How to Calculate Your Next Period
Add your average cycle length to the first day of your last period. That is the core of most period predictions — including this site’s period calculator.
The short answer
Estimate your next period by counting forward from the first day of your last period by your average cycle length:
Next period start ≈ last period start + average cycle length
If your last period started on October 1 and your average cycle is 28 days, the estimate is October 29. If October 29 has already passed, add another cycle to find the next expected date. The calculator projects forward automatically.
What you need
Two pieces of information are enough for a basic estimate:
- The first day of your last period. Use the first day you bled, not the last day.
- Your average cycle length. Count from the first day of one period to the first day of the next.
A cycle length of 21 to 35 days is commonly described as typical, but cycles outside that range can still be normal for some people. If you have tracked several cycles, an average is usually more useful than one unusually short or long cycle.
A worked example
Suppose your last period began on March 3 and your average cycle is 30 days. March 3 + 30 days is April 2, so April 2 is the estimated next period. If your next period actually starts on April 4, your cycle for that month was 32 days, and your average will gradually adjust as you track more cycles.
The same cycle math can estimate ovulation as about 14 days before the next expected period. The ovulation calculator and fertile window calculator show that wider estimate.
Not a contraceptive method
Calendar estimates cannot reliably identify days when pregnancy is impossible. Do not use them as contraception. Talk with a healthcare professional about methods that fit your situation.
What can shift the date
A predicted date is a planning estimate, not a promise. Common factors that can change cycle timing include:
- Stress, poor sleep, illness, or travel
- Significant weight change or changes in exercise
- Starting, stopping, or changing hormonal medication
- Pregnancy, breastfeeding, perimenopause, and other hormonal changes
- Conditions that affect ovulation, such as polycystic ovary syndrome (PCOS) or thyroid problems
You do not need to diagnose the cause yourself. If your cycles are consistently very irregular or have changed significantly, a clinician can assess what is happening.
How accurate is the estimate?
For people with fairly regular cycles, a calendar estimate can be a useful planning tool. It is less reliable when cycles vary widely. Period apps may use different assumptions, logged history, or different averages, so a few days of difference between tools is common. The method here is transparent: last period start plus your average cycle length.
When to talk to a healthcare professional
Consider seeking advice if your periods are very irregular, very heavy, painful enough to disrupt daily life, or if you bleed between periods or after sex. Also seek advice if your periods stop and pregnancy is possible, or if something feels wrong. A calculator cannot assess these symptoms.
Related guides
Questions about period estimates
Can I calculate my next period if my cycles are irregular?
You can enter an average, but the estimate may be less reliable. If your cycles vary widely, track several cycles or talk with a healthcare professional. A single average cannot capture large cycle-to-cycle variation.
Why is my period app showing a different date?
Apps may use your logged history, different assumptions about the luteal phase, or a different average. The method here is last period start plus your average cycle length. Neither result is a guarantee.
Can I use this to avoid pregnancy?
No. Calendar-based predictions are not a reliable method of contraception. They cannot reliably identify days when conception is impossible.
General references
These organizations provide general health information. They are external sites with their own content and policies, and this site is not affiliated with them.