Period Calculator
Quickly find your Menstrual cycle with our free Period Calculator. Simple, fast, and accurate.
Your cycle
Upcoming periods
What is a Period Calculator?
A Period Calculator predicts when your next period will start based on the first day of your last menstrual period (LMP) and your average cycle length. It helps you track your menstrual cycle, plan ahead, and identify patterns in your cycle regularity.
How to calculate your next period manually (the formula)
Next period start date: LMP + cycle length.
Simply add your average cycle length (in days) to the first day of your most recent period. For example, if your LMP was 1 June and your cycle is 28 days, your next period starts on 29 June.
Example calculation
LMP = 1 April 2025, cycle = 33 days. Period 2 = 1 April + 33 = 4 May. Period 3 = 4 May + 33 = 6 June. Period 4 = 6 June + 33 = 9 July. Period 5 = 9 July + 33 = 11 August.
Common mistakes
- Using the period end date instead of the start date — Always use the first day of bleeding (not the last day) as your starting point for calculating the next cycle.
- Assuming every cycle is identical — Cycle length varies naturally. Your actual period may come a few days earlier or later than predicted. Track for several months to find your average.
- Counting from day 1 of bleeding each time — The first day of full flow (not spotting) counts as day 1. Spotting a day or two before is considered part of the previous cycle.
Frequently asked questions
How do I calculate my next period?
Add your average cycle length to the first day of your last period. For a 28-day cycle starting 1 March, the next period starts on 29 March.
What is a normal cycle length?
A normal cycle ranges from 21 to 35 days for adults and 21 to 45 days for teens. The average is 28 days, but variation is common and normal.
Why is my cycle irregular?
Irregular cycles can be caused by stress, significant weight changes, hormonal imbalances, polycystic ovary syndrome (PCOS), thyroid disorders, or approaching menopause.
How many days does a period typically last?
A typical period lasts 3 to 7 days. Flow is usually heaviest in the first 2 days and then tapers off.
Can I get pregnant during my period?
It is unlikely but possible, especially if you have a short cycle. Sperm can survive up to 5 days, so intercourse near the end of a period could overlap with early ovulation.
When should I see a doctor about my cycle?
Consult a doctor if your cycle is consistently shorter than 21 days or longer than 35 days, if you bleed for more than 7 days, or if you experience severe pain that interferes with daily life.