Due Date Calculator
Estimate your baby’s due date based on your last period.
Due Date Calculator – Find Your Baby’s Estimated Due Date Instantly
What Is a Due Date Calculator?
A Due Date Calculator is a free online tool that estimates the date your baby is expected to be born — known as the Estimated Due Date (EDD) — based on the first day of your Last Menstrual Period (LMP) and your average menstrual cycle length. It is the first calculation most women want to make after discovering they are pregnant.
Your due date is one of the most important dates in your pregnancy. It determines the timing of every prenatal appointment, screening test and trimester milestone. It guides your midwife’s or obstetrician’s entire care plan, establishes the gestational age your baby will reach at birth, and gives you — and everyone who loves you — a date to look forward to, plan around and prepare for.
Unlike basic due date calculators that assume a standard 28-day cycle for every woman, our free Due Date Calculator accounts for your actual average cycle length — producing a personalised, more accurate estimated due date whether your cycle is 21 days, 35 days or anything in between.
Use our free online Due Date Calculator above to instantly find your estimated due date.
How to Calculate Your Due Date
Due date calculation is based on Naegele’s Rule, a clinical formula developed in the early 19th century by German obstetrician Franz Karl Naegele and still universally used in obstetrics today. It assumes that pregnancy lasts 280 days (40 weeks) from the first day of the LMP, based on the average 28-day menstrual cycle.
Standard Naegele’s Rule Formula (28-day cycle)
EDD = LMP + 280 days
Or equivalently:
EDD = LMP + 9 months + 7 days
Adjusted Formula for Non-Standard Cycle Lengths
Because not all women have a 28-day cycle, our calculator adjusts the due date based on your actual average cycle length. The adjustment shifts the estimated ovulation date — and therefore the EDD — forward or backward relative to the 28-day assumption:
Cycle Length Adjustment = Actual Cycle Length − 28 days Adjusted EDD = LMP + 280 days + Cycle Length Adjustment
Or in a single step:
Adjusted EDD = LMP + 252 days + Actual Cycle Length
Example 1 — Standard 28-Day Cycle:
LMP: 1 January 2025 | Cycle Length: 28 days
EDD = 1 January 2025 + 280 days = 8 October 2025
Example 2 — Longer 35-Day Cycle:
LMP: 1 January 2025 | Cycle Length: 35 days
Adjustment = 35 − 28 = +7 days Adjusted EDD = 8 October 2025 + 7 days = 15 October 2025
Example 3 — Shorter 21-Day Cycle:
LMP: 1 January 2025 | Cycle Length: 21 days
Adjustment = 21 − 28 = −7 days Adjusted EDD = 8 October 2025 − 7 days = 1 October 2025
The adjustment reflects the fact that women with shorter cycles ovulate earlier and women with longer cycles ovulate later — shifting both conception and the expected delivery date accordingly.
Due Date by Cycle Length — Reference Table
The table below shows how your estimated due date shifts based on cycle length, using 1 January as the example LMP:
| Average Cycle Length | Ovulation Day | EDD (from 1 Jan LMP) | Shift from 28-Day |
|---|---|---|---|
| 21 days | Day 7 | 1 October | −7 days earlier |
| 24 days | Day 10 | 4 October | −4 days earlier |
| 26 days | Day 12 | 6 October | −2 days earlier |
| 28 days | Day 14 | 8 October | Standard |
| 30 days | Day 16 | 10 October | +2 days later |
| 32 days | Day 18 | 12 October | +4 days later |
| 35 days | Day 21 | 15 October | +7 days later |
This illustrates why entering your actual cycle length produces a more accurate EDD than simply using the standard 28-day formula — a difference of up to one week or more depending on your cycle.
Pregnancy Term Classifications Around the Due Date
Your estimated due date is the centre of a clinical birth window. Deliveries within this window are all considered term pregnancies, though the specific week of delivery affects neonatal outcomes:
| Classification | Gestational Age | Clinical Significance |
|---|---|---|
| Preterm | Before 37 weeks | Baby may require neonatal intensive care support |
| Early Term | 37 weeks 0 days – 38 weeks 6 days | Higher risk of complications than full-term |
| Full Term | 39 weeks 0 days – 40 weeks 6 days | Optimal gestational age for delivery |
| Late Term | 41 weeks 0 days – 41 weeks 6 days | Increased monitoring; induction often discussed |
| Post-Term | 42 weeks or beyond | Induction typically recommended; placental function monitored |
The EDD falls at 40 weeks 0 days — the midpoint of the full-term window. Only approximately 4–5% of babies are born on their exact due date; the vast majority are born within two weeks either side.
