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Conception calculator

Subtract 266 days from an estimated due date to find the conventional estimated conception date.

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Answer

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Subtract 266 days from an estimated due date to find the conventional estimated conception date.

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Formula and assumptions
The 266-day interval is the 280-day pregnancy-dating convention minus the assumed 14 days before ovulation. This is an approximate calendar back-calculation, not the known date of fertilization or intercourse. Actual ovulation and pregnancy duration vary. It cannot establish paternity; use your clinician's dating for care.
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Using Conception calculator
Subtract 266 days from an estimated due date to find the conventional estimated conception date.
The 266-day interval is the 280-day pregnancy-dating convention minus the assumed 14 days before ovulation. This is an approximate calendar back-calculation, not the known date of fertilization or intercourse. Actual ovulation and pregnancy duration vary. It cannot establish paternity; use your clinician's dating for care.
Next: Pregnancy calculator
Next: Period calculator
A worked example
For estimated due date (yyyy-mm-dd) = 2026-10-08, conventional estimated conception date = 2026-01-15.
Before you use the result
The 266-day interval is the 280-day pregnancy-dating convention minus the assumed 14 days before ovulation. This is an approximate calendar back-calculation, not the known date of fertilization or intercourse. Actual ovulation and pregnancy duration vary. It cannot establish paternity; use your clinician's dating for care.
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ToolOctopus. “Conception calculator.” Updated 2026-10-03. https://tooloctopus.com/conception-calculator.
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