How to calculate your cycle length
Your cycle length is the number of days from the first day of one period to the first day of the next. Only the start dates matter. How long you bleed doesn’t change the answer.
Say your last three periods started on June 2, June 30 and July 29. June 2 to June 30 is 28 days. June 30 to July 29 is 29 days. Your average cycle length is 28.5 days, and the calculator rounds that to 29.
The mistake people make most is counting from the day their bleeding stops. That leaves out the period itself, so the answer comes out about five days short. A 28-day cycle then looks like a worrying 23-day one.
What is a normal cycle length?
The NHS says regular cycles anywhere from 21 to 35 days are normal. That’s the range this calculator checks against. Mayo Clinic gives 24 to 38 days. Very few cycles are exactly 28 days. In a study of more than 600,000 cycles, only about 13 percent were.
Where your number sits in that range matters less than whether it stays there. Our guide on normal menstrual cycle length covers what shifts it, including age, birth control and the years before menopause.
Cycle length and period length are different numbers
Your period length is how many days you bleed. For most people it’s 2 to 7 days. Your cycle length is the whole gap from one period to the next, bleeding included. A 5-day period inside a 28-day cycle is common. If you were looking for how long your period lasts, our period length guide has the numbers.
What if my cycle length is different every month?
A few days of difference is normal. The calculator shows your shortest and longest cycle so you can see how far yours moves. Once those two are a week or more apart, your average stops being a good guess for your next period.
That’s the job of the irregular period calculator. It takes the same dates and gives you a window for your next period instead of one day. If you already know your cycle length and want the dates ahead, the period calculator lays out your next six periods and fertile windows.
Can a period app track my cycle length?
Plum, a period tracker app for iPhone, does it for you. You log the day each period starts. Plum measures every cycle as it ends, so there are no dates to type back in. The Insights tab shows your average cycle length, how much it varies and a chart of each cycle. That part of the app is free.
When to see a doctor about your cycle length
The NHS says to see a GP if your periods are irregular or your usual pattern changes. So cycles that often land outside 21 to 35 days are worth a visit. So is a cycle that stayed put for years and then shifted, even if it’s still inside the range. Bring the cycle list from this page with you. It turns “my periods are all over the place” into dates a doctor can use.
