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How Do I Calculate My Due Date from Ovulation Date?

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Quick answer: How do I calculate my due date from my ovulation date? Add 266 days (38 weeks) to the date of ovulation or conception. Example: ovulation on 4 April 2026 → due date 26 December 2026. If you only remember your last period, add 14 days first (for a 28-day cycle), then 266 more. An early ultrasound remains the most accurate dating.

Enter the first day of your last period — your estimated due date, how far along you are, and your trimester dates appear instantly.

Estimated due date
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You are
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Conception (est.)
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Trimester ends
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Based on Naegele's rule (LMP + 280 days, adjusted for cycle length). This is an estimate for planning — your doctor or midwife will confirm dating with an early ultrasound.

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How do I calculate my due date from my ovulation date

Three steps: (1) pin down your ovulation or conception date — from ovulation tracking, test strips, or (for a regular 28-day cycle) 14 days after your last period started; (2) add 266 days (38 weeks); (3) that is your estimated due date.

Worked example: ovulation on 4 April 2026 → 4 April + 266 days = 26 December 2026.

Only remember your last period? The free calculator above estimates your ovulation date automatically (last period + cycle length − 14 days) and counts 280 days from the period instead. For irregular cycles an ovulation-day guess is less reliable — enter your average cycle length above, and know that a first-trimester ultrasound is the most accurate dating there is.

How the due date is calculated

Doctors count pregnancy as 40 weeks (280 days) starting from the first day of your last menstrual period — even though conception happens about two weeks later. This is called Naegele's rule. If your cycle is longer or shorter than 28 days, the estimate shifts: each extra cycle day pushes ovulation (and the due date) roughly one day later.

The three trimesters

How accurate is it?

Think of the due date as the middle of a range, not an appointment. Only about 1 in 20 babies arrives exactly on the date; most come within two weeks on either side. A first-trimester ultrasound measures the baby directly and is the most accurate way to date a pregnancy — it can shift the estimate by several days.

Frequently asked questions

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

How do I calculate my due date from my ovulation date? Add 266 days (38 weeks) to the ovulation or conception date — the same as counting 280 days from your last period for a textbook 28-day cycle. The calculator above does this automatically when you enter your period date and cycle length.

Can I use my ovulation date with an irregular cycle?

You can, but an ovulation-day guess is less reliable when cycles vary. Enter your average cycle length in the calculator above so it can adjust the estimate — and for the most accurate dating, a first-trimester ultrasound measures the pregnancy directly and can shift the estimate by several days.

How is a pregnancy due date calculated?

Doctors use Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your average cycle length. This calculator applies it automatically.

What is Naegele's rule?

Naegele's rule estimates the due date as the first day of the last period plus one year, minus three months, plus seven days — equivalent to 280 days for a 28-day cycle.

How accurate is the due date?

It is an estimate: only about 5% of babies arrive exactly on their due date. Most are born within two weeks before or after. An early ultrasound gives the most accurate dating.

What are the three trimesters?

First trimester: weeks 1–13. Second trimester: weeks 14–27. Third trimester: weeks 28–40. Each brings different development milestones and symptoms.

Is this due date calculator free?

Yes. It is 100% free with no signup required.

Why is pregnancy counted as 40 weeks instead of 9 months?

Because obstetrics counts 280 days from the last menstrual period — exactly 40 weeks. Calendar months vary in length, so '9 months' is approximate; weeks give every provider and test schedule a precise common language.

What if I conceived through IVF — which date do I use?

Use the embryo transfer date: a day-5 embryo transfer counts as 19 days of pregnancy already (14 + 5), so add 261 days to the transfer date. This calculator's ovulation-date option (transfer date minus embryo age) approximates the same result.

Can my due date change later in pregnancy?

Yes, slightly. If an early ultrasound (ideally weeks 8–12) measures the baby more than a few days off from the LMP-based date, providers usually adopt the ultrasound date — it is the most accurate dating available.

How due dates are calculated

Obstetrics counts pregnancy as 280 days — exactly 40 weeks — from the first day of the last menstrual period (LMP), not from conception (which usually happens ~14 days later, hence the "two weeks pregnant at conception" quirk). The classic shortcut is Naegele's rule: LMP + 7 days − 3 months + 1 year. If you know the ovulation or conception date instead, add 266 days (38 weeks). The date anchors everything else: trimesters (weeks 1–13, 14–27, 28–40), screening schedules, and growth charts all count from it.

Worked example: LMP March 10, 2026

Add 280 days to March 10, 2026: March has 21 days remaining after the 10th, then April (30), May (31), June (30), July (31), August (31), September (30), October (31), November (30) = 265 days, leaving 15 → December 15, 2026. (Naegele's rule gives December 17 — the two-day gap is just calendar rounding; both mean "mid-December.") Week 40 lands exactly on a Tuesday, 40 × 7 days later, same weekday as the LMP.

Practical tips and limits

Only about 5% of babies arrive on the due date — think "due month," with 37–42 weeks all full-term. An early ultrasound (weeks 8–12) dates pregnancy more precisely than LMP and your provider may adjust the date by a few days. Track milestones by week, not by the single date, and you will never be misled.

Trimesters and what happens when

The due date anchors the whole schedule. First trimester (weeks 1–13): dating ultrasound, first screening bloods. Second trimester (14–27): the anatomy scan around week 20, glucose screening near week 24–28. Third trimester (28–40): growth checks, Group B strep test, birth planning. Key milestones count from the same LMP: quickening (first movements) ~weeks 16–20, viability ~week 24. When a provider says "you're 24 weeks," they mean 24 weeks from the LMP date this calculator starts from — one consistent clock for every appointment, test, and decision.

Key takeaways

  • 280 days (40 weeks) from the first day of the last period; ~266 days from ovulation.
  • Only ~5% arrive on the date — think "due month."
  • Early ultrasound (weeks 8–12) gives the most precise dating.

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