Period Calculator · Practical guide
When Is My Next Period Due?
Add your cycle length to the first day of your last period — that is your predicted start date.
At a glance
The Basic Rule
Your next period is due about one full cycle length after the first day of your last period. Most cycles run 21 to 35 days, with 28 days as the classic average. If your last period started on August 20 and your cycle is 28 days, your next period is due around September 17. With a typical 5-day period, it would run through about September 21.
Interactive tool
Period Calculator
Enter your own values, then compare the result with the examples below.
Cycle Prediction
How Cycle Length Shifts the Date
Same last period (August 20), different cycle lengths:
| Cycle Length | Next Period Due |
|---|---|
| 21 days | September 10 |
| 25 days | September 14 |
| 28 days | September 17 |
| 30 days | September 19 |
| 35 days | September 24 |
What Else the Prediction Includes
Along with the start and end dates, the calculator above estimates your ovulation date — roughly 14 days before your next period starts (September 3 in the example above) — and counts down the days remaining until your next period. Enter your own last period date, average cycle length, and typical duration for dates tailored to you.
Why Predictions Are Estimates
Stress, travel, illness, and hormonal changes can shift a cycle by several days, and cycles that vary in length from month to month are harder to predict. The estimate is most reliable when your cycles are regular and you use your true average cycle length rather than the textbook 28 days.
Frequently Asked Questions
How do I count my cycle length?
Count from the first day of one period to the first day of the next. Track three or more cycles and average them for the best prediction.
What counts as a late period?
A period is generally considered late when it is more than about 5 to 7 days past your expected date, or when your cycle stretches beyond 35 days.
Can stress make my period late?
Yes. Stress, travel, illness, and significant weight or exercise changes can all delay ovulation, which pushes your period back.
Is this calculator a substitute for medical advice?
No. It is an informational estimate based on regular cycles. See a healthcare provider for concerns about irregular or missed periods.