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.
- • Mon, Aug 17, 2026
- • Mon, Sep 14, 2026
- • Mon, Oct 12, 2026
Educational estimate only — not a substitute for medical advice or contraception.
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.