Quick start
- Open the Due Date Calculator.
- Enter the first day of the last menstrual period.
- Use the first example, "LMP Apr 1, 2026: 28-day cycle: Apr 1 + 280 days", if you want to see a filled-out calculation before entering your own values.
- Calculate, read the formula line, then copy the result only after the units and assumptions look right.
Best uses
Start here if one of these sounds like your job. The examples below show which inputs matter most.
- Estimate an expected due date from the first day of the last menstrual period.
- Adjust the due-date estimate for shorter or longer usual cycle length.
- See estimated conception timing, gestational age today, and trimester context.
- Use a simple planning date before clinical dating is confirmed.
What this calculator is for
The Due Date Calculator focuses on expected delivery date from the first day of the last menstrual period and usual cycle length.
Use it when you want to: Estimate an expected due date from the first day of the last menstrual period. Adjust the due-date estimate for shorter or longer usual cycle length.
What to enter
Good answers start with clean inputs. Before calculating, check the labels, units, and dates so the tool is solving the same problem you actually have.
- Enter the first day of the last menstrual period.
- Set the usual cycle length if it differs from 28 days.
- Use the result as a planning date until clinical dating is confirmed.
Example walkthrough
Try the calculator example: LMP Apr 1, 2026: 28-day cycle: Apr 1 + 280 days. The example result is Estimated due Jan 6, 2027; conception around Apr 15.
- With Apr 1 as LMP and a 28-day cycle, the calculator adds 280 days.
- A longer cycle moves estimated ovulation later, so the date can shift later.
Formula and steps
In plain language: The calculator uses Naegele-style dating: due date = first day of LMP + 280 days + (cycle length - 28 days). It estimates conception near LMP + cycle length - 14 days and counts gestational age from LMP to today. Read the result together with the notes on the page, because health and fitness numbers often need personal context.
Read the formula note when you need to understand where the number came from, especially before comparing results over time.
How to read the answer
Due dates are estimates. Many healthy pregnancies deliver before or after the estimated date.
- A due date is an estimate; many healthy pregnancies deliver before or after it.
- The conception estimate is backward math from the due date, not a confirmed event date.
Common mistakes to avoid
Most bad results come from a small input mistake or from using a rough estimate for a decision it cannot safely answer.
- Do not enter ovulation date into an LMP field.
- Do not treat the due date as an appointment guarantee.
- Do not ignore clinician updates after ultrasound.
What to try next
A related health tool can help check the same topic from another angle, but one number should not replace proper care.
- Use Pregnancy Calculator for gestational age and trimester too.
- Use Pregnancy Conception Calculator to work backward from a due date.
Sources and safety notes
This guide uses public-health, clinical, or peer-reviewed references where the calculator needs a specific formula or interpretation boundary.
Source links are provided for transparency, but they do not turn the calculator into medical advice or a replacement for professional care.
Worked examples for Due Date Calculator
Estimated due Jan 6, 2027; conception around Apr 15
Estimated due Nov 25, 2026; conception around Mar 4
Estimated due Feb 7, 2027; conception around May 17
Estimated due Mar 24, 2027; conception around Jul 1
FAQ in plain language
When should I use the Due Date Calculator?
Use it for simple educational checks, trend tracking, or planning tasks like these: Estimate an expected due date from the first day of the last menstrual period. Adjust the due-date estimate for shorter or longer usual cycle length. It can help you understand a number, but it cannot explain your whole health situation.
What do the main Due Date Calculator inputs mean?
Enter the first day of the last menstrual period, not the last day of bleeding. Cycle length means the usual number of days from one period start to the next. Use 28 only if that is close for you, and use clinician, ultrasound, or IVF dating if that has already been assigned.
What is the Due Date Calculator doing with my inputs?
In plain language: The calculator uses Naegele-style dating: due date = first day of LMP + 280 days + (cycle length - 28 days). It estimates conception near LMP + cycle length - 14 days and counts gestational age from LMP to today. Read the result together with the notes on the page, because health and fitness numbers often need personal context.
How should I read the Due Date Calculator result?
Read the due date as an estimated delivery date from LMP calendar math, not a guarantee of when birth will happen. The gestational-age, conception, and trimester lines are planning references only. An early ultrasound, IVF date, or clinician review can override the calculator.
Why does the due date calculator add 280 days?
A common LMP-based pregnancy estimate counts about 280 days, or 40 weeks, from the first day of the last menstrual period. That is gestational-age dating, so it starts before estimated conception.
Why does cycle length change the due date?
Cycle length shifts the estimated ovulation timing. A 32-day cycle moves the estimate about four days later than a 28-day cycle, while a 26-day cycle moves it about two days earlier. Irregular cycles make calendar dating less reliable.
Should I enter the first or last day of my period?
Enter the first day bleeding started for the last menstrual period. The formula is built around the period start date, not the last bleeding day, ovulation day, positive test date, or appointment date.
Related tools
- Pregnancy CalculatorEstimate due date, pregnancy week, conception timing, and trimester from LMP.
- Pregnancy Conception CalculatorEstimate conception date, window, and LMP from a due date.
- Ovulation CalculatorEstimate ovulation date and fertile window from cycle details.
Keep exploring
If this guide is close but not exact, these links keep you near the same kind of problem.
- Health & FitnessBrowse the full category for related tools that help with the same job.
- All free toolsSearch the complete Access Free Tools library by task, category, or tool name.
- All calculator and utility guidesFind more plain-language examples, formulas, mistakes, and result explanations.
- Free calculator resourcesStart here when you are not sure which calculator page fits.
Privacy and copying results
Recent answers stay visible only while you work in the current browser tab. They are not sent to a server.
Use Copy answer when you want to save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.