How this due date calculator works
We use the same rule most maternity services start with: Naegele's rule, adjusted for the length of your cycle. Nothing is guessed silently — here is the exact arithmetic behind every number on this page.
Estimated due date = LMP + 280 days + (cycle length − 28)
Current pregnancy = today − LMP (shown in weeks + days)
Estimated conception = LMP + 14 days
- LMP is the first day of your last menstrual period, counted as day 1 — not the last day of bleeding.
- 280 days is 40 weeks, the standard length of a pregnancy when counted from the LMP.
- Cycle adjustment: if your cycle is longer than 28 days, ovulation likely happened later, so the due date moves later by the same number of days. A 35-day cycle adds 7 days; a 24-day cycle subtracts 4 days.
- Gestational age is counted from the LMP (today minus LMP) and is shown as weeks and days, the format used in antenatal notes.
- Trimesters are split as 0–13 weeks 6 days (first), 14–27 weeks 6 days (second) and 28 weeks onwards (third).
- All date maths uses whole UTC calendar days, so daylight-saving changes can never shift your result by a day.
Milestones such as 12 weeks or 20 weeks are counted forward from the LMP in seven-day steps, because gestational weeks are calendar weeks rather than months.
How accurate is this?
An estimated due date is a midpoint, not an appointment. Only about 4 in 100 babies are born on their estimated date, and the large majority arrive within the two weeks before or after it.
A due date calculated from your last period assumes a regular cycle and ovulation around day 14. If your cycles are irregular, very long, or you were taking hormonal contraception, that assumption can be off by a week or more. A first-trimester dating ultrasound measures the baby directly and is generally the more accurate method — which is why your midwife may revise the date you see here.
Our formula follows the approach described in the NHS due date calculator and in ACOG's guidance on estimating gestational age. The cycle-length adjustment is stated openly above so you can check it against the date your clinician gives you.
What to do next
Use this estimate to plan, not to decide. The next step is a conversation with a clinician.
- Book your first antenatal appointment. Tell your midwife or obstetrician the first day of your last period and your usual cycle length; they will confirm or adjust the date.
- Ask about a dating ultrasound. A scan in the first trimester is the usual way to confirm gestational age, especially if your cycle is irregular or longer than 35 days.
- Write the date down. Download the result card and take it to your appointment so you can compare numbers.
- Know when to call sooner. Contact your maternity service straight away — do not wait for the calculator — if you have bleeding, severe pain, reduced movement, or any symptom that worries you.
Not medical advice. Baby Tools is an educational calculator. It cannot diagnose a pregnancy, date it reliably, or detect problems. Always rely on your midwife, doctor or emergency service for medical decisions.
Frequently asked questions
How is my due date calculated?
This calculator uses Naegele's rule: it adds 280 days (40 weeks) to the first day of your last menstrual period, then adjusts for your cycle length by adding the difference between your cycle and the standard 28-day cycle. A 35-day cycle, for example, moves the estimated due date 7 days later.
How accurate is an estimated due date?
Only about 4 in 100 babies are born on their estimated due date, and most arrive within the two weeks before or after it. A first-trimester dating ultrasound is usually more accurate than a date based on your last period, because it measures the baby directly.
Does my cycle length change the result?
Yes. Ovulation happens later in a longer cycle, so the estimated due date moves later too. This calculator accepts cycle lengths from 20 to 45 days and shifts the due date by the difference from 28 days.
Is this due date calculator medical advice?
No. It is an educational estimate based on public guidance from the NHS and ACOG. Always confirm your due date with your midwife or obstetrician, who may change it after an ultrasound or if your cycle is irregular.
Sources and disclaimer
Primary sources used for the method and the accuracy notes on this page:
This page is informational only and is not a medical device. It does not create a clinician–patient relationship, and it must not be used to make medical decisions. If you think you may be pregnant, or if you are concerned about your pregnancy, contact a healthcare professional.
Your dates are processed only in this browser tab. Nothing is uploaded, stored or shared.
More baby tools
The Baby Tools home page lists everything we are building, including an ovulation calculator, baby percentile charts, a feeding chart, a contraction timer and corrected age for premature babies. The due date calculator is the first tool — the rest are on the way.