How to use the period calculator
- Enter the first day of your last period: the first day of real bleeding, not light spotting before it.
- Enter your average cycle length, from the first day of one period to the first day of the next. If you track your cycles in an app, it will show this number.
- Enter how many days your period usually lasts.
- Read the date of your next period, and the table of your next six periods with the fertile days in each cycle.
How periods are predicted
Each later period is one more cycle length on. The fertile days use the usual assumption that ovulation happens about 14 days before the next period. The time after ovulation, the luteal phase, varies much less than the first part of the cycle. The six-day fertile window reflects how long sperm (up to 5 days) and an egg (about a day) can survive. If you know your own luteal phase, the ovulation calculator lets you enter it.
Worked example
The calculator opens with a last period that started 14 days before today, a 28-day cycle and a 5-day period. On October 29, 2026, that period started on October 15, 2026.
- Next period: October 15 + 28 days = November 12, 2026, lasting until November 16
- Fertile days this cycle: ovulation around October 29 (14 days before November 12), so October 24 to October 29
- The period after that: November 12 + 28 = December 10, 2026
- The sixth period in the table: April 1, 2027
What counts as a regular cycle?
For adults, a cycle anywhere from about 21 to 35 days is considered typical, and periods usually last 2 to 7 days. Being regular means your cycle length stays similar month to month, not that it is exactly 28 days. Only a minority of people have a 28-day cycle every time.
| Cycle length | Next period after a Jan 1 start | Ovulation around |
|---|---|---|
| 24 days | Jan 25 | Jan 11 |
| 28 days | Jan 29 | Jan 15 |
| 30 days | Jan 31 | Jan 17 |
| 35 days | Feb 5 | Jan 22 |
It is common for a period to arrive a few days earlier or later than predicted. Stress, illness, travel, weight change, intense exercise, breastfeeding and some medicines can all shift it. Teenagers, people who have recently given birth and people approaching menopause often have less regular cycles.
When to talk to a doctor
See your doctor if your periods are very heavy (soaking through a pad or tampon every hour for several hours in a row), very painful, last more than 7 days, or stop for three months or more when you are not pregnant. See them too if your cycles are often shorter than 21 days or longer than 35. Bleeding between periods or after sex is also worth checking. If your period is late and you could be pregnant, take a pregnancy test first.
Frequently asked questions
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, or simply the number of days between the two start dates. Do this for three to six cycles and take the average for the best prediction.
Can I use the fertile days to avoid pregnancy?
No. These are calendar predictions, and ovulation can come earlier or later than expected, so you could get pregnant on a day marked as not fertile. If you don't want to get pregnant, talk to a doctor, nurse or pharmacist about reliable contraception.
Why was my period late this month?
A late period is often caused by stress, illness, travel or changes in sleep, weight or exercise. Pregnancy is a common cause too, so take a test if there's a chance. If periods stay irregular or stop for several months, talk to your doctor.
Does the calculator work if my cycles are irregular?
It gives a rough guide based on your average, but the predictions will be less reliable, especially further ahead. Re-enter your latest period each month to keep the dates current.
Related calculators
- Ovulation Calculator — Your estimated ovulation date and fertile window for the next 6 cycles, from your last period and cycle length.
- Pregnancy Conception Calculator — Estimate your conception date and fertile window from your last period or due date, with the early pregnancy timeline.
- Due Date Calculator — Your estimated due date by Naegele's rule, adjusted for cycle length, or from a conception or IVF transfer date.
Results are estimates for general information and are not medical advice. Talk to a doctor or other qualified professional about your health. See our disclaimer.