Key Dates and Milestones Around Your Due Date
Once you know your EDD, you can calculate every major milestone of your pregnancy. The table below shows key dates relative to your due date:
| Milestone | Weeks Before EDD | What to Expect |
|---|---|---|
| First day of LMP | 40 weeks before EDD | Starting point for all gestational age calculations |
| Estimated conception | 38 weeks before EDD | Approximate ovulation and fertilisation date |
| Positive pregnancy test | ~36–37 weeks before EDD | hCG detectable from approximately 4 weeks gestation |
| First trimester ends | 27 weeks before EDD | Highest-risk developmental period complete |
| Anatomy scan (20-week) | 20 weeks before EDD | Major structural assessment of baby |
| Viability threshold | 16 weeks before EDD | Baby may survive outside womb from 24 weeks |
| Third trimester begins | 12 weeks before EDD | Rapid weight gain; birth preparation begins |
| Full-term window opens | 1 week before EDD | 39 weeks — optimal delivery zone begins |
| Estimated Due Date | 0 | 40 weeks — expected birth date |
| Late-term threshold | 1 week after EDD | 41 weeks — increased monitoring and induction discussion |
| Post-term threshold | 2 weeks after EDD | 42 weeks — induction typically recommended |
How Accurate Is a Due Date Calculator?
Understanding the accuracy of your EDD helps you approach it with the right expectations:
| Dating Method | Accuracy | When Used |
|---|---|---|
| LMP-based calculation (28-day cycle) | ±2 weeks | When LMP is known; used from the first appointment |
| LMP-based calculation (adjusted cycle) | ±1–1.5 weeks | Our calculator — more accurate for irregular cycles |
| First-trimester ultrasound (Weeks 8–13) | ±3–5 days | Crown-rump length measurement — most accurate clinical method |
| Second-trimester ultrasound (Weeks 14–20) | ±7–10 days | Less accurate than first-trimester scan for dating |
| Third-trimester ultrasound | ±2–3 weeks | Least accurate for dating; used for growth monitoring |
The most accurate due date comes from combining an LMP-based calculation with a first-trimester ultrasound. When these two dates agree within 5–7 days, the LMP-based EDD is typically retained. When they differ by more than 7 days, the ultrasound date takes precedence because it is based on direct foetal measurement.
Why Your Due Date Matters
Your estimated due date is far more than a date circled on a calendar — it drives the entire clinical management of your pregnancy:
- Establishes the gestational age framework — every prenatal appointment, blood test, ultrasound and screening is scheduled relative to your gestational age, which is counted backwards from your EDD
- Determines screening windows — critical tests such as the nuchal translucency scan (weeks 11–14) and the anatomy scan (weeks 18–21) must be performed within specific gestational age windows; missing these windows means missing the test entirely
- Guides maternity leave planning — most employment maternity leave provisions are calculated relative to the EDD; an accurate EDD allows you to plan your leave start date correctly
- Sets induction thresholds — if labour does not begin spontaneously, induction is typically offered at 41–42 weeks; knowing your EDD means you can discuss and plan for this possibility with your care team well in advance
- Informs foetal growth monitoring — growth scans in the third trimester compare your baby’s measurements against expected values for gestational age; these comparisons are only meaningful if gestational age is correctly established
- Supports birth preparation — knowing your EDD allows you to time antenatal classes, hospital bag preparation, nursery completion and support arrangements with family and friends
- Provides emotional anchoring — a due date gives expectant parents a tangible milestone to look forward to, prepare for and share with those who care about them
Limitations of the Due Date Calculator
Our Due Date Calculator is more accurate than standard tools because it adjusts for your cycle length. However, all LMP-based due date calculations share inherent limitations:
- LMP recall may be imprecise — the accuracy of the EDD depends entirely on the accuracy of the LMP date entered. Women with irregular cycles, recent hormonal contraceptive use or breastfeeding-related amenorrhoea may find their LMP date less reliable as a starting point
- Ovulation does not always occur at the expected point in the cycle — even in women with consistent cycle lengths, the timing of ovulation can vary by several days from cycle to cycle, shifting the true EDD by an equivalent amount
- Only 4–5% of babies arrive on their due date — the EDD is a statistical midpoint, not a prediction. Approximately 80% of babies are born within two weeks of the EDD, and perfectly healthy pregnancies vary significantly in length
- The formula does not adjust for individual pregnancy duration variation — some women consistently carry pregnancies to 41 weeks; others consistently deliver at 38–39 weeks. The calculator cannot account for this individual variation
- Multiple pregnancies follow different timelines — average gestational length for twins is approximately 36–37 weeks, not 40; the calculator is designed for singleton pregnancies
- Assisted reproduction requires different dating — IVF and IUI pregnancies have known fertilisation dates that allow more precise gestational age dating than LMP-based methods
Who Should Use the Due Date Calculator?
Our free Due Date Calculator is useful for:
- Women who have just taken a positive pregnancy test — finding your due date is the very first thing most newly pregnant women want to know
- Women with irregular or non-standard cycle lengths — our cycle-length-adjusted calculation is more accurate than basic 28-day tools for anyone whose cycle differs from the standard
- Partners and family members — quickly calculate the due date to understand the pregnancy timeline and begin planning
- Women in the early weeks of pregnancy — before your first midwife appointment, this calculator gives you an accurate reference EDD to plan from
- Anyone who wants a second opinion on their due date — cross-checking a due date given by a healthcare provider helps confirm the calculation or identify a potential discrepancy worth discussing
This calculator is a reference tool only. It does not replace:
- Confirmation of your due date by your midwife or obstetrician
- First-trimester ultrasound dating, which is the most clinically accurate method available
- Personalised prenatal care tailored to your individual health history and pregnancy
Frequently Asked Questions (FAQ)
How accurate is a due date calculator?
