Formula
Add 280 days to the first day of the last period and adjust by the difference between the entered cycle length and 28 days.
UK pregnancy reference
UK Pregnancy Due Date Calculator
Estimate due date and pregnancy week from the first day of the last period and cycle length. NHS care and scan dating may change the date.
Estimated due date
Enter values to calculate Calculation basis date: 17 May 2026. Last period plus 40 weeks, adjusted for cycle length.Calculation Basis and Official Sources
Calculation basis date: 17 May 2026. Uses the common pregnancy estimate from the first day of the last period.
For pregnancy confirmation, pain, bleeding, medication, symptoms or appointment timing, use NHS, midwife or medical care.
Formula and Interpretation
How to read it
Use it to estimate timing. NHS care may revise dating after scans and clinical review.
This calculator and its examples are for general information and rough estimation only. They are not medical diagnosis, treatment advice, pregnancy care advice, nutrition advice, emergency guidance, legal advice, tax advice, insurance advice or any other professional advice.
Actual health, pregnancy and nutrition decisions can differ because of symptoms, age, pregnancy status, medical history, medication, clinical measurements, ultrasound results and a clinician's judgement. Use a qualified health professional or medical service before acting on the result.
Calcitda does not guarantee accuracy, completeness, currentness or suitability for your situation. To the fullest extent permitted by law, we disclaim liability for losses, damages, claims or decisions arising from use of, or reliance on, this page.
Before You Rely on the Result
Practical examples
Enter the first day of the last period to estimate the due date.
Use a longer cycle if cycles are usually longer than 28 days.
Do not use the result for urgent symptoms or medication decisions.
What this includes
- Estimated due date
- Estimated pregnancy week and days
- Days left to the estimated due date
What this does not include
- NHS scan dating and clinical history
- IVF transfer dates, ovulation confirmation and multiple pregnancy
- Diagnosis, emergency advice and treatment
When to check official sources
Use NHS, a midwife or medical service for diagnosis, symptoms, medication, appointment timing or official dating.