To calculate your next period, count your cycle length: the days from the first day of one period to the first day of the next. Add that number to the first day of your last period. For example, if your last period started on 3 March and your cycle is 29 days long, your next period is due around 1 April. Averaging several cycles gives a better estimate than using one.
Step 1: count your cycle length
Day 1 of your cycle is the first day of real bleeding, not spotting. Your cycle ends the day before your next period starts. The average cycle is about 28 days, but anything from 21 to 40 days is common in adults.
Step 2: average a few cycles
One cycle can be shorter or longer than usual, so use an average:
| Cycle | Length |
|---|---|
| January | 28 days |
| February | 31 days |
| March | 29 days |
| Average | (28 + 31 + 29) ÷ 3 ≈ 29 days |
Three to six cycles give a good average. Add it to the first day of your last period to get the expected start date.
Step 3: add your period length
Count how many days you usually bleed, often 3 to 7. The estimated period runs from the expected start date for that many days. Our Try our free period calculator does steps 1 to 3 for you and projects your next few periods.
Why the date can move
A period calculator gives an estimate, not a guarantee. Ovulation can come earlier or later because of stress, illness, travel, weight changes, breastfeeding, perimenopause or stopping hormonal contraception. When ovulation moves, the period moves with it. That is why it helps to think of a range of a few days rather than one exact date.
Irregular cycles
If your cycle length varies a lot, an average is less useful. Instead:
- Note your shortest and longest cycle: your next period will most likely fall between those two lengths.
- Track ovulation signs such as LH tests or basal body temperature: your period usually comes 10 to 16 days after ovulation.
- Keep logging; the range becomes clearer with more cycles.
Our guide on irregular period tracking covers the common causes and what to log.
When to see a doctor
- Your periods suddenly become irregular and you are under 45.
- Your cycle is shorter than 21 days or longer than 35 days.
- Your periods last longer than 7 days.
- There is a big difference, 20 days or more, between your shortest and longest cycles.
- Your period is late and you could be pregnant: take a test first.
Predicting your period in Bloom Private
Bloom Private estimates your next period and fertile window from the cycles you log, and the estimate becomes more personal as you add cycles. Your records stay on your iPhone. The app is not a medical device and not a contraceptive method.
Frequently Asked Questions
Add your average cycle length to the first day of your last period. The cycle length is the number of days from the first day of one period to the first day of the next.
Three to six cycles give a good average. With irregular cycles, use the range between your shortest and longest cycle instead.
If cycles are shorter than 21 days or longer than 35 days, if periods last more than 7 days, or if your cycles suddenly become irregular.