Conception Calculator by Last Period
Find your likely conception date and fertile days from your last period.
Find your likely conception date and fertile days from your last period.
How this calculator works
Enter your dates and the calculator applies standard obstetric rules: a pregnancy lasts about 280 days from the first day of the last period, ovulation usually happens about 14 days before the next period, and the fertile window covers the five days before ovulation plus ovulation day.
Frequently asked questions
How accurate is this calculator?
It gives an estimate based on average cycle and pregnancy length. Dating by ultrasound from your doctor is more precise.
Is my data stored?
No. Everything is calculated in your browser and nothing is sent anywhere.
What if my cycles are irregular?
Estimates are less reliable with irregular cycles. Ovulation tests or tracking basal body temperature can help, and your doctor can advise.
This tool is for information only and is not medical advice. Talk to a doctor or midwife about your pregnancy or fertility.
Related calculators
- Conception Date Calculator
Find the most likely conception date from your due date.
- Conception Calculator by Birth Date
Estimate when someone was conceived from their date of birth.
- Due Date Calculator
Estimate your due date from the first day of your last period.
- Ovulation Calculator
Predict ovulation days for your next three cycles.
- Fertile Window Calculator
See the days you are most likely to get pregnant.
- IVF Due Date Calculator
Calculate your due date from an embryo transfer date.
Further reading
- How Conception Works, Step by Step
- Signs of Ovulation: How to Tell When You're Ovulating
- Trying to Conceive With Irregular Periods
- Ovulation Predictor Kits: How to Use LH Tests
- How Conception Works, Step by Step
- How Due Dates Are Calculated (Naegele's Rule)
- Signs of Ovulation: How to Tell When You're Ovulating