Conception Calculator
Estimate the most likely date of conception based on the start of your last period and your cycle length.
How to use the Conception Calculator
- Enter your inputs into the Conception Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The conception date estimate
Conception ≈ LMP + (Cycle length − 14)
For a standard 28-day cycle, conception is most likely on day 14 (the day of ovulation). For different cycle lengths, adjust the offset accordingly. Sperm can survive 3–5 days, so conception could occur within a 5-day window.
Worked example
LMP June 1 with a 30-day cycle: conception likely around June 17 (day 16). For a 28-day cycle, the same LMP gives June 15. The 5-day fertile window spans June 11–16 for a 28-day cycle.
Frequently asked questions
Is the estimated conception date exact?
No — it's a window. Sperm can fertilize an egg up to 5 days after intercourse, so the act of conception (intercourse) and the moment of fertilization can be several days apart.
Why does cycle length matter?
Ovulation timing varies. Most cycle length variability is in the first half (before ovulation); the second half (luteal phase) is more constant at ~14 days.
Does this work after IVF?
For IVF, conception equals the embryo transfer date plus a small offset. Use your clinic's embryo transfer date instead of the LMP-based estimate.