How the calculator works
- Next period: the first day of your last period plus your cycle length, repeated for the next three cycles.
- Ovulation: estimated 13 days before each expected period, the same 13-day luteal phase Apple uses in its calendar-method predictions for Cycle Tracking on iPhone. The NHS notes that in most people ovulation happens around 10 to 16 days before the next period, so your real day may differ.
- Fertile window: the six days ending on the estimated ovulation day. You're most likely to conceive around ovulation, and sperm can survive for up to 7 days.
- Today's phase: menstrual during your period, follicular until the fertile window, ovulation during the window, then luteal until your next period.
When the estimates won't fit
- Irregular cycles. Cycles usually come 21 to 35 days apart and vary a little. If yours vary a lot, calendar estimates become unreliable, and very irregular periods are worth raising with your doctor. See why is my period late?
- Hormonal contraception. The combined pill stops ovulation, so ovulation and fertile-window estimates don't apply. See how to track your period on the pill.
- Trying to conceive. Ovulation tests and tracking your own cycles over a few months give a far better picture than a calendar estimate.
What each phase means
Each phase has its own hormone pattern, which is why energy, mood and sleep shift through the month. Our plain-English guide to the four phases of the menstrual cycle explains what to expect in each.
Get predictions that learn from you
A calculator assumes every cycle is the same length. SheSync learns from the periods you actually log, flags each cycle as regular or irregular against your own baseline, and tells you which phase you're in every day. Tracking and predictions are free, and on the free plan your logs never leave your phone. With SheSync Pro you can also share your cycle with a partner, who gets the context too.
Frequently asked questions
How do I calculate when my next period is due?
Count your usual cycle length from the first day of your last period. If your last period started on 1 March and your cycles are 28 days, your next period is due around 29 March. Cycles between 21 and 35 days are common, and they can vary from month to month.
How is ovulation estimated?
This calculator uses the calendar method: ovulation is estimated 13 days before your next expected period (the same 13-day figure Apple uses for Cycle Tracking), and the fertile window shown is the six days ending on that day. In reality ovulation happens anywhere from about 10 to 16 days before the next period, so treat it as an estimate.
Can I use an ovulation calculator as contraception?
No. Calendar estimates can be wrong by several days, and sperm can survive for up to 7 days. Never rely on a calculator or period tracker app to prevent pregnancy.
Does this work if I'm on the pill?
No. The combined pill stops ovulation, and the bleed during your inactive pills is a withdrawal bleed, so cycle and ovulation estimates don't apply. See how to track your period on the pill.
Sources
- Menstruation (periods), healthdirect
- Periods and fertility in the menstrual cycle, NHS
- Track your period with Cycle Tracking, Apple Support
- Irregular periods, healthdirect
- Hormonal contraceptives and periods, healthdirect
Last updated 7 October 2026. SheSync is a tracking and education tool, not a medical device or contraceptive method. For medical questions about your cycle, talk to your doctor.