How the calendar is built
A cycle runs from the first day of one period to the first day of the next. Add the cycle length to your last period's start date and you have the next; keep adding and you have the year. The period length sets each row's end date, and the fertile window and ovulation day come from the standard 14-day luteal phase (ovulation 14 days before the next period). The example below uses a 28-day cycle and a 5-day period starting April 6, 2026.
| Cycle | Period starts | Period ends | Fertile window |
|---|---|---|---|
| 1 | May 4, 2026 | May 8, 2026 | May 13, 2026 – May 19, 2026 |
| 2 | June 1, 2026 | June 5, 2026 | June 10, 2026 – June 16, 2026 |
| 3 | June 29, 2026 | July 3, 2026 | July 8, 2026 – July 14, 2026 |
| 4 | July 27, 2026 | July 31, 2026 | August 5, 2026 – August 11, 2026 |
| 5 | August 24, 2026 | August 28, 2026 | September 2, 2026 – September 8, 2026 |
| 6 | September 21, 2026 | September 25, 2026 | September 30, 2026 – October 6, 2026 |
What a normal cycle looks like
The 28-day cycle is an average, not a rule. Adult cycles of 21 to 35 days are normal, and variation of up to a week or so between cycles is common. Teenagers' cycles are often longer and less regular for the first few years; cycles also change in the years before menopause. Persistent departures from these ranges — very heavy bleeding, cycles under 21 or over 35 days, bleeding between periods, or periods that stop — are worth a conversation with a clinician.
| Feature | Adults | Teens | Worth discussing |
|---|---|---|---|
| Cycle length | 21–35 days (adults) | 21–45 days (teens) | Varying by more than 7–9 days |
| Period length | 2–7 days | Over 8 days | |
| Blood loss | 30–40 mL (2–3 tbsp) | Over 80 mL; soaking a pad/tampon hourly | |
| Cycle-to-cycle variation | Up to 7–9 days | Skipped periods, or under 21 / over 35 days |
Periods per year
Cycle length changes how many periods a year brings: thirteen for a 28-day cycle, fewer than eleven for a 35-day one. The calculator also totals the days of bleeding a year implies, which is useful for budgeting supplies and for noticing when a change in pattern has crept in.
| Cycle length | Periods per year | Days of bleeding (5-day period) |
|---|---|---|
| 21 days | 17.4 | 87 |
| 24 days | 15.2 | 76 |
| 26 days | 14.0 | 70 |
| 28 days | 13.0 | 65 |
| 30 days | 12.2 | 61 |
| 32 days | 11.4 | 57 |
| 35 days | 10.4 | 52 |
Phases of the cycle
The cycle has two halves around ovulation. In the follicular phase an egg matures and estrogen rebuilds the uterine lining; it begins with the period itself. After ovulation the luteal phase, run by progesterone, prepares the lining for implantation and, if none occurs, ends with the next period. Premenstrual symptoms belong to the late luteal phase. The follicular phase is what varies between people and cycles; the luteal phase is close to 14 days for most.
| Phase | Days | What happens |
|---|---|---|
| Menstrual | Days 1–5 | Period; lining shed |
| Follicular | Days 1–13 | Follicle matures; estrogen rises; lining rebuilds |
| Ovulation | Day 14 | Egg released |
| Luteal | Days 15–28 | Progesterone; PMS symptoms; lining ready |
Getting better predictions
The prediction is only as good as the cycle length you enter. Average the last three to six cycles rather than using one. If your cycles vary by more than a few days, run the calculator with your shortest and longest lengths to see the earliest and latest the period could start. Tracking start dates for a few months — in a notebook or an app — gives you the number. Hormonal contraception, breastfeeding, significant weight change, stress, travel and illness all shift cycles; a late period is not unusual, and a missed one after unprotected sex warrants a test.
Related tools
The ovulation calculator focuses on the fertile window in detail with luteal-phase adjustment; the due date and pregnancy calculators take over once a test is positive; and the conception calculator works backwards from a due date or birth date.