Due Date Calculator
Estimate a pregnancy due date from the first day of the last menstrual period and an optional cycle length. The calculator starts with the conventional 280-day, 40- week estimate and adjusts for a cycle longer or shorter than 28 days. It is not a diagnosis, an ultrasound measurement, or a substitute for prenatal care. Ovulation can vary, dates can be uncertain, and a clinician may revise the estimate. The date calculation happens locally in your browser.
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Add to Chrome — freeWhat it does
- Last-period method
- Cycle-length adjustment
- Estimated date
- Local calculation
How to use Due Date Calculator
- 1
Enter the first day
Select the first day of the last menstrual period, not the day bleeding ended.
- 2
Add cycle length
Leave 28 for the conventional cycle or enter a different typical length when known.
- 3
Discuss the estimate
Use the displayed date as a planning estimate and confirm it with a qualified prenatal-care provider.
How it works
Keep the assumptions visible
Write down the selected period date, cycle length, and calculation method with the result. If the date is uncertain, preserve that uncertainty rather than treating the displayed day as exact. Cycle irregularity, ovulation timing, ultrasound dating, and clinical history can all change the estimate. Use this page to prepare questions and calendar reminders, not to assess symptoms or replace prenatal care. A provider's documented dating method should take precedence when it differs from the conventional 280-day calculation.
The page parses the selected local date at midday to avoid common daylight-saving date boundary surprises, adds 280 days, and then adds the difference between entered cycle length and 28 days. The output is formatted with the browser's locale. It does not use conception date, embryo measurements, timezone location, or clinical records.
Use it as a planning estimate
The result can help organize questions, appointments, or a calendar, but it should not be used to make medical decisions. Keep the source date and cycle assumption with the estimate, because changing either changes the answer. A clinician's documented dating method should take precedence when the two disagree.
Date boundaries
Planning checklist
Keep the selected last-period date and cycle length with the estimate, and mark which assumption is uncertain. A remembered date, irregular cycle, or later clinical dating can move the estimate. Use the result for calendar planning and questions, not for deciding whether a symptom is urgent or whether prenatal care can be delayed.
The page uses the calendar date you select and does not know your timezone history or the exact time a period began. Leap years are handled by the browser's date arithmetic. If the last-period date is uncertain, irregular, or remembered only approximately, keep that uncertainty visible when discussing the result. A due-date estimate is a useful calendar marker, not a precision timestamp or a prediction of the actual birth date.
For planning, save the input date and cycle assumption with the result instead of saving only the final date. That makes it easier to explain why another method differs. The calculator is intentionally modest: it gives a conventional calendar estimate and leaves clinical dating, examination, and follow-up to the care team.
Examples
Standard 28-day cycle
The method adds 280 days to the selected first day.
Longer cycle adjustment
The adjustment reflects the simple assumption that ovulation is later by the cycle difference.
Frequently asked questions
How accurate is a due date calculator?
It is an estimate based on a conventional 280-day pregnancy and the date supplied. Ovulation, implantation, cycle irregularity, and date memory can all change the real timeline. Ultrasound and clinical assessment may produce a different estimate, and only a small portion of births occur exactly on the calculated date.
Why does cycle length change the estimated date?
The simple adjustment assumes ovulation occurs later in a cycle longer than 28 days and earlier in a shorter one. It shifts the conventional date by the difference in days, but it cannot measure your actual ovulation. Use the cycle value that reflects your usual pattern and discuss uncertainty with your provider.
Does this calculator replace prenatal care?
No. It cannot assess pregnancy health, gestational complications, fetal growth, or urgent symptoms. Contact a qualified clinician for confirmation and care, and seek urgent medical help for symptoms your care team identifies as emergencies rather than relying on an online date estimate.