Period Calculator · Practical guide
Period Calculator by Cycle Length
A 21-day cycle and a 35-day cycle produce period dates two weeks apart — your cycle length drives every prediction.
At a glance
Why Cycle Length Is the Key Input
Every period prediction comes down to one number: your average cycle length, counted from the first day of one period to the first day of the next. Normal cycles range from 21 to 35 days. The 28-day cycle is only an average — using your real cycle length is the single biggest improvement you can make to a prediction.
Interactive tool
Period Calculator
Enter your own values, then compare the result with the examples below.
Cycle Prediction
Same Last Period, Different Cycle Lengths
If your last period started August 20, here is how the prediction moves with cycle length:
| Cycle Length | Next Period Starts | Estimated Ovulation |
|---|---|---|
| 21 days | September 10 | August 27 |
| 24 days | September 13 | August 30 |
| 28 days | September 17 | September 3 |
| 30 days | September 19 | September 5 |
| 35 days | September 24 | September 10 |
Across the normal range, the predicted start date swings by a full two weeks — and ovulation moves with it, always about 14 days before the next period.
How to Find Your Real Cycle Length
Write down the first day of each period for at least three months, count the days between consecutive starts, and average them. If your cycles were 27, 29, and 28 days, your average is 28 — enter that. Slight variation of a day or two is normal and barely moves the prediction.
When Cycle Length Signals Something More
Cycles consistently shorter than 21 days or longer than 35 days, cycles that suddenly change pattern, or periods that disappear for months are worth a conversation with a healthcare provider. So are very heavy or severely painful periods. This calculator is an informational tool, not a medical diagnosis.
Frequently Asked Questions
What is a normal menstrual cycle length?
Most cycles fall between 21 and 35 days, with 28 days as the commonly cited average. Your personal normal may be anywhere in that range.
Is a 35-day cycle normal?
It sits at the long end of the normal range. If your cycles are consistently around 35 days, that pattern is generally considered normal for you.
Why does my cycle length change from month to month?
Stress, travel, illness, poor sleep, and changes in exercise or weight can all delay ovulation, lengthening that month's cycle by a few days.
When should I see a doctor about my cycle?
See a healthcare provider if cycles are consistently under 21 or over 35 days, if periods stop for several months, or if bleeding is very heavy or painful.