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How a gestational age calculator turns an LMP date into weeks — LMP March 10, 2026 gives a due date of December 15, 2026 and 24w 0d on August 25.

Method & verification

How This Tool Works

fact-checked against our calculation methodology

Calculation Method

Computes Gestational age from LMP month, LMP day and LMP year.

Site methodology

BoringToolsKit methodology and verification standard

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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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A Worked Example: LMP March 10, 2026

Step 1: the tool adds 280 days to the LMP. March 10, 2026 falls on day 69 of the year (31 for January, 28 for February, 10 for March). Adding 280 gives day 349, which lands on December 15, 2026. That is the estimated due date, and it matches the tool exactly because the same 280-day constant drives both.

Step 2: gestational age. If today is August 25, 2026, the elapsed count is 168 days (day 237 minus day 69). Dividing: 168 / 7 = 24 remainder 0, so the display reads 24w 0d. Try a three-day offset instead: August 28 is 171 days out, and 171 / 7 = 24 remainder 3, so it reads 24w 3d.

Notice the floor function at work: the tool never rounds up. A count of 167 days still reads 23w 6d, not 24w, because 167 / 7 = 23 with remainder 6. Dates before the year 2020 or after 2035 are rejected outright, so the tool cannot produce a negative age by accepting a future LMP.

Where Naegele's Rule Breaks Down

The 280-day constant assumes a 28-day cycle with ovulation on day 14. Cycle-length studies put average cycle length near 29.3 days, and only roughly 30 percent of women fall inside the 26 to 32 day window the rule quietly assumes. A 35-day cycle shifts ovulation about 7 days later, which moves the true due date roughly a week past what the calculator reports.

The calendar arithmetic adds a second bias: February. A pregnancy spanning February ages about 1 day slower in Naegele's terms than one that does not, because the tool counts real calendar days (365-day years, 28-day February) rather than 30-day months.

In practice, first-trimester ultrasound (ideal window 11w0d to 13w6d) reassigns the due date whenever it disagrees with the LMP estimate by more than about 5 to 7 days, and about 40 percent of LMP-based dates get revised that way. Treat this tool as a day-one estimate, and let a crown-rump length measurement supersede it.