Due Date Calculator
Estimate your baby due date and how far along you are from your last menstrual period.
Enter Your Values
Table of Contents
Complete Guide to Due Dates
Estimated due dates help plan prenatal visits and parental leave. Most calculators add 280 days (40 weeks) to the first day of the last menstrual period (LMP).
Why LMP Is Used
Ovulation date is often unknown, but LMP is usually remembered. Ultrasound dating in the first trimester can refine the estimate.
Understanding Menstrual Cycle Phases
Follicular phase
From the first day of bleeding until ovulation. Estrogen rises and the uterine lining rebuilds.
Luteal phase
After ovulation, typically about 14 days before the next period if pregnancy does not occur.
Pregnancy Timeline Overview
Gestational age is often expressed in weeks from LMP. Trimesters divide prenatal care schedules: first trimester through week 12, second through week 27, third until birth.
- Week 4-5: missed period may prompt a home test.
- Week 8-10: first prenatal visit often includes ultrasound.
- Week 20: anatomy scan in many care plans.
- Week 37+: full-term window begins for many providers.
When to Talk to a Clinician
Cycle-based estimates vary with stress, illness, and hormonal conditions. Seek medical guidance for irregular bleeding, suspected pregnancy, or fertility treatment planning.
- Ultrasound dating overrides calendar estimates when available.
- Track several cycles for better ovulation prediction.
- Emergency contraception and prenatal vitamins are separate decisions from this tool.
Privacy
Calculations run when you submit the form. Do not enter personal identifiers you do not want stored in server logs.
Frequently Asked Questions
How accurate is this online calculator?
Results follow standard formulas used in textbooks and industry guides. Always verify critical projects with a licensed professional or measured field data.
Can I reuse my entries?
After submitting the form, your values remain in the fields so you can adjust one input and recalculate quickly.
Which units should I use?
Use the unit selectors where available and stay consistent within one calculation. The backend converts measurements before applying formulas.
Safety and Disclaimer
This page is for educational and planning purposes. Building codes, medical decisions, and engineering sign-off require qualified experts.
Calendar vs gestational age
Gestational age counts from LMP even though conception usually occurs about two weeks later. That is why due dates are labeled as approximately 40 weeks from LMP, not from conception day.
Regular cycles
28-day cycles align reasonably well with the 14-day luteal assumption.
Irregular cycles
Ovulation and due date estimates vary more; use clinical dating.
Due Date Formula
Due date = LMP + 280 days
Cycle length field helps related fertility tools; standard rule still uses 280 days from LMP.
Variables at a Glance
| Symbol | Meaning | Typical unit |
|---|---|---|
| x | Input you provide | As labeled in the form |
| y | Computed output | Shown in results cards |
Substitute your measured values into the formula box above, then compare with the calculator output to verify your manual work.
Limitations of calendar methods
Calendar formulas assume ovulation occurs a fixed number of days before the next period. Polycystic ovary syndrome, breastfeeding, perimenopause, and recent hormonal contraception can disrupt that pattern.
- Combine calendar estimates with ovulation predictor kits when trying to conceive.
- Do not use fertile window estimates alone for contraception without understanding failure rates.
- Emergency care is appropriate for severe pain or bleeding, not this calculator.
How to Use This Calculator
- Enter the first day of your last menstrual period.
- Enter your average cycle length if not 28 days.
- Review due date and current gestational age if LMP is in the past.
- Confirm with your clinician and ultrasound when available.
Pro tips
- Round only at the end to avoid rounding error stacking.
- Write down intermediate values when checking homework.
- If an error appears, read the message and fix the labeled field first.
Common mistakes
- Mixing unit systems without converting.
- Using negative values where the model expects positive sizes.
- Forgetting optional fields that change the mode of calculation.
Types of Dating Methods
LMP rule
Quick estimate from last period.
Ultrasound
Crown-rump length most accurate early.
IVF transfer date
Embryo age added to transfer day.
Physical exam
Less precise than early ultrasound.
Choosing the right scenario
Match your real project or lab setup to the closest type listed above, then adjust inputs such as spacing, waste factor, or gravity to fit your conditions.
Glossary
- LMP
- Last menstrual period, day one of bleeding.
- Gestational age
- Weeks counted from LMP used in prenatal care.
- Fertile window
- Days when intercourse is most likely to lead to pregnancy.
Practical Examples
LMP January 1 → due date approximately October 8 (280 days later).
If today is 70 days after LMP → about 10 weeks pregnant.
Worksheet Practice
Change one input at a time and predict how the result should move before clicking Calculate. This builds intuition faster than memorizing formulas alone.
Checklist before you trust a number
- Units match the formula.
- Inputs are positive where required.
- Result magnitude makes physical sense.
Related planning
Use the sidebar links to jump to complementary calculators on the same site category.
Tracking tips
- Log period start dates in a calendar app for three months before relying on predictions.
- Note symptoms such as mittelschmerz or cervical mucus changes to refine fertile days.
- Discuss prenatal vitamins and folic acid at a preconception visit.
- Understand that stress, travel, and illness can shift ovulation.
Resources to discuss with your provider
Bring your LMP date, cycle length history, and any home test results to appointments. Only ultrasound or clinical assessment can confirm gestational age or ovulation with medical certainty.