Period Calculator
This period calculator estimates upcoming period start dates from the first day of your last period and your average cycle length. An optional period length adds a bleeding window. Calendar estimate only — not birth control or medical advice.
How to use
- Enter the first day of your last period.
- Enter average cycle length in days. A common default is 28. That span runs from one period start to the next, not the bleeding days.
- Optionally enter how many days you usually bleed. Leave it blank to show start dates only.
- Read the next period start dates. This is a calendar estimate for a regular cycle, not contraception or medical advice.
Formula
Next period start = last period start + cycle length in days. Each following start adds that same number of days. If you enter a period length, the estimated end is the start date plus period length minus 1 day. The tool does not predict ovulation or a fertile window.
Example. A 28-day cycle that started March 1 is next due on March 29. The following start is April 26. A 5-day period that begins March 29 runs through April 2.
Frequently asked questions
How is the next period estimated?
Add average cycle length (often about 28 days) to the first day of your last period. A 28-day cycle that started March 1 is due around March 29. Each later start adds that same number of days.
What is cycle length?
Days from the first day of one period to the first day of the next, not the number of bleeding days. A 5-day period on a 28-day cycle still uses 28 as the cycle length.
Does this work with irregular cycles?
Estimates are less reliable if length varies a lot. Use a range or track several cycles before treating one number as typical.
Is this the same as the ovulation calculator?
No. This page projects period start dates. The ovulation calculator estimates a fertile window. Neither is contraception or medical advice.
Is my data uploaded?
No. Dates stay in your browser.
Is this medical advice?
No. It is a calendar estimate for regular cycles, not a diagnosis or birth-control method.
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