About the Period Calculator
This period calculator predicts when your next periods are likely to start and end, based on the first day of your last period, your average cycle length and how many days your period usually lasts. It lays out up to 12 upcoming cycles in a table so you can plan trips, events or appointments months ahead.
For each cycle it also estimates your ovulation day and fertile window — the six days ending on ovulation day plus the day after — which is useful whether you are trying to conceive or simply want to understand your cycle. If you became pregnant this cycle, it shows the estimated due date too.
Predictions assume a regular cycle and that ovulation happens about 14 days before the next period, which is typical but not guaranteed. Stress, illness, travel, breastfeeding, hormonal contraception and conditions such as PCOS can shift dates. Calendar predictions are not a reliable method of contraception.
How to use the period calculator
- 1Enter the first day of your most recent period.
- 2Enter your average cycle length (check a few past cycles if you can).
- 3Enter how many days your period usually lasts.
- 4Choose how many future cycles to list.
- 5Read your next period, ovulation and fertile dates, and the table for later cycles.
Formula and method
Each predicted period start is the first day of your last period plus a whole number of cycle lengths, and it is expected to last the number of days you enter. Ovulation is estimated at 14 days before the next period, because the luteal phase (after ovulation) is fairly constant at around 12–16 days while the follicular phase varies — so longer cycles move ovulation later, not the period earlier.
Sperm can survive up to about five days, and an egg about a day, so the fertile window covers the five days before ovulation, ovulation day and the day after. The due date uses Naegele’s rule (280 days from the last period) adjusted by the difference between your cycle length and 28 days.
- last period
- First day of your most recent period
- cycle length
- Days from one period start to the next
Worked examples
28-day cycle, last period 1 Sep 2026
Adding 28 days to 1 September gives a next period on 29 September, lasting until about 3 October. Ovulation is 14 days earlier, around 15 September, so the fertile window is 10–16 September.
32-day cycle, 6-day period, last period 15 Jan 2027
15 January + 32 days is 16 February. With a longer cycle, ovulation shifts later to day 18 — around 2 February — and the fertile window starts on 28 January. A due date from this cycle would be 284 days after the last period.
Frequently asked questions
How do I calculate when my next period is due?+
Add your average cycle length to the first day of your last period. For a 28-day cycle that started on 1 March, the next period is due on 29 March. Counting from the first day of bleeding, not the last, is the key step.
What is a normal cycle length?+
For adults a cycle anywhere from 21 to 35 days is considered normal, and it is common for it to vary by a few days from month to month. Teenagers and people approaching menopause often have longer or less predictable cycles.
When am I most fertile?+
You are most fertile in the few days leading up to and including ovulation, which usually happens about 14 days before your next period. Sperm can live up to five days, so sex in the days before ovulation can lead to pregnancy.
Why is my period late?+
Common reasons include pregnancy, stress, significant weight change, intense exercise, illness, travel, breastfeeding and starting or stopping hormonal contraception. If you could be pregnant, take a test; if periods stop for three months or more, see a doctor.
Can I use a period calculator as birth control?+
Not on its own. Calendar predictions assume ovulation always happens on schedule, and it often does not. Fertility awareness methods that also track basal body temperature and cervical mucus are more reliable, but still less effective than most contraceptives.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.