone rise that triggers ovulation 24–36 hours later.
Frequently asked questions
Can I rely on this for contraception?
No. Calendar estimates are not reliable enough to prevent pregnancy.
How can I confirm ovulation?
Ovulation (LH) tests, basal body temperature charts and cervical mucus changes.
What if my cycles are irregular?
Predictions are less reliable; ovulation tests help.
Sources
Guides on this topic
- How to Calculate Your Due Date: LMP, Conception, IVF and Ultrasound: Add 280 days to your last period, or 266 to conception. Here is how each method works, when an early scan changes the date, and what the trimesters look like.