Reverse Due Date Calculator (When Did I Conceive?)
Work backwards from a due date or birth date to the likely conception window.
Because sperm survive up to five days and ovulation timing varies, conception is a window, not a date — intercourse days before ovulation can still be 'the day'. If you're working from an actual birth date rather than the EDD, remember the baby may have arrived early or late against the 266-day clock.
Formula
For general information only. Pregnancy and infant care decisions must be guided by your treating clinician or midwife — always defer to their assessment.
Work backwards from a due date or birth date to the likely conception window. The Reverse Due Date Calculator (When Did I Conceive?) is a free, private reverse due date tool — every result computes instantly in your browser with no sign-up and no data upload.
About Reverse Due Date Calculator (When Did I Conceive?)
Because sperm survive up to five days and ovulation timing varies, conception is a window, not a date — intercourse days before ovulation can still be 'the day'. It applies conception ≈ EDD − 266 days (±3 for ovulation timing; sperm survive ~5 days). Use the Reverse Due Date Calculator (When Did I Conceive?) to get an instant, clearly-explained result with the working shown step by step — free, private and with the source method cited.
How to use Reverse Due Date Calculator (When Did I Conceive?)
- 1Enter your details in the Reverse Due Date Calculator (When Did I Conceive?) input fields above.
- 2The result updates instantly with the working and reference bands shown.
- 3Adjust any value to explore how it changes the outcome — it's free and unlimited.
Why use Reverse Due Date Calculator (When Did I Conceive?)?
- ✓Instant reverse due date result that recomputes as you type — no waiting, no page reloads
- ✓100% client-side: your health data never leaves your browser
- ✓Shows the actual formula and your numbers substituted in, so you can see exactly how the result is reached
- ✓Based on published, citable sources (Wilcox AJ et al., NEJM 1995 — timing of conception relative to ovulation)
- ✓Free forever with no sign-up, account or app install
Frequently asked questions
How does the Reverse Due Date Calculator (When Did I Conceive?) work?+
Because sperm survive up to five days and ovulation timing varies, conception is a window, not a date — intercourse days before ovulation can still be 'the day'. The calculation uses the formula: conception ≈ EDD − 266 days (±3 for ovulation timing; sperm survive ~5 days). Everything runs instantly in your browser as you type — your inputs are never uploaded.
Is the Reverse Due Date Calculator (When Did I Conceive?) accurate, and what is it based on?+
The method is traceable to authoritative sources: Wilcox AJ et al., NEJM 1995 — timing of conception relative to ovulation. Results are estimates — individual variation always applies, so treat the output as a well-grounded starting point.
Is the Reverse Due Date Calculator (When Did I Conceive?) free and private?+
Yes. It's completely free with no sign-up, and all computation happens locally in your browser, so none of your health data ever leaves your device.
Can I use this for medical decisions?+
This tool is informational only. Always defer to your treating clinician or midwife for pregnancy and infant care decisions.
What can I use the Reverse Due Date Calculator (When Did I Conceive?) for?+
It's commonly used for reverse due date, when did i conceive, conception date from due date. Work backwards from a due date or birth date to the likely conception window.
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