Due Date Calculator
Estimate your due date from last period, conception date or ultrasound.
About Due Date Calculator
Due date is estimated with Naegele's rule: 280 days (40 weeks) from the first day of your last menstrual period, adjusted if your cycle differs from 28 days.
Formula
Naegele's rule
EDD = LMP + 280 days + (cycle length − 28 days)
- 28
- Standard cycle length assumed by Naegele's rule
- 280
- Standard gestational period in days (40 weeks)
- EDD
- Estimated due date
- LMP
- First day of last menstrual period
- C
- Average cycle length in days
Worked example
With the default values below, Due Date Calculator returns Estimated Due Date of —. Change any input above and every figure here is recalculated in your browser.
Inputs used
- First Day of Last Period
- Average Cycle Lengthaccepts 20–45 days
- 28 days
Results
- Estimated Due Date
- —
- Gestational Age Today
- Enter your last period date
- Trimester
Step by step
- 1Waiting for the first day of your last periodYour due date is counted forward from that date, so it's the one input this calculator can't estimate for you.
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- This is an estimate only and not a substitute for professional prenatal care. Always confirm your due date with your doctor or midwife.
- This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Due Date Calculator — full guide
Counting forward from one date
Pregnancy dating starts from the first day of your last menstrual period, not the day you suspect conception happened, because the period is the event with a date you can actually recall. Clinicians use the same convention, which is why the gestational age you are quoted at an appointment is measured from that day.
Naegele's rule
EDD = LMP + 280 days + (cycle length − 28 days)
The 280 days are 40 weeks of gestation. The second term is the cycle adjustment: Naegele's rule assumes a 28-day cycle with ovulation around day 14, so a longer cycle implies later ovulation and pushes the date back, while a shorter cycle pulls it forward. With the default 28-day cycle that term is zero and the rule is simply LMP plus 280 days.
Worked example
Last period beginning 5 January 2026, average cycle 32 days:
- Base date: 5 January 2026 + 280 days = 12 October 2026
- Cycle adjustment:
32 − 28 = +4days - Estimated due date: 12 October + 4 days = 16 October 2026
The same first day with a 28-day cycle stays on 12 October; with a 24-day cycle it moves to 8 October. Each day of cycle length is worth exactly one day of due date.
Gestational age and trimester
Gestational age is the plain day count since your LMP, split into weeks and days. From 5 January to 16 March 2026 is 70 days, shown as 10 weeks, 0 days. The trimester label follows the completed-week count: under 13 weeks is the first trimester, 13 to 26 weeks the second, and 27 weeks onwards the third.
What the arithmetic cannot know
- Ovulation does not shift neatly with cycle length. The adjustment is a reasonable proxy, not a measurement of when you ovulated.
- Irregular cycles weaken the whole approach, because there is no single cycle length to subtract 28 from.
- A first-trimester ultrasound measures the embryo directly and is more precise than any date arithmetic. If a scan disagrees with this result, the scan date is the one your care team will usually keep.
- Conception after fertility treatment is dated from the transfer or insemination, so LMP-based rules do not apply.
This is an estimate, not a substitute for prenatal care — confirm your date with a doctor or midwife.
To count weeks between appointments or work out an exact interval, use the date duration calculator; for a precise age in years, months and days, the age calculator. Fluid needs rise during pregnancy, and the water intake calculator gives a baseline to discuss.
This is an estimate only and not a substitute for professional prenatal care. Always confirm your due date with your doctor or midwife.
Frequently Asked Questions
How accurate is the due date?
Only about 4% of babies arrive exactly on their due date. It's a best estimate — most births occur within two weeks either side.
What if my cycle isn't 28 days?
This calculator shifts the due date by the difference between your average cycle length and the standard 28 days, which improves the estimate for longer or shorter cycles.
My cycle runs 35 days — how far does that move the date?
Seven days later, because the adjustment is cycle length minus 28. An LMP of 5 January 2026 gives 12 October 2026 at a 28-day cycle and 19 October 2026 at 35 days. A 21-day cycle would pull the same LMP forward to 5 October. The shift is exactly one day per day of cycle length.
Why can't I just enter the date I think I conceived?
Naegele's rule is defined from the first day of bleeding, so this page counts 280 days from there. If you only know a likely conception date, add roughly 14 days to approximate an LMP for a 28-day cycle, then enter that — and flag the uncertainty to your clinician, who can date the pregnancy by scan instead.
What makes the trimester label flip over?
Completed weeks since your LMP, not the due date. Below 13 weeks the page shows first trimester, 13 through 26 weeks second, and 27 weeks and beyond third. At 70 days — 10 weeks exactly — you are still in the first trimester, with the switch arriving 21 days later.
Does the result change if I am carrying twins?
No. The page applies the same 280-day rule regardless, because Naegele's rule has no multiples adjustment. Twin pregnancies are commonly delivered earlier than singletons, so the calculated date is best treated as a reference point while your care team sets a realistic delivery window.