Estimate the conception date and the last period from a known due date. Enter your values; the result updates as you type, with the method shown below.
Formula
Conception ≈ due date − 266 days
Worked example
Due date 8 May 2027: conception around 15 August 2026.
Frequently asked questions
How precise is it?
A few days either side; ultrasound dating is more accurate.
Why 266 days?
Pregnancy lasts about 38 weeks from conception.
Can it tell paternity?
No, it is only an estimate.
Related calculators
- Target Heart Rate Calculator (Training Zones)
- Pregnancy Weight Gain Calculator
- All calculators in this category
Results are estimates for information only. Check important figures with a professional.