How to use this calculator
- Enter or choose first day of your last period.
- Enter or choose usual cycle length.
- Enter or choose period length.
- Enter or choose periods to show.
- The next period starts updates as you type. Use Copy result or Share to keep it.
Method
Period n starts = LMP + n × cycle length; ends = start + period length − 1; ovulation ≈ start − 14 days
- LMP
- first day of the last menstrual period
- n
- 1 for the next period, 2 for the one after, and so on
Worked example
- Next period: 5 Jan + 28 days = 2 Feb 2026.
- It lasts 5 days, so it ends on 6 Feb (2 Feb + 4 days).
- The one after: 5 Jan + 56 days = 2 Mar 2026.
With a 28-day cycle that started on 5 January 2026, the next period is due on 2 February and the one after on 2 March.
What the result means
A period arriving a few days early or late is common. If periods stop for three months or more, become very irregular or are unusually heavy or painful, it is worth seeing a doctor or nurse.
Assumptions and limits
- Your cycle length stays the same from month to month.
- Ovulation is estimated 14 days before each period.
- Predictions are less reliable for irregular cycles, in the first years after periods start, around menopause, after pregnancy or when changing contraception.
- Not suitable for avoiding pregnancy.
Questions
How do I work out my cycle length?
Count from the first day of one period to the day before the next one starts; average several cycles for a better figure.
Is a 28-day cycle normal?
It is the average, but longer and shorter cycles are also normal. What matters most is what is typical for you.
Sources
Please note: This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health.