Period calculator
Enter the first day of your last period and how long your cycle usually is. The calculator counts forward to show when the next six periods may start and when the next one ends. It assumes a regular cycle and is not contraception.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Enter the first day of your last period.
- Enter your usual cycle length and how many days your period lasts.
- Read the estimated start dates of the next six periods and when the next one ends.
How it works
Each predicted start date is the previous start date plus your cycle length: start₁ = last start + cycle, start₂ = last start + 2 × cycle, and so on. The end of the next period is its start plus the period length minus one day, because the first day is day 1.
Worked example
If the last period began on 1 January 2026 and the cycle is 28 days, the next six periods are predicted to begin on 29 January, 26 February, 26 March, 23 April, 21 May and 18 June. With a 5-day period, the first of these ends on 2 February.
Limits and safety
- It assumes every cycle has the same length. Real cycles vary, so treat the dates as a guide and check them against your own records or a tracking app.
- This is not a contraception or fertility tool.
- This is general information, not medical advice. See a doctor if your periods stop, become very irregular, very heavy or painful, or if you have bleeding between periods.
Frequently asked questions
How do I count my cycle length?
Count from the first day of one period (day 1) to the day before the first day of the next one. If your periods began on 1 and 29 January, your cycle is 28 days. Averaging several cycles gives a better number than a single one.
How accurate are the dates?
Only as accurate as your cycle is regular. Cycles vary by a few days for most people, and the difference adds up, so the dates for later months are less certain than the next one. Stress, illness, travel, hormonal birth control, breastfeeding and perimenopause can all change timing.
Can I use this to avoid pregnancy?
No. This calculator only predicts period dates and does not estimate fertile days. Ovulation can vary from cycle to cycle, so counting days is not reliable contraception. Use a proper method of birth control and ask a doctor or clinic about options.
What is a normal cycle length?
A cycle of 21 to 35 days is commonly described as normal for adults, and periods usually last between two and seven days. Teenagers often have longer, more irregular cycles in the first years. Talk to a doctor if your cycle changes suddenly or periods are very heavy or painful.
What if my period is late?
A late period has many possible causes, including pregnancy. If you could be pregnant, take a pregnancy test; if periods are repeatedly missing, see a doctor.