How to use the Conception Calculator
The conception calculator estimates your likely ovulation and fertile window from your cycle.
Open the Conception Calculator →
What you need to enter
- First day of last period
- Cycle length (days)
The formula
Ovulation ≈ cycle length − 14 days after the first day of your period. The fertile window is the 5 days before and the day of ovulation.
Worked example
Using the default values in the calculator:
- First day of last period: 2026-10-13
- Cycle length (days): 28
| Likely ovulation / conception | Tue, 27 Oct 2026 |
| Fertile window | Thu, 22 Oct 2026 – Wed, 28 Oct 2026 |
| Estimated due date | Tue, 20 Jul 2027 |
Tips for better results
- Cycles vary.
- Combine with ovulation tests.
Common mistakes to avoid
- Assuming a 28-day cycle for everyone.
- Expecting exact timing.
Please note: Health calculators and guides give general estimates, not medical advice. Talk to a doctor or other qualified professional about your own health. See our Disclaimer.