The short answer
A period calculator predicts your next period by adding your average cycle length to the first day of your last period. With a 28-day cycle, your next period starts 28 days after the last one began. The tool also lists your next 12 periods and shows whether your period is late.
Formulas and sources are named on this page. How we calculate.
How this is calculated
The calculator counts forward from the first day of your last period. Each cycle runs from the first day of one period to the day before the next period starts, so the next period begins exactly one cycle length after the last one began.
Next period starts = first day of last period + cycle length
Period ends = start date + (period length − 1) days
Each following period = previous start + cycle length
Estimated ovulation = next period start − 14 days
Worked examples
28-day cycle, 5-day period, last period started 1 October 2026. Next period = 1 October + 28 days = 29 October, lasting until 2 November. The period after that starts on 26 November. Ovulation in the cycle that ends on 29 October is estimated for 15 October.
35-day cycle, same start date. Next period = 1 October + 35 days = 5 November.
Late period. With a 28-day cycle and a last period on 1 October, a period due on 29 October that has not started by 5 November is 7 days late. At that point a pregnancy test is the next step.
What counts as a normal cycle?
The American College of Obstetricians and Gynecologists (ACOG) describes a typical adult cycle as 21 to 35 days, with a typical period lasting 2 to 7 days. The NHS gives the average as about 28 days, with a period usually lasting about 5 days. In the first few years after periods start, cycles are often longer and less regular, commonly 21 to 45 days.
Why predictions can be off
The calculator assumes your next cycle will be the same length as your average. In reality the length of a cycle can change from month to month. Stress, illness, travel, changes in weight or exercise, breastfeeding, some medicines and hormonal birth control can all move a period earlier or later. The further into the future a date is, the less certain it becomes, which is why the calendar is best used as a planning guide and refreshed each time a new period starts.
The four phases of a cycle
The cycle has two halves either side of ovulation. The follicular phase runs from the start of your period until ovulation, and the luteal phase runs from ovulation until the next period. The luteal phase is fairly steady at around 14 days, so changes in cycle length mostly come from the first half.
Next period date by cycle length
Examples for a last period that started on 1 October 2026| Cycle length | Next period starts | Estimated ovulation |
|---|
| 21 days | 22 October | 8 October |
| 24 days | 25 October | 11 October |
| 26 days | 27 October | 13 October |
| 28 days | 29 October | 15 October |
| 30 days | 31 October | 17 October |
| 32 days | 2 November | 19 October |
| 35 days | 5 November | 22 October |
Ovulation is estimated 14 days before the next period starts, so it falls on cycle day 8 in a 21-day cycle and day 22 in a 35-day cycle (day 1 is the first day of the period).
Frequently asked questions
How do I calculate when my next period is due?
Count from the first day of your last period. Add your average cycle length to that date. For a 28-day cycle, the next period is due 28 days after the last one started. The calculator does this for you and also lists the following 11 periods.
What is a normal cycle and period length?
ACOG describes a typical adult cycle as 21 to 35 days, with a period that lasts 2 to 7 days. The NHS gives an average of about 28 days between periods and about 5 days of bleeding. Teens often have longer cycles of 21 to 45 days while their cycles settle.
When should I take a pregnancy test if my period is late?
The NHS says you can do most pregnancy tests from the first day of a missed period. A positive result is almost certainly correct, but a negative result is less reliable, especially if taken too early. If you get a negative result and your period has not started, wait a few days and test again.
Why is my period late if I am not pregnant?
Common causes include stress, illness, a large change in weight or exercise, polycystic ovary syndrome (PCOS), thyroid problems, breastfeeding, perimenopause and some medicines such as hormonal birth control. A single late period is often nothing to worry about, but it is worth talking to a doctor if it keeps happening.
How accurate is a period calculator?
It is most accurate when your cycles are regular and you enter an average based on several recent cycles. The prediction is only as steady as your cycle, so a period can still arrive a few days early or late. Enter the new start date each month to keep the dates current.
When should I see a doctor about my periods?
See a doctor if you have missed three periods in a row, your periods stop for more than three months, bleeding lasts more than seven days, you soak a pad or tampon every one to two hours, or your cycle is often shorter than 21 or longer than 35 days. Seek help if you feel dizzy or faint with heavy bleeding.
Disclaimer and editorial note
For information only, not medical advice. This calculator gives general estimates from the numbers you enter. It cannot diagnose, treat or prevent any condition and does not replace advice from a doctor, midwife, registered dietitian or other qualified professional who knows your history.
If you are worried about your periods, a possible pregnancy or heavy bleeding, speak with a doctor, nurse or midwife. This calculator cannot diagnose a medical problem and is not a contraceptive.
Written and maintained by the HealthFitnessHQ editorial team. We name the source of every formula and category used, and we review each calculator when guidance changes. Read how we work in our editorial policy and methodology.