An LMP-based due date calculator is accurate to within approximately 1–2 weeks for women with regular cycles. Our calculator improves on basic tools by adjusting for your actual cycle length, narrowing the accuracy range. The most accurate EDD comes from combining an LMP-based calculation with a first-trimester ultrasound, which can date gestational age to within 3–5 days.
Does my cycle length really change my due date?
Yes, significantly. For every day your average cycle length differs from 28 days, your estimated due date shifts by one day in the same direction. A woman with a 35-day cycle has a due date approximately 7 days later than the standard Naegele calculation would suggest. A woman with a 21-day cycle has a due date approximately 7 days earlier. Over many pregnancies and many women, this variation is one of the main reasons due dates based on standard 28-day assumptions are often slightly inaccurate.
When will my healthcare provider confirm my due date?
Your midwife or GP will calculate an initial EDD at your booking appointment, typically around 8–10 weeks of pregnancy. This date will usually be confirmed or revised at your first ultrasound scan, which in most healthcare systems takes place between 10 and 13 weeks of gestation. If the ultrasound date differs from the LMP-calculated date by more than 5–7 days, the ultrasound date is used.
What happens if I go past my due date?
Going past your EDD is common — the majority of first-time mothers deliver after 40 weeks. At 41 weeks, your midwife or obstetrician will typically recommend increased monitoring (a cardiotocograph and amniotic fluid assessment) and discuss induction. At 41 weeks and 3–5 days, most UK and US guidelines recommend offering induction to avoid the increasing risks associated with post-term pregnancy, including reduced placental function and foetal distress.
Can I change my due date after it has been set?
In clinical practice, due dates are sometimes revised — most commonly after a first-trimester ultrasound that produces a different gestational age estimate from the LMP calculation. Once a due date has been confirmed by ultrasound, it is generally not changed again, even if later scans produce different measurements, because first-trimester dating is the most accurate. Discuss any discrepancies you notice with your healthcare provider.
What is the difference between a due date and a birth date?
Your due date (EDD) is an estimate of when birth is statistically most likely to occur. Your birth date is the actual date your baby is born. The two will coincide in only 4–5% of pregnancies. The EDD is best understood as the midpoint of a normal distribution of birth dates — most healthy deliveries fall within a window of approximately two weeks before to two weeks after the EDD, with full-term births most concentrated in the 39–40 week range.
My ultrasound gave a different due date than this calculator — which is right?
If your first-trimester ultrasound (performed between weeks 8 and 13) gives a due date that differs from this calculator’s result, the ultrasound date is more accurate and should be used. The crown-rump length measurement performed in early ultrasound is the gold standard for gestational age dating and is more reliable than LMP-based calculation for women with cycle irregularity, late or early ovulation, or uncertain LMP dates. Always use the date confirmed by your healthcare provider.
What to Do After Calculating Your Due Date
Finding your EDD is the start of a series of important steps. Here is what to do next:
- Book your first prenatal appointment immediately — contact your GP or midwife as soon as you have a positive test. Most booking appointments take place at around 8–10 weeks. Do not wait until your second trimester
- Start taking folic acid today — if you are not already taking it, begin 400 mcg of folic acid daily immediately. It is most critical in the first 12 weeks of pregnancy for reducing neural tube defect risk
- Calculate your key pregnancy dates — use our Pregnancy Calculator to find your current gestational age and trimester, and use our Pregnancy Conception Calculator to estimate when conception occurred
- Schedule your first-trimester screening — the nuchal translucency scan must be performed between weeks 11 and 14. Book it in advance to avoid missing the window
- Understand your due date window — your baby is most likely to arrive between 38 and 42 weeks. Begin birth preparation — antenatal classes, hospital bag and birth plan — from around week 32–34
- Track your pregnancy week by week — use our Pregnancy Calculator regularly to see what your baby is developing right now and what to expect in the coming weeks
- Review your weight gain guidance — use our Pregnancy Weight Gain Calculator to understand the recommended weight gain for your pre-pregnancy BMI across each trimester
Final Thoughts
The Due Date Calculator is one of the most significant free online tools you will use during your pregnancy. Your estimated due date organises everything that follows — from your first midwife appointment and prenatal blood tests to your anatomy scan, birth preparation classes, maternity leave and the moment you finally meet your baby.
By accounting for your actual cycle length rather than assuming a standard 28 days, our calculator gives you a more accurate and more personalised EDD than basic tools — a small but meaningful improvement that can shift your due date by up to a week or more if your cycle differs from the standard.
Use it alongside the full free pregnancy toolkit on CalcoraTools — the Pregnancy Calculator for week-by-week tracking, the Pregnancy Conception Calculator to find when conception occurred, the Ovulation Calculator to understand your fertile window, the Period Calculator for cycle planning, and the Pregnancy Weight Gain Calculator for trimester-by-trimester weight guidance.
Calculate your due date now — free, instant and no sign-up required.