Period Calculator

Estimate a next-period date using your usual cycle length. Enter your own values below to see estimated next period and check the method behind it.

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Your inputs

Change the example values to match your situation.

Your result

Let’s work it out.

Fill in your inputs and select Calculate.
Your result and breakdown will appear here.

How this calculator works

Next period = last period start + cycle length. Estimated ovulation = next period − luteal phase length.

Before you use the result

  • Calendar estimates are uncertain, especially with irregular cycles. Do not use them as a contraceptive method.

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What is Period Calculator?

Period Calculator helps you estimate a next-period date using your usual cycle length.

The main result is estimated next period, with a breakdown of estimated fertile window. The formula, example and assumptions below explain how to interpret it.

How to use Period Calculator

Use the example to get familiar with the form, then enter your own figures. Any rates in the example are illustrative. Check the field labels and units before calculating.

  1. Gather first day of last period, usual cycle length and estimated luteal phase. Use values from the same situation or reporting period.
  2. Replace the example first day of last period with your own value, then complete the other fields. Check the displayed units.
  3. Select Calculate and read estimated next period with its result breakdown. Change one input and calculate again to compare a second scenario. Reset restores the original example.
  • Select the first day of last period as a calendar date. The example uses 2026-09-14.
  • Enter usual cycle length, at least 21 and at most 45. Use a whole number.
  • Enter estimated luteal phase, at least 10 and at most 18. Use a whole number.

Period Calculator: a worked example

With the inputs below, the result is 2026-10-12 (estimated next period). Follow the example, then replace these illustrative values with your own.

Example inputs and output
Input or outputExample value
First day of last period2026-09-14
Usual cycle length28
Estimated luteal phase14
Estimated next period2026-10-12
Estimated fertile window2026-09-23 to 2026-09-29

Understanding the Period result

Read estimated next period alongside estimated fertile window. These describe different parts of the same calculation.

Displayed results use a readable number of decimal places. When checking a result by hand, keep extra precision until the last step. To compare scenarios, change one input at a time and keep the others fixed.

  • Example Estimated fertile window: 2026-09-23 to 2026-09-29

Period Calculator: assumptions and common mistakes

A useful estimate starts with the right inputs. Check the following assumptions and limits before applying the result to your situation.

  • Calendar estimates are uncertain, especially with irregular cycles. Do not use them as a contraceptive method.
  • Health estimates are context-dependent. Age, pregnancy, illness, medication and individual measurements can affect interpretation; these tools do not diagnose a condition.

For wider guidance, visit World Health Organization. The formula and scope of this specific tool are stated on this page.

Period Calculator FAQs

What does Period Calculator calculate?

Period Calculator helps you estimate a next-period date using your usual cycle length. Its main output is estimated next period.

What inputs do I need for Period Calculator?

You need first day of last period, usual cycle length and estimated luteal phase. Use the field labels and units to match your figures to the formula.

Which formula does Period Calculator use?

Next period = last period start + cycle length. Estimated ovulation = next period − luteal phase length.

What is a worked example for Period Calculator?

Example inputs: First day of last period: 2026-09-14; Usual cycle length: 28; Estimated luteal phase: 14. The result is 2026-10-12 (estimated next period).

How should I enter first day of last period?

Select the first day of last period as a calendar date. The example uses 2026-09-14.

How do I choose usual cycle length?

Enter usual cycle length, at least 21 and at most 45. Use a whole number. The example uses 28; replace it if your circumstances differ.

How should I read estimated next period?

Read estimated next period alongside estimated fertile window. These describe different parts of the same calculation.

Why might my Period Calculator result differ from my records?

Calendar estimates are uncertain, especially with irregular cycles. Do not use them as a contraceptive method. Differences in entered values, units or rounding can also change the comparison.

Can I compare two scenarios in Period Calculator?

Yes. Note your first estimated next period, change first day of last period or another input, and select Calculate again. Editing a field clears the old result so it is not mistaken for a new calculation.

Is Period Calculator free, and where are my inputs calculated?

This calculator is free to use without signing in. Its formula runs in your browser; calculator inputs are not sent to a calculation API. The inputs reset to the example values when you reset the form or start a fresh page.