How to use the period calculator
- Enter the first day of your last period – the first day of real bleeding, not spotting.
- Choose your average cycle length: the days from the start of one period to the start of the next. If you are not sure, count the last three cycles and take the average.
- Choose how many days your period usually lasts.
- Press Calculate for your next period, today's cycle day, a three-month colour calendar and the dates of your next 12 periods.
How the dates are predicted
A cycle starts on the first day of a period. Assuming your cycles are regular, each period starts one cycle length after the last:
\text{next period} = \text{first day of last period} + \text{cycle length}Ovulation happens about 14 days before the next period, so the calculator places it on day cycle length − 14 and marks the fertile window as the five days before ovulation, ovulation day and the day after.
The phases of the menstrual cycle
| Phase | Days (28-day cycle) | What happens |
|---|---|---|
| Period (menstruation) | 1–5 | The lining of the womb sheds. |
| Follicular phase | 1–13 | An egg matures; oestrogen rises and the lining rebuilds. |
| Ovulation | about 14 | An egg is released; the most fertile time. |
| Luteal phase | 15–28 | Progesterone rises. PMS symptoms can appear in the last week. |
What is a normal cycle?
- Cycle length: 21–35 days in adults (21–45 days in the first years after periods start). It is normal for the length to vary by a few days.
- Period length: 2–7 days.
- Blood loss: usually 30–40 ml over a period; soaking a pad or tampon every hour for several hours is heavy.
See a doctor if your cycles are often shorter than 21 days or longer than 35, if periods last more than 7 days, stop for 3 months or more without pregnancy, or are very painful or heavy.
Why a period can be late
Pregnancy is the first thing to rule out – a home test is reliable from the day the period was due. Other common causes are stress, illness, travel, big changes in weight or exercise, breastfeeding, some contraceptives and polycystic ovary syndrome (PCOS). Cycles also become irregular in the years before menopause.
Predictions are only as regular as your cycles, and this calculator is not a method of contraception. For the most fertile days in more detail, use the ovulation calculator.
Frequently asked questions
How do I calculate my next period?
Add your cycle length to the first day of your last period. With a 28-day cycle and a period that started on March 1, the next is due on March 29.
How do I work out my cycle length?
Count the days from the first day of one period to the day before the next one starts. Average the last three cycles for a better estimate.
Is a 35-day cycle normal?
Yes. Adult cycles of 21–35 days are considered normal, and a few days of variation from month to month is common.
How late can a period be before I should worry?
A few days late is common. If it is a week or more late, take a pregnancy test; if periods stop for three months or more, see a doctor.
Can I get pregnant on my period?
It is unlikely but possible, especially with short cycles, because sperm can survive up to five days and ovulation may come early.