Period Calculator
Predict your next three period start and end dates based on your last period date and average cycle length. All calculations happen in your browser — nothing is stored or transmitted.
This calculator provides general estimates for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.
How It Works
Menstrual cycle tracking is based on two inputs: the start date of the last period and the average cycle length. The cycle length is measured from the first day of one period to the first day of the next.
Prediction formula:
Next period start = LMP + cycle length
Following period = LMP + (2 × cycle length)
Third period = LMP + (3 × cycle length)
Period end date = Period start + period duration − 1
Average cycle statistics:
- Average cycle length: 28 days (range: 21–35 days)
- Average period duration: 2–7 days (most commonly 3–5 days)
- Cycles shorter than 21 days or longer than 35 days may warrant medical evaluation
These predictions assume a regular cycle. In reality, cycle length can vary by several days from cycle to cycle due to stress, exercise, illness, hormonal fluctuations, and other factors. Tracking actual dates over several months gives you a personalised average.
Note: This calculator is for informational and planning purposes only. It is not a contraceptive method and should not be used for family planning without additional contraception.
Frequently Asked Questions
What is a normal cycle length?
The average menstrual cycle is 28 days, but cycles between 21 and 35 days are considered normal for adult women. Cycles outside this range, or significant variation from cycle to cycle, may be worth discussing with a healthcare provider.
Why is my period irregular?
Irregular periods can be caused by stress, significant weight changes, intense exercise, thyroid disorders, polycystic ovary syndrome (PCOS), perimenopause, and other hormonal conditions. If your cycles are regularly unpredictable, consult a healthcare provider.
Can I use this calculator as birth control?
No. Predictions are based on averages and cannot account for cycle variability. This calculator is for planning and awareness purposes only, not for contraception.
Is my data private?
Yes. All calculations are performed entirely in your browser using JavaScript. No dates or personal information are transmitted to or stored on any server.