What Is a Period Calculator?
A period calculator predicts when your next menstrual period is likely to start, along with your estimated ovulation day and fertile window, based on the date of your last period and your average cycle length. Tracking your cycle helps you anticipate your period, understand your body, plan around symptoms, and identify your most and least fertile days. Enter two simple pieces of information and the calculator maps out your upcoming cycle.
How to Use the Period Calculator
- First day of last period — the date your most recent period began.
- Average cycle length — the number of days from the start of one period to the start of the next (commonly 28, but 21–35 is normal).
- Calculate — see your predicted next period date, ovulation day, and fertile window.
How the Menstrual Cycle Works
The menstrual cycle is counted from the first day of one period to the first day of the next. A typical cycle is around 28 days but varies from person to person. Ovulation — when an egg is released — usually occurs about 14 days before the next period starts. The fertile window spans roughly the five days before ovulation plus the day of ovulation, since sperm can survive several days.
Cycle Phases Overview
| Phase | Approximate Timing (28-day cycle) |
|---|---|
| Menstruation | Days 1–5 |
| Follicular phase | Days 1–13 |
| Ovulation | Around day 14 |
| Luteal phase | Days 15–28 |
What Affects Cycle Length
- Stress, sleep, and significant lifestyle changes
- Diet, weight changes, and intense exercise
- Hormonal contraception and certain medications
- Health conditions such as PCOS or thyroid issues
Cycles that are consistently shorter than 21 days, longer than 35 days, or highly irregular are worth discussing with a healthcare provider.
Tracking for Family Planning
If you are trying to conceive, the fertile window is your key opportunity. If you are avoiding pregnancy, remember that cycle-based prediction is an estimate and not a reliable form of contraception on its own. Tracking over several months improves accuracy as you learn your personal average.
Note: This calculator provides estimates for general informational purposes and is not a substitute for medical advice.
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, your next period is expected about 28 days after the previous one started. The calculator does this automatically.
When am I most fertile?
Your fertile window is roughly the five days before ovulation plus the day of ovulation. Ovulation usually happens about 14 days before your next period begins.
What is a normal cycle length?
A normal menstrual cycle ranges from 21 to 35 days. Around 28 days is average, but consistent cycles within that range are all considered healthy.
Why is my period irregular?
Stress, weight changes, exercise, travel, hormonal birth control, and certain health conditions can all affect cycle regularity. Persistent irregularity is worth raising with a doctor.
Is this period calculator free?
Yes — it is completely free, requires no signup, and predicts your cycle from just two inputs.