CalculatorX

OBGYN; Reference

Period Calculator

Period Calculator adds a cycle length, in days, to a start date. A date plus 28 days lands four weeks later. Real cycles move. The page does not predict ovulation or diagnose a missed period.

Period Calculator adds a cycle length, in days, to a start date. A date plus 28 days lands four weeks later. Real cycles move. The page does not predict ovulation or diagnose a missed period.

Reviewed 2026-11-08. The result is an educational estimate, not professional advice.

Enter your values

This calculation stays in your browser. Nothing is sent when you press Calculate.

Result

Enter values to calculate.

Formula

Next date = start date + cycle length in days.

Example

A start date plus 28 days is four weeks later. Cycles vary.

Illustration for the Period Calculator used by CalculatorX.
Illustration only. Use the formula and the result box for the number.

How to use Period Calculator

Use the start date you actually know and the cycle length you want to test. Try 26, 28, and 30 if you are comparing.

The formula that actually runs is: Next date = start date + cycle length in days.

A start date plus 28 days is four weeks later. Cycles vary.

Where this estimate stops

Do not use the next date as a fertility plan or as proof of pregnancy.

The calculation stays in your browser. CalculatorX does not save the numbers as an account.

If you need a different figure, look at Safe Period Calculator, Menstrual Cycle Calculator, Notice Period Calculator.

How to use this calculator

  1. Start with the numbers you already have. Use the start date you actually know and the cycle length you want to test. Try 26, 28, and 30 if you are comparing.
  2. Press Calculate. The result uses only the fields on this form and the formula printed under it.
  3. Read the limit. Do not use the next date as a fertility plan or as proof of pregnancy.

Questions about Period Calculator

How does Period Calculator get its result?

Use the start date you actually know and the cycle length you want to test. Try 26, 28, and 30 if you are comparing.

What formula is on the period calculator page?

Next date = start date + cycle length in days.

Can I trust “period calculator” as a final decision?

Do not use the next date as a fertility plan or as proof of pregnancy.

Is this stored or sent anywhere?

No. The page calculates in your browser and does not create an account.

What this helps you do

  • Estimate Period from the inputs you already have.
  • Check the result against the formula and the worked example.
  • Change one value at a time and compare the new estimate.

Limits to keep in mind

  • The figure is only as complete as the inputs on this form.
  • Rounding, fees, and local rules can change the real-world number.
  • Use it to learn and compare. Confirm important finance or health decisions with a qualified professional.