How the period calculator works
The calculator assumes your cycles are regular and simply repeats your average cycle length forward from the first day of your last period.
Period end = period start + period length − 1 day
Ovulation ≈ next period start − 14 days
Day 1 of a cycle is the first day of proper bleeding, not spotting. Your cycle length is the number of days from one day 1 to the next. If today is during a predicted period, that period is shown first; otherwise the list starts with the next one.
Each row also shows an estimated fertile window: the six days that end on ovulation day. That comes from a 1995 study in the New England Journal of Medicine by Wilcox and colleagues, which found that conception happened only after sex during those six days. Ovulation is placed 14 days before the following period because the second half of the cycle (the luteal phase) is fairly steady at about 10 to 16 days, while the first half varies more. If you know your own luteal phase length, enter it.
Worked examples
A regular 28-day cycle. Your last period started on 1 October and usually lasts 5 days. The next period is due on 1 October + 28 days = 29 October, ending around 2 November. Ovulation is estimated 14 days before the period after that (26 November), so on 12 November, with a fertile window of 7 to 12 November.
A longer 32-day cycle. Your last period started on 10 March and lasts 4 days. Next period: 10 March + 32 = 11 April, ending 14 April. In a 32-day cycle ovulation falls around cycle day 19 (32 − 14 + 1), not day 14. This is why counting “two weeks after your period” can be misleading for anyone whose cycle is not 28 days.
Checking a late period. Your last period started on 1 October and your cycle is 28 days, so the next was due on 29 October. If it is now 2 November and nothing has started, your period is about 4 days late. The calculator tells you this and still projects future cycles, in case you simply forgot to log a period.
What is a normal cycle?
| Measure | Typical range | Source |
|---|---|---|
| Cycle length (adults) | 21–35 days | ACOG |
| Cycle length (common) | 21–40 days, average 28 | NHS |
| Cycle length (teens, first years) | 21–45 days | ACOG |
| Period length | 2–7 days | NHS, ACOG |
| Luteal phase | About 10–16 days | NHS |
Cycles are often irregular in the first years after periods start and again in the years before menopause. Small differences of a few days from month to month are normal.
The four phases of the cycle
- Menstruation (period). The lining of the womb sheds. This is cycle day 1 until bleeding stops.
- Follicular phase. Rising oestrogen thickens the lining while an egg matures in a follicle. It starts on day 1 too and lasts until ovulation, so it is the part that changes most in length.
- Ovulation. A surge of luteinising hormone (LH) releases the egg, usually about 14 days before the next period. The egg survives for about a day.
- Luteal phase. The empty follicle makes progesterone. If no pregnancy happens, hormone levels fall and the next period starts. Premenstrual symptoms, if you get them, usually show up here.
The result tells you which phase you are probably in today, based on your cycle day.
Tips for better predictions
- Track three to six cycles. Write down the first day of each period, then use the average gap as your cycle length.
- Use the right day 1. Count from the first day of real flow. Light spotting the day before does not count.
- Re-enter your latest period. When a new period starts, update the date. A recent starting point is far more accurate than one projected forward several months.
- Expect some drift. Stress, illness, travel, big changes in weight or exercise, breastfeeding and stopping hormonal contraception can all shift a period.
- For fertility, add body signs. Ovulation test kits, cervical mucus and basal body temperature show what your body is actually doing, which a calendar cannot.
Not a form of contraception
These dates are averages. Ovulation can happen earlier or later than predicted, and sperm can live for up to 7 days, so there is no reliably “safe” day from a calendar estimate alone. Fertility awareness methods taught by a trained practitioner combine several signs, and the NHS notes they are much less effective if the instructions are not followed exactly. If you do not want to get pregnant, use contraception.
When to see a doctor
Speak to a GP, doctor or nurse if any of these apply:
- Your periods are often less than 21 days or more than 35 days apart, or the gap changes a lot from month to month.
- Bleeding lasts more than 7 days, or you bleed through a pad or tampon every hour or two.
- Period pain stops you doing normal activities.
- You bleed between periods, after sex or after menopause.
- Your periods stop for 3 months or more and you are not pregnant.
Related calculators
If you are trying to conceive, the ovulation calculator focuses on your most fertile days and shows a due date for each cycle. If you are already pregnant, the pregnancy due date calculator works out how many weeks along you are. To count days between any two dates, use the date calculator.