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
January 6, 2027
How far along today
19w 3d
Current trimester
Second
Days remaining
144
Estimated conception date
April 15, 2026
24-week viability milestone
September 16, 2026
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter first day of last period.
- 2Enter average cycle length (days).
- 3Read your estimated due date on the right — it updates as you type.
- 4Hit Share to keep the scenario or send it to someone.
About this calculator
Naegele's rule adds 280 days to the first day of the last menstrual period. Only about 4% of babies arrive on the estimated date; the 37–42 week window is the realistic range.
Worked example
Using the values the calculator loads with:
Inputs
- First day of last period: 2026-04-01
- Average cycle length: 28 days
Results
- Estimated due date: January 6, 2027
- How far along today: 19w 3d
- Current trimester: Second
- Days remaining: 144
- Estimated conception date: April 15, 2026
- 24-week viability milestone: September 16, 2026
What each field means
Inputs
- First day of last period
- The reference date for this calculation. Defaults to today unless you change it.
- Average cycle length (days)
- The average cycle length used in the calculation, measured in days. Starts at 28 days so you have a working example on load. Accepted range: 20–45 days.
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.
- How far along today
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Current trimester
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Days remaining
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Estimated conception date
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- 24-week viability milestone
- Returned as a plain value. 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 adds 280 days to the first day of the last menstrual period. Only about 4% of babies arrive on the estimated date; the 37–42 week window is the realistic range. The underlying maths is: Due date = Last period + 280 days (adjusted for cycle length).
What do I need to enter?
2 values: first day of last period, and average cycle length. 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/due-date
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/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/due-date) (2026).