Sperm can survive up to five days, so the most fertile days are the five before ovulation and the day of ovulation itself.
How to use the Ovulation Calculator
- First day of last period
- Average cycle length
- Luteal phase
Results update as you type, and every field accepts the units shown in its menu.
How it works
The calculator counts forward one cycle and back 14 days for the luteal phase.
Inputs explained
| Field | What to enter |
|---|---|
| First day of last period | A date |
| Average cycle length | In days |
| Luteal phase | In days |
What each result means
- Estimated ovulation = [First day of last period] + [Average cycle length] – [Luteal phase]
- Fertile window starts = [Estimated ovulation] – 5
- Fertile window ends = [Estimated ovulation] + 1
- Next period = [First day of last period] + [Average cycle length]
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 lengthOvulation ≈ next period − 14 daysFertile window = ovulation − 5 days to ovulation + 1 dayWorked example
Inputs — First day of last period: 2026-10-01; Average cycle length: 28 days; Luteal phase: 14 days.
Results — Estimated ovulation = Thu, October 15, 2026; Fertile window starts = Sat, October 10, 2026; Fertile window ends = Fri, October 16, 2026; Next period = Thu, October 29, 2026.
Key terms
- Luteal phase
- Days between ovulation and the next period.
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 test kits and basal body temperature tracking help.
What formula does the Ovulation Calculator use?
Next period = LMP + cycle length.