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 ovulation
2026-09-02
Fertile window start
2026-08-28
Fertile window end
2026-09-03
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.
- 2Enter cycle length (days).
- 3Read your estimated ovulation on the right — it updates as you type.
- 4Hit Share to keep the scenario or send it to someone.
About this calculator
Estimates the next ovulation date and fertile window from the last period and cycle length.
Worked example
Using the values the calculator loads with:
Inputs
- First day of last period: 2026-08-19
- Cycle length: 28 days
Results
- Estimated ovulation: 2026-09-02
- Fertile window start: 2026-08-28
- Fertile window end: 2026-09-03
What each field means
Inputs
- First day of last period
- The reference date for this calculation. Defaults to today unless you change it.
- Cycle length (days)
- The 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 ovulation
- 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.
- Fertile window start
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Fertile window end
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the ovulation calculator work?
Estimates the next ovulation date and fertile window from the last period and cycle length.
What do I need to enter?
2 values: first day of last period, and 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 ovulation 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). Ovulation Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/ovulation
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/ovulation" target="_blank" rel="noopener">Ovulation Calculator — RevenueLab</a> (2026).</p>
Source: [Ovulation Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/ovulation) (2026).