LMP month

LMP day

LMP year

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General estimate for planning only — not medical advice. Consult a doctor about your specific situation.

Calculation details

Every figure above is calculated locally in your browser from the assumptions shown. No inputs are sent anywhere. See the methodology section below for how this tool works.

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How the 280 day due date estimate is calculated — 280 days from LMP puts a January 8 LMP at October 15.

Method & verification

How This Tool Works

fact-checked against our calculation methodology

Calculation Method

Computes Estimated due date from LMP month, LMP day and LMP year.

Site methodology

BoringToolsKit methodology and verification standard

No unrelated authority link is added merely to make this page look sourced. Tool-specific external sources are shown only when they directly support the method, rate, or rule.

Assumptions to Verify

These tools provide general estimates for planning and education only.
They are not medical advice and not a substitute for a qualified healthcare professional.
Always consult your doctor or another healthcare provider about your specific situation.

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Worked example: LMP of January 8, 2026

Enter January 8, 2026 as the last menstrual period. The calculator adds 280 days to that date, which lands on October 15, 2026. That is the estimated due date, and it is simply a date-translation of Naegele's rule: add one year, subtract three months, add seven days to the first day of the LMP (Jan 8, 2027, minus 3 months = Oct 8, plus 7 days = Oct 15). Both methods agree because they encode the same 280-day interval.

The 280 days come from a 28-day cycle: 14 days to ovulation plus 266 days from conception to birth. If your cycle runs 32 days, ovulation happened around day 18 instead of day 14, so the clinical estimate shifts about 4 days later, to roughly October 19. The tool also reports current gestational age: on February 19, 2026, exactly 42 days have elapsed since January 8, which is 6.0 weeks, displayed as week 6.

Where the estimate drifts in practice

Only about 4 in 100 babies arrive on the projected date, and roughly 70 in 100 deliver within 10 days either side of it. First pregnancies average closer to 40 weeks and 5 days; subsequent births average nearer 40 weeks and 2 days. The formula also assumes a 28-day cycle with ovulation on day 14, so anyone with cycles of 35 days or more can be miscategorized by a week or more on the calendar alone.

This is why first-trimester ultrasound dominates real-world dating. A crown-rump length measured between weeks 7 and 13 predicts gestational age within about 5 to 7 days, versus 10 to 14 days for LMP dating, and clinicians reassign the due date when the two disagree by more than about 7 days before week 9. The LMP method also fails outright when the date is uncertain, cycles were irregular, hormonal contraception was recently stopped, or bleeding in early pregnancy was mistaken for a period; in those cases the ultrasound number is the only reliable one.