- Is my data sent to a server or stored in an account?
- No. Everything — your last period date, cycle length, and history — is stored only in your browser's
localStorage. It never leaves your device. There is no account, no login, and no server. If you clear your browser data or switch browsers, the history will be gone, so use the CSV export to back it up.
- How accurate is the next period date calculation?
- The estimate assumes a consistent average cycle length. Real cycles vary by a few days due to stress, illness, weight changes, or hormonal shifts. The more cycles you log, the more precisely you can tune your average. This tool is for planning reference — not medical diagnosis. If your cycles are very irregular (varying by more than 7–9 days), consult a healthcare provider.
- What is the fertile window and why does it span 5 days?
- Ovulation releases an egg that survives about 12–24 hours. However, sperm can survive in the reproductive tract for up to 5 days. The fertile window therefore covers the 2 days before ovulation through 2 days after (and ovulation day itself), giving 5 days total where conception is most likely. The ovulation day itself carries the highest probability.
- Why does the calculator subtract 14 days to find ovulation?
- The luteal phase — from ovulation to the next period — is remarkably consistent at about 14 days across most people. What varies between individuals is the follicular phase (period to ovulation). So ovulation = next period date − 14 days. For a 28-day cycle this puts ovulation at day 14; for a 35-day cycle, at day 21.
- How do I log a past cycle to improve my average?
- Enter the start date of that past period in the "Last period start date" field and click Log This Cycle. Up to 12 past cycles are stored. Once you have several cycles logged, calculate the average by adding up all cycle lengths and dividing by the count, then enter that as your "Average cycle length".