Two figures in, a clear answer out. The period calculator uses cycle duration, my last period started on… and period duration to produce first day, last day and ovulation day, applying the standard relationship for this problem in the order the calculation actually needs it.
What Is the Period Calculator?
The period calculator gives you an estimate of first day, last day and ovulation day based on cycle duration, my last period started on... and period duration.
It is a quick reference figure rather than a diagnosis, and it is most useful for tracking how a number moves over time rather than for interpreting a single reading.
How the Period Estimate Is Worked Out
The calculation follows published reference relationships, so it is a reasonable approximation when the inputs are accurate. It assumes an average adult and does not account for individual variation.
If you are comparing two people, or the same person over a period of weeks, keep the method and the inputs identical. A change in the method can easily look like a change in the person.
What This Calculator Does Not Do
It does not diagnose anything, and it does not replace advice from a clinician. If a figure comes out at an extreme, or if it differs sharply from what you expect, that is a reason to speak to a professional rather than to adjust the inputs.
Nothing you type here is stored or sent anywhere. The whole calculation happens in your browser.
A Worked Example
Working through an example, with cycle duration set to 21.000000000000000, my last period started on... set to 10, period duration set to 3.000000000000000, the calculator returns first day of 1,021, and last day of -, and ovulation day of 1,007. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out first day?
Enter cycle duration, my last period started on... and period duration. The calculator then works through the relationship and reports first day, last day and ovulation day.
What does the period actually calculate?
It takes cycle duration, my last period started on... and period duration and converts them into first day, last day and ovulation day using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter cycle duration?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Why does a small change in one value move the answer so much?
That happens when an output depends on a ratio, because dividing by a small number magnifies any change in it. Check the units first, then check whether the value you entered is in the range the relationship was designed for.
Is the answer exact?
It is as exact as the formula and the inputs allow. The method itself is the standard one for this problem, so the only sources of error are the figures you enter and the precision you ask for.