Period CalculatorTracking your cycle length over time is one of the most powerful things you can do for your hormonal health. A single cycle tells you little — but a series of cycles reveals patterns, irregularities, and trends that can help you predict your period, plan your fertility, and detect health concerns early.
Cycle Length Tracker
Log your menstrual cycles, automatically calculate your average cycle length, and visualize trends on an interactive chart. Data saves to your device — private and persistent.
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Why Track Your Cycle Length?
What Is a Normal Cycle Length?
| Cycle Length | Classification | What It May Mean |
|---|---|---|
| Less than 21 days | Polymenorrhea | Short luteal phase; may indicate low progesterone or thyroid issues |
| 21–35 days | Normal | Healthy range; most women fall here |
| 36–45 days | Oligomenorrhea | Infrequent periods; may be related to PCOS, thyroid, or high stress |
| More than 45 days | Amenorrhea risk | Medical evaluation recommended |
Pro Tip: Track at least 3–6 cycles before drawing conclusions. Single-cycle anomalies due to stress or illness are common and not always a cause for concern.
Cycle Variation: What's Normal?
Not every cycle is the same length, and that's perfectly normal. A variation of 2–7 days between your shortest and longest cycle is considered typical. However, consistently unpredictable cycles with a variation of 9+ days may warrant investigation.
Common causes of cycle variation:
• Seasonal changes (cycles can lengthen in winter)
• Travel across time zones
• Significant stress or emotional events
• Illness or infection
• Changes in exercise intensity or diet
• Perimenopause (women 40+)
When to see a doctor: If your cycle is consistently shorter than 21 days, longer than 35 days, or you experience sudden changes in cycle regularity lasting more than 3 months, speak with a healthcare provider.
Frequently Asked Questions
How many cycles should I track before the data is meaningful?
At least 3 cycles to establish a baseline average, and 6+ cycles to identify reliable patterns or detect irregularities. The more data you log, the more accurate your insights become.
Does my data get saved if I close the browser?
Yes! Your cycle log is saved to your device's localStorage. It persists across browser sessions on the same device. However, clearing browser data or switching devices will reset it.
My cycle varies a lot — is that a problem?
Variation of up to 7 days is usually normal. Use our Irregular Period Analyzer to assess your specific data and get a more detailed assessment with guidance on when to consult a doctor.