How to use the Pregnancy Calculator
Enter the first day of last period, average cycle length and as of date and the result appears straight away. Everything runs in your browser, so nothing you type is uploaded.
The formula
Due date = first day of last period + 280 days (adjusted for cycle length)
Examples
- Last period started 1 Sep 2026, 28-day cycle
Estimated due date: Tue, June 8, 2027 Gestational age: 10 weeks 0 days (trimester 1) Days to go: 210 End of first trimester (13 weeks): Tue, December 1, 2026
- Same date with a 32-day cycle
Estimated due date: Sat, June 12, 2027 Gestational age: 9 weeks 3 days (trimester 1) Days to go: 214 End of first trimester (13 weeks): Sat, December 5, 2026
- Last period 15 Aug 2026, 28-day cycle
Estimated due date: Sat, May 22, 2027 Gestational age: 12 weeks 3 days (trimester 1) Days to go: 193 End of first trimester (13 weeks): Sat, November 14, 2026
Limitations and assumptions
Estimate only. Only about 5% of babies arrive on the due date. A dating scan gives a more accurate date. General estimate for adults, not medical advice. Speak to a doctor or registered dietitian before making significant changes to diet, exercise or health routines.
Frequently asked questions
How is the due date calculated?
It adds 280 days (40 weeks) to the first day of your last period, adjusted if your cycle is longer or shorter than 28 days.
Why does pregnancy start before conception?
Gestational age is counted from the last period because the date of conception is rarely known precisely.
How many people deliver on their due date?
Only a small share do. Most births happen within a couple of weeks either side.
Should I rely on this for care?
No. Your midwife or doctor will confirm dates, often with an ultrasound.