Due Date
Calculator
40 weeks from your last period.
Open the due date calculator →
Pregnancy
Period Calculator
Enter your last period and cycle length to predict your next several periods and ovulation.
Next period
—
Predicted start date.
| Following period | — |
|---|---|
| Third period | — |
| Estimated ovulation | — |
| Cycle day today | — |
Show our math
Enter your figures above to see the step-by-step working.
Estimate — cycle length variesEverything computed client-side — nothing leaves this page
How the Period Calculator Works
Your period calculator predicts upcoming periods by adding your average cycle length to the first day of your last period, then repeating. A cycle is counted from the first day of one period to the first day of the next; 28 days is the textbook average, but anywhere from about 21 to 35 days is common and normal.
The calculator also estimates ovulation — roughly 14 days before the next period — and shows which day of your cycle today is. Tracking a few cycles gives a more personal average than any single prediction, since cycle length naturally varies from month to month.
next period = first day of last period + cycle length
Note: these are estimates based on standard averages. Every pregnancy is different — your doctor or midwife, using an ultrasound, gives the most accurate dates.
Frequently Asked Questions
How do I predict my next period?
Add your average cycle length to the first day of your last period. For a 28-day cycle, the next period is expected 28 days later. This calculator projects the next several periods for you.
What is a normal cycle length?
Most cycles run between 21 and 35 days, measured from the first day of one period to the first day of the next. Some variation month to month is normal.
How does it estimate ovulation?
Ovulation typically occurs about 14 days before the next period, so the calculator subtracts 14 days from the predicted next-period date. It’s an estimate, not a precise fertility guarantee.
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Calculator
When conception occurred.
Open the conception date calculator →