How to use the period calculator
- Enter the first day of your last period, the day bleeding started (not spotting).
- Enter your average cycle length. If you do not know it, keep 28 days and update it after tracking a few cycles.
- Enter how many days your period usually lasts, often 3 to 7 days.
- See your next period date and a calendar of the next six periods with the estimated fertile window of each cycle.
How the next period is calculated
Ovulation ≈ next period − 14 days
Fertile window = 5 days before ovulation + ovulation day
Example
If your last period started on 3 March and your cycle is 30 days, your next period is expected on 2 April. Ovulation is likely around 19 March, so the fertile window is about 14 to 19 March.
What is a normal cycle?
- Cycle length: 21 to 35 days in adults is considered normal; only a minority of people have exactly 28 days.
- Variation: a cycle that changes by up to 7–9 days from month to month is still within the normal range.
- Period length: bleeding usually lasts 2 to 7 days.
- Teenagers and perimenopause: cycles are often irregular in the first years after the first period and in the years before menopause.
Why your period may be late
Common reasons are pregnancy, stress, illness, travel, big changes in weight or exercise, breastfeeding, hormonal contraception and conditions such as polycystic ovary syndrome (PCOS) or thyroid problems. If your period is more than a week late and you could be pregnant, take a home pregnancy test. See a doctor if you miss three periods in a row, bleed for more than 7 days, or have very heavy or painful periods.
Planning a pregnancy?
Use our ovulation calculator to see your most fertile days in detail, and the due date calculator once you are pregnant.
Frequently asked questions
How accurate is a period calculator?
It is accurate if your cycles are regular. If your cycle length changes, use the average of your last three to six cycles and expect your period within a few days of the predicted date.
Can I use this to avoid pregnancy?
No. Ovulation can move from cycle to cycle, so a calendar estimate is not a reliable method of contraception.
How do I count my cycle length?
Count from the first day of one period up to the day before the next period starts. For example, periods starting on 1 May and 29 May mean a 28-day cycle.
Do you store my dates?
No. Everything is calculated in your browser and nothing is sent to our servers.
Sources: American College of Obstetricians and Gynecologists (ACOG), "Menstruation in girls and adolescents: using the menstrual cycle as a vital sign", 2015; NHS, "Periods"; Bull JR et al., "Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles", npj Digital Medicine, 2019.
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Last reviewed: 2026-11-10. These tools give general information, not medical advice. Talk to a doctor about your health. See our medical disclaimer.