Conception Date Calculator
If you know your due date or the first day of your last period, you can work backwards to estimate when conception happened. Conception takes place at fertilisation, usually within a day of ovulation, and pregnancy is dated from the last period — so conception is typically about two weeks after the first day of that period, or 266 days before the due date. This calculator gives you the single most likely date and a few-day window around it, because sperm can survive for several days before an egg is released, and ovulation timing varies. It is a curious-minded planning tool: it cannot determine paternity or the exact day, and an early ultrasound is better at dating a pregnancy.
How to use the conception date calculator
- Choose whether you know your due date or the first day of your last period.
- Enter that date, and your usual cycle length if you chose last period.
- Read the estimated conception date, the likely window and the matching due date or period date.
- Treat the result as a best estimate — your clinician’s ultrasound dating takes precedence.
Formula
From the due date: conception ≈ due date − 266 days. From the last period: conception ≈ ovulation ≈ first day of last period + (cycle length − 14) days. The window shown spans three days before to one day after the estimate. Pregnancy dating counts from the last period, so gestational age at conception is about two weeks.
Worked examples
Working back from a due date
A due date of 8 October 2026 gives an estimated conception date of Thursday, January 15, 2026, within a window of Monday, January 12, 2026 – Friday, January 16, 2026.
From the last period
A last period beginning 1 January 2026 and a 28-day cycle gives Thursday, January 15, 2026, with a due date of Thursday, October 8, 2026.
A longer cycle moves conception later
With a 35-day cycle and the same period date, the estimate moves to Thursday, January 22, 2026.
How dating and conception relate
Doctors count pregnancy from the first day of your last period because it is observable, while conception usually takes place about two weeks later. That is why a pregnancy of “40 weeks” is really around 38 weeks from conception. If your cycle is not 28 days, ovulation and conception shift accordingly. Compare dates with the due date calculator and map your cycle with the ovulation calculator.
Practice problems
Try these yourself first, then check your answer. The answers come from the calculator above.
- The due date is 20 December 2026. When was conception likely?
Show answer
Estimated conception date: Sunday, March 29, 2026 · Likely conception window: Thursday, March 26, 2026 – Monday, March 30, 2026
Common mistakes to avoid
- Expecting a single exact day. Sperm survive up to five days, so intercourse earlier than the estimate can still be the cause.
- Ignoring cycle length when using the last-period method.
- Using the result to assign paternity or settle disputes. It is not a legal or clinical tool.
- Forgetting that IVF and assisted conception have known dates, so no estimate is needed.
- Relying on the estimate over an ultrasound. Scans date a pregnancy more accurately.
Frequently asked questions
How do I calculate my conception date?
Subtract 266 days from your due date, or add your cycle length minus 14 days to the first day of your last period.
How accurate is a conception date estimate?
It is accurate to within a few days at best. It depends on when you ovulated, and sperm survive up to five days.
Why is pregnancy dated from the last period?
It is the one date most people know. Conception is generally about two weeks later, so by convention gestational age includes those two weeks.
Does cycle length matter?
Yes, because ovulation happens about 14 days before the next period; in a longer cycle ovulation, and conception, are later.
Can this tell me who the father is?
No. It cannot determine paternity — only a DNA test can.
Is this medical advice?
No. It is an educational estimate; consult your healthcare provider for prenatal care.
Sources and further reading
Reviewed September 26, 2026 by the SolveCalcPro editorial team. Found a mistake? Tell us; see our editorial policy.