Period Calculator
Free period calculator and menstrual cycle tracker — predict your next period start date, ovulation day, and fertile window based on your last period and average cycle length.
Short answer: Period Calculator — Predict your next period and fertile window. Free period calculator and menstrual cycle tracker — predict your next period start date, ovulation day, and fertile window based on your last period and average cycle length. It answers “period calculator next period fertile window”. Who it is for: Anyone tracking their menstrual cycle; People planning around their next period dates; Anyone learning their fertile window. It runs entirely in your browser — no sign-up, no account, no email and no upload — and every result can be exported as a dated PDF report.
- ✔ Price: free, unlimited use, no rate limits
- ✔ Sign-up: not required
- ✔ Privacy: all math runs client-side, nothing is uploaded
- ✔ Export: one-click PDF report (2026 rates)
- • Thu, Dec 10, 2026
- • Thu, Jan 7, 2027
- • Thu, Feb 4, 2027
Educational estimate only — not a substitute for medical advice or contraception.
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Who is this tool for?
- ✔ Anyone tracking their menstrual cycle
- ✔ People planning around their next period dates
- ✔ Anyone learning their fertile window
- ✔ People noticing an irregular cycle and wanting dates to show a clinician
- ✔ Anyone who wants cycle dates without an app account
How to use Period Calculator
- Enter the first day of your last period.
- Set your average cycle length (default 28 days).
- Set your average period length (default 5 days).
- Instantly see your next 3 period dates, ovulation day, and fertile window.
Frequently asked questions
How is my next period date calculated?
We add your average cycle length (typically 28 days) to the first day of your last period. Cycles usually range from 21 to 35 days.
When is my ovulation day?
Ovulation typically occurs 14 days before your next period starts. This calculator estimates it based on your cycle length.
What is the fertile window?
The 6 days ending on ovulation — sperm can survive up to 5 days, so the days before ovulation are the most fertile.
How accurate is this period calculator?
Very accurate for regular cycles. If your cycles vary, use your average over the last 3–6 months for the best prediction.
Why is my period late?
Stress, travel, weight change, illness, hormonal shifts, or pregnancy can delay a period. See a doctor if it is more than 2 weeks late.
Is my data saved anywhere?
No — everything runs in your browser. Nothing is sent to any server or stored online.
Period calculator next period fertile window?
Use the Period Calculator above: Predict your next period and fertile window. Free period calculator and menstrual cycle tracker — predict your next period start date, ovulation day, and fertile window based on your last period and average cycle length.