Due Date Calculator
Estimate your baby's due date from your last menstrual period, or work backward from a due date.
About this calculator
This calculator uses Naegele's rule — the standard method used by most healthcare providers — which estimates a due date as 280 days (40 weeks) from the first day of your last menstrual period (LMP). It also works in reverse: enter a due date to estimate the corresponding LMP. This is an estimate based on average cycle length; only about 5% of babies are actually born on their exact due date.
Frequently asked questions
- How accurate is a due date calculated from LMP?
- It's an estimate based on a standard 28-day cycle with ovulation on day 14. Actual cycle length varies, so an early ultrasound is generally considered more accurate than LMP-based dating alone — your provider may adjust your due date based on ultrasound measurements.
- What if I don't remember my last menstrual period exactly?
- Use your best estimate. If you know your due date from an ultrasound instead, use the reverse mode to work backward — that will generally be more accurate than a remembered LMP date.
- Why 280 days?
- 280 days (40 weeks) is counted from the first day of your last period, not from conception — even though conception typically happens about two weeks after that. This is the standard convention used in prenatal care.
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🔒 All calculations run entirely in your browser — nothing you enter is ever sent anywhere. These are estimates based on standard averages, not medical advice — consult your healthcare provider for guidance specific to you.