Pregnancy Calculator
Estimate your due date, conception date, and current pregnancy week using your last menstrual period.
Reviewed October 7, 2026 Runs in your browser — inputs are never stored
This tool provides general educational estimates only and is not medical advice. It does not diagnose, treat, or replace guidance from a qualified healthcare provider. Consult a doctor for decisions about your health.
About the Pregnancy Calculator
A pregnancy due date calculator is a planning tool that estimates your baby's arrival and your current pregnancy week from the first day of your last menstrual period. It is designed for expectant parents eager to track milestones, partners and family preparing for the new arrival, and anyone organising maternity leave, appointments, or a nursery. Using Naegele's rule, the calculator adds 280 days to your last period and adjusts for cycle length, then shows your estimated due date, conception date, current week and day, and current trimester. This gives you a clear timeline for booking scans, planning announcements, and preparing practical arrangements. Because only about four percent of babies arrive on the exact calculated date, the result is a guide rather than a guarantee, and most births fall within two weeks of it. The tool runs entirely in your browser for privacy. Always confirm dates and any health concerns with your midwife or obstetrician, who can refine the estimate using ultrasound measurements.
How to use it
- 1Enter the first day of your last menstrual period (LMP).
- 2Adjust average cycle length if it is not 28 days.
- 3Calculate estimated due date, conception window, and current week.
- 4Note trimester boundaries for planning appointments.
- 5Confirm dates with your midwife or clinician and ultrasound.
Worked example
LMP on 1 January with a 28-day cycle → estimated due date around 8 October (Naegele’s rule: LMP + 280 days). At 1 April you are roughly 12 weeks 6 days, late first trimester. Only a small share of births land on the exact EDD — plan a two-week window.
How it works
How it works
The standard estimate (Naegele’s rule) adds 280 days (40 weeks) to the first day of your last menstrual period, adjusted for cycle length.
Trimesters
- First: weeks 1–12
- Second: weeks 13–26
- Third: weeks 27–40
Who it is for
Expectant parents tracking milestones and preparing for the baby, partners and family planning announcements and leave, and anyone organising scans, a nursery, or maternity arrangements.
Why it helps
Only about 4% of babies arrive on the exact calculated date, but the estimate gives a clear timeline for booking scans, planning practical arrangements, and recognising when to seek care.
Tips for better results
- Enter the first day of your last period as accurately as possible — a few days off shifts the due date by the same amount.
- Adjust the cycle length if yours isn’t 28 days; longer cycles push the due date later, shorter ones earlier.
- Use the week and trimester readout to anticipate when to book scans and maternity leave.
- Confirm the date with a first-trimester ultrasound, which is more accurate than a calendar estimate.
Common mistakes to avoid
- Entering the date of conception or last intercourse instead of the first day of the last menstrual period.
- Leaving the cycle length at 28 when yours is longer or shorter, which misdates the estimate.
- Treating the due date as a deadline — most births fall within two weeks either side of it.
- Using the estimate to make medical decisions instead of confirming dates with a midwife or obstetrician.
Frequently asked questions
A:It is an estimate. Most births occur within two weeks either side of the calculated date.
A:Enter your average cycle length and the estimate is adjusted accordingly.
A:No. Use it for general guidance and confirm all dates with your doctor or midwife.
Reviewed for accuracy · Last updated October 7, 2026
Formulas follow recognised clinical references — including WHO BMI classifications, the Mifflin-St Jeor equation, and Naegele’s rule — and are periodically checked against published reference values.
CalcWisee Editorial Team
Calculator development and formula review
CalcWisee is an independent site founded by Ayman. Every calculator is built from a published formula or official definition, tested against hand-worked examples before it goes live, and corrected promptly when a reader reports an issue to ayman@calcwisee.com.
Expertise: Financial mathematics, Health and fitness formulas, Applied mathematics, Unit conversion standards
References & sources
Related guides
Pregnancy Due Date Calculator: Estimate Your Baby's Arrival
Estimate your baby's expected delivery date, track pregnancy week by week, and understand your baby's development — with our free Pregnancy Due Date Calculator.
Age Calculator: Calculate Exact Age in Years, Months & Days
Calculate your exact chronological age in years, months, weeks, days, hours, and minutes instantly — with full support for leap years, varying month lengths, and any reference date.
Understanding BMI and a Healthy Weight Range
Learn how BMI is calculated, what each category means, a worked example, and the real limits of BMI as a health measure.