The calendar method
This calculator uses the standard calendar-based method for cycle tracking, which assumes a relatively consistent luteal phase (the time from ovulation to the next period) of about 14 days, with the earlier follicular phase varying more between individuals and cycles.
How the estimates are made
Days Until Next Period is your average cycle length minus how far you are into the current cycle. Estimated ovulation is placed 14 days before the next expected period, and the fertile window is estimated as the 5-6 days ending on ovulation day, since sperm can survive in the reproductive tract for several days before an egg is released.
- Days Until Next Period = Cycle Length − Days Already Elapsed
- Estimated Ovulation Day = Cycle Length − 14 days
This tool provides general estimates for planning and educational purposes only, based on average cycle assumptions. Actual cycles vary and can be affected by stress, illness, travel and many other factors; it is not a method of contraception or a medical diagnostic tool.