Conception Calculator
Estimate when conception likely happened, based on your last period and cycle length.
PregnancyScience-backed formula
Enter your details
Fill in the fields, then calculate
Please choose a date.
Enter your cycle length.
Awaiting Data
Enter your details and tap Calculate to see your results appear here.
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ResultYour result appears here.
How Conception Is Estimated
Conception usually happens around ovulation, near the middle of your cycle. From your last period, that lands roughly two weeks later for a 28-day cycle.
Key dates line up into an easy week by week timeline.
A longer or shorter cycle moves ovulation, so the tool adjusts the estimate.
Sperm can survive several days, so a fertile window surrounds the date.
How to Use This Calculator
- Choose the first day of your last period.
- Set your cycle length.
- Tap Calculate for the conception estimate.
The Formula Used
Conception ≈ last period + (cycle − 14) days
- last period
- First day of your last period.
- cycle
- Your average cycle length.
Understanding Your Results
Fertile window opens
About 5 days before
Sperm can wait for the egg.
Most likely day
Around ovulation
Peak chance of conception.
Window closes
About 1 day after
The egg's short lifespan.
Tips to Improve
- Track cycles to spot your pattern.
- Watch for ovulation signs like cervical mucus.
- Time intimacy across the fertile window.
- See a provider for a precise date.
Frequently Asked Questions
It is an estimate built on cycle averages. Actual ovulation can vary by a few days.
Ovulation tests and ultrasounds narrow it down better than dates alone.
It is the date most people know, and clinical dating starts there.
Scientific References
- Wilcox, A. J., et al. (1995). Timing of intercourse in relation to ovulation. New England Journal of Medicine, 333(23), 1517–1521.
- Dunson, D. B., et al. (2002). Changes with age in the level and duration of fertility. Human Reproduction, 17(5), 1399–1403.
- American College of Obstetricians and Gynecologists. (2017). Committee Opinion No. 700.