Due Date Calculator
Calculate your Pregnancy due date in seconds with the free Due Date Calculator. No registration needed.
Last period
Due date
What is a Due Date Calculator?
A Due Date Calculator estimates the date your baby is likely to arrive, based on the first day of your last menstrual period (LMP) and your average cycle length. It uses Nägele’s rule, the standard clinical method for estimating gestational age.
How to calculate due date manually (the formula)
Nägele’s rule: EDD = LMP + 280 days − (28 − cycle length) days.
For a 28-day cycle: simply add 280 days (40 weeks) to the first day of your LMP. For a shorter cycle, subtract the difference. For a longer cycle, add the difference.
Example calculation
LMP = 15 March 2025, cycle = 28 days. EDD = 15 March + 280 days = 22 December 2025. If the cycle were 32 days instead: EDD = 15 March + 280 + (32 − 28) = 26 December 2025.
Common mistakes
- Using the conception date instead of LMP — The due date is calculated from the first day of your last period, not the date of conception. Conception typically occurs about 2 weeks after LMP.
- Ignoring cycle length — A 35-day cycle shifts ovulation and conception later. Using the default 28-day assumption for a long cycle gives an earlier due date than expected.
- Treating the due date as exact — Only 4–5% of babies are born on their exact due date. The 40-week mark is a midpoint, not a deadline.
Frequently asked questions
How is my due date calculated?
The most common method is Nägele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a standard 28-day cycle.
What if my cycle is longer or shorter than 28 days?
You can adjust for cycle length: for each day your cycle deviates from 28 days, shift the due date by the same number of days. This calculator lets you enter your cycle length.
How accurate is the estimated due date?
Only about 5% of babies arrive exactly on their due date. A normal pregnancy can range from 37 to 42 weeks. The due date is a best estimate, not a guarantee.
Can an ultrasound change my due date?
Yes. If a first-trimester ultrasound shows a significant discrepancy (usually 7+ days) from your LMP-based date, the ultrasound date is considered more accurate.
When does the third trimester start?
Pregnancy is divided into three trimesters: first (week 1–12), second (week 13–26), and third (week 27–40). Your current trimester is shown alongside your due date.
What is Nägele’s rule?
Nägele’s rule: subtract 3 months from the first day of your LMP, then add 7 days. For example, LMP of 15 March → subtract 3 months (15 December) → add 7 days = 22 December.