Due Date Calculator by Ovulation Date or Last Period

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This is a general estimate based on standard published formulas, not a medical assessment — it doesn’t account for your individual health history. Results are informational only and aren’t a substitute for advice from a doctor or qualified professional. Read the full disclaimer.

Due Date by Ovulation Date

If you know when you ovulated, your due date is 266 days — 38 weeks — after it. The more familiar “40 weeks” counts from the first day of your last period, about two weeks before ovulation, so both methods land on the same day when your cycle is a textbook 28 days. They differ when you ovulate earlier or later than day 14: someone who ovulated on day 18 would get a due date four days too early from the last-period method alone. That's why this tool lets you start from whichever date you actually know.

Two Related Estimates, Clearly Labeled

Due date estimation and ovulation window estimation both come from the same cycle-length math, so this tool covers them together — a due date from either your last period or a known conception date, with trimester progress shown visually, plus a separate ovulation window estimate for cycle planning. Every result here is a mathematical estimate, not a diagnosis or confirmed date.

How to Use This Tool

1Choose Due Date or Ovulation WindowTwo related but separate estimates.
2Enter the date you knowLast period start date (with your average cycle length), or your ovulation or conception date if you know it.
3Read the estimateDue date, current week and trimester, or the estimated fertile window.

Good to Know

Naegele's rule (adding 280 days to the last period) is a well-established starting estimate used widely in early pregnancy, but it assumes a standard 28-day cycle — this tool adjusts for a different average cycle length if entered.
An ultrasound dating scan, especially early in pregnancy, is generally more accurate than a date-based estimate alone, since it measures the fetus directly rather than working from cycle assumptions.
The fertile window estimate assumes a reasonably regular cycle — for those with irregular cycles, this estimate is considerably less reliable.
This tool is for general estimation and planning only — it does not replace a doctor's assessment, confirmed dating, or personalized medical guidance.

Frequently Asked Questions

How do I calculate my due date from my ovulation date?

Add 266 days (38 weeks) to your ovulation date. For example, ovulation on 1 March 2026 gives an estimated due date of 22 November 2026. Choose "From Ovulation Date" above and the tool does this for you, along with your current week and trimester.

Is a due date from ovulation more accurate than one from my last period?

It can be, if you know your ovulation date reliably — from ovulation test strips, basal body temperature tracking, or a fertility treatment timeline. The last-period method assumes ovulation around day 14 of a 28-day cycle, which isn't true for everyone. Either way it's an estimate; an early ultrasound dating scan is generally the most accurate.

How accurate is the due date estimate?

It's a well-established mathematical starting point (Naegele's rule), but only about 5% of babies are actually born exactly on their estimated due date — actual delivery commonly falls within a couple of weeks either side. An ultrasound dating scan is generally considered more precise.

What if my cycle isn't a regular 28 days?

Enter your actual average cycle length — the calculation adjusts the estimated ovulation and conception timing accordingly, though very irregular cycles make any date-based estimate meaningfully less reliable.

Should I use this instead of seeing a doctor?

No — this is for general estimation and planning only. For confirmed dates, medical guidance, and monitoring, please consult a doctor or healthcare provider.

Is this free to use?

Yes, completely free, with no account and no data saved anywhere.