Rex says
Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.
Try a scenario
Click to load — tweak from there.Inputs
Result
Estimated due date
2027-05-26
Current week
0
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter first day of last period.
- 2Read your estimated due date on the right — it updates as you type.
- 3Hit Share to keep the scenario or send it to someone.
About this calculator
Naegele's rule: due date is the first day of the last menstrual period + 280 days.
Worked example
Using the values the calculator loads with:
Inputs
- First day of last period: 2026-08-19
Results
- Estimated due date: 2027-05-26
- Current week: 0
What each field means
Inputs
- First day of last period
- The reference date for this calculation. Defaults to today unless you change it.
Results
- Estimated due date
- Returned as a plain value and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Current week
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the pregnancy due date calculator work?
Naegele's rule: due date is the first day of the last menstrual period + 280 days.
What do I need to enter?
Just one value: first day of last period. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the estimated due date result mean?
It is returned as a plain value and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
Who is this tool for?
It's built for anyone tracking a health or training target and wanting the standard formula applied correctly, without a signup or an app.
Accuracy and limitations
- Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
- Results do not account for medical conditions, medication, or pregnancy.
- Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Pregnancy Due Date Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/pregnancy-due-date
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/pregnancy-due-date" target="_blank" rel="noopener">Pregnancy Due Date Calculator — RevenueLab</a> (2026).</p>
Source: [Pregnancy Due Date Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/pregnancy-due-date) (2026).