Due month

Due day

Due 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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Bar chart showing how many days each dating method can drift — The 266-day rule gives a fertile window of November 16 to 27, 2025.

Method & verification

How This Tool Works

fact-checked against our calculation methodology

Calculation Method

Computes Estimated conception from Due month, Due day and Due 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.

Privacy: The calculation runs in this browser. Shared links contain the page address only, not your inputs or results.

Worked example: a due date of August 15, 2026

Say the ultrasound gives a due date of August 15, 2026. Day-of-year arithmetic: August 15 is day 227 of 2026. Subtracting 266 days lands you 39 days before January 1, which is day 326 of 2025, or November 22, 2025. That is the estimated conception date the tool returns.

The fertile window adds 6 days back and 5 days forward: November 22 minus 6 days is November 16, plus 5 days is November 27. So the full output reads: conception November 22, 2025, fertile window November 16 to 27, 2025. If your due date instead came from a first-trimester ultrasound rather than a last period, this window narrows in practice to a few days, because a 9-week scan pins the due date to within about 5 to 7 days while period-based dating can be off by 2 weeks or more.

Where this estimate breaks down

The fixed 266-day gap assumes every pregnancy lasts exactly 38 weeks from conception, but real deliveries cluster in a distribution: in a large cohort, only about 4 to 5 percent of babies arrive exactly on the due date, and roughly 70 percent within 10 days either side. Sperm survival pushes the true conception day up to 5 days earlier than the ovulation window's start, since sperm can live that long before fertilizing an egg.

Two edge cases matter most. First, an early due date guess built from a remembered last period is wrong for a sizable share of users: studies find ovulation day varies by 7 or more days from the textbook day 14 in a large fraction of cycles, which shifts the entire window. Second, the date-only math ignores leap years only inside its own calendar object, so inputs crossing February 29 are handled correctly, but a due date before 2020 or after 2035 is rejected outright. If your dates came from IVF transfer, use a calculator built for that; embryo age changes the offset entirely.