Due Date CalculatorFree online fitness & health calculator — instant results, no sign-up
Estimated due date from your last period and cycle length.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the Due Date Calculator
This calculator estimates an expected delivery date based on the start date of your last menstrual period and the average duration of your menstrual cycle. It also calculates the remaining days between today and that projected delivery date. Expecting parents and healthcare providers routinely use this method as a baseline timeline for prenatal planning.
- First day of last period
- The calendar date marking the first day of bleeding in your most recent menstrual cycle.
- Cycle length
- The typical duration of your menstrual cycle from the first day of one period to the first day of the next, measured in days (typically 21 to 35 days, default 28).
Formula and method
Due Date = LMP + 280 days + (Cycle Length − 28 days) Days to Go = Due Date − Today
- LMPDate of the first day of the last menstrual period
- Cycle LengthAverage length of the menstrual cycle in days
- TodayCurrent calendar date
The tool applies Naegele's rule adjusted for menstrual cycle duration by adding 280 days (40 full weeks) to the first day of your last period, then shifting the date forward or backward by the difference between your cycle length and a standard 28-day cycle. 'Days to go' is calculated as the integer difference in calendar days between today's date and the calculated estimated due date.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- First day of last period1969-11-02
- Cycle length28 days
Results
- Due dateSun, August 9, 1970
- Days to go-20,539
How to use the Due Date Calculator
- 1Select the first day of your last menstrual period (LMP).
- 2Enter your typical menstrual cycle length in days.
- 3Compute the adjusted gestation span by adding (280 + Cycle Length − 28) days to the LMP.
- 4Subtract the current date from the calculated due date to determine days remaining.
Reading your result
The 'Due date' output displays the estimated day you reach 40 completed weeks of gestation based on menstrual timing. 'Days to go' indicates how many calendar days remain until that estimated delivery target. Reaching 37 to 42 weeks is generally considered full term.
Limitations
The calculation assumes regular menstrual cycle lengths and that ovulation occurs roughly 14 days before the end of each cycle. It does not account for irregular cycles, leap years or time zones differently than standard date math, or clinical adjustments determined via early first-trimester ultrasound.
Common questions
Why does cycle length change the due date?+
A standard 28-day cycle assumes ovulation happens around day 14. If your cycle is longer, ovulation typically occurs later, meaning conception happened later and shifting the due date further out.
What if my ultrasound date differs from this date?+
First-trimester ultrasound measurements of crown-rump length are considered the most accurate method for establishing gestational age. If an early ultrasound differs significantly from the period-based date, clinical guidelines recommend relying on the ultrasound date.
How many babies are actually born on their exact due date?+
Only about 4% to 5% of babies are born on their precise estimated due date. Most spontaneous births occur within a window spanning two weeks before to two weeks after the estimated date.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.