Predicting your next period
The menstrual cycle runs from the first day of one period to the first day of the next. If you know when your last period started and your average cycle length, you can estimate when the next ones will come. This calculator predicts the next three periods, when the next one will end, and your next ovulation.
The formulas
- Next period = first day of the last period + cycle length
- End of the next period ≈ next period + period length − 1
- Following periods: add the cycle length again
- Ovulation ≈ next period − 14 days
Worked example
Your last period started on October 24, 2026, with a 28-day cycle and 5-day periods.
- Next period: October 24 + 28 days = November 21, 2026, ending around November 25.
- The following ones: December 19, 2026 and January 16, 2027.
- Next ovulation: around November 7.
- On November 3, the next period is 18 days away.
What is a typical cycle?
| Measure | Typical range in adults |
|---|---|
| Cycle length | 21 to 35 days |
| Period length | 2 to 7 days |
| Variation between cycles | up to 7 to 9 days |
| Blood loss per period | about 2 to 3 tablespoons (30 to 40 mL) |
Teenagers in the first years after their first period, and people approaching menopause, often have less regular cycles.
Finding your average cycle
Note the first day of each period for three to six months, count the days between them and take the average. If your cycles vary a lot, the prediction will have a wider margin of error.
When a period is late
Pregnancy is one reason, and a home pregnancy test is reliable from the first day of a missed period. Other common causes include stress, illness, travel and time-zone changes, significant weight loss or gain, intense training, breastfeeding, and some medicines and hormonal contraceptives.
When to see a doctor
See a health care provider if your periods stop for three months or more without pregnancy, if cycles are regularly shorter than 21 or longer than 35 days, if periods last more than 7 days, if bleeding soaks a pad or tampon every hour, or if pain stops you from doing everyday activities.
A note on medical advice
This calculator gives estimates based on averages. It does not replace advice from your doctor, midwife or health care provider, who will date your pregnancy and follow it with exams and ultrasounds.