A period usually lasts 2 to 7 days, about 5 on average (NHS), and a menstrual cycle usually lasts 21 to 35 days (NHS, ACOG). You count the cycle from the first day of bleeding to the day before your next period starts.
Predictions are estimates, not medical advice or a diagnosis, and not a method of contraception.
At a glance
The usual figures, from the NHS (the UK health service), ACOG (the American College of Obstetricians and Gynecologists) and the World Health Organization (WHO):
- Blood lost in a period
- about 20 to 90 ml, which is about 1 to 5 tablespoons (NHS)
How long does a period last?
The NHS says a period can last between 2 and 7 days and will usually last about 5 (NHS). ACOG gives the same range (ACOG). The bleeding tends to be heaviest in the first 2 days (NHS) or the first 3 days (ACOG). Over a whole period women lose about 20 to 90 ml of blood, which is about 1 to 5 tablespoons (NHS).
A girl’s first periods can be different. ACOG says a first period may last only a few days and be very light, and that anywhere from 2 to 7 days is normal (ACOG).
How long is a cycle, and how do you count it?
Your cycle is not the same as your period. Day 1 of the cycle is the first day of your period, the first day you bleed. The cycle ends on the day before your next period starts (NHS, ACOG). For example:
- Your period starts
- , which is day 1
- Your next period starts
- Your cycle
- 28 days long: day 28 was , the day before the next period
- In the Hijri calendar (Umm al-Qura)
- day 1 was
Most women have a cycle of 21 to 35 days (NHS, ACOG, WHO). The average is about 28 days. A cycle does not have to be 28 days long to be regular. The NHS says regular cycles that are shorter or longer, from 21 to 35 days, are normal (NHS).
How to find your own average
Write down the first day of each period and work out each cycle as above. Add up your last few cycle lengths, then divide by the number of cycles. This sum is our own example, not from a source. Cycles of 27, 29 and 31 days give (27 + 29 + 31) ÷ 3 = 29 days. A diary, a calendar or an app can help you keep track of your periods (NHS, ACOG).
Period Calc does the sum for you: it averages the last six cycles you have logged. In the period calculator, enter your average. If your cycles often change a lot, see irregular periods.
What counts as a heavy or long period?
The NHS says you may have heavy periods if you (NHS):
- need to change your pad or tampon every 1 to 2 hours;
- need to empty your menstrual cup (a silicone cup that collects the blood, says the NHS) more often than is recommended;
- need to use 2 types of period product together, such as a pad and a tampon;
- have periods lasting more than 7 days;
- pass blood clots (lumps of blood) larger than about 2.5 cm across;
- bleed through to your clothes or bedding;
- avoid daily activities, or take time off work, because of your periods;
- feel tired or short of breath a lot.
ACOG’s list is similar. It also names changing pads or tampons during the night, and soaking through one or more every hour for several hours in a row (ACOG). The NHS and ACOG both count bleeding that lasts more than 7 days (NHS, ACOG).
Heavy periods are common, and the NHS says treatment can help if they affect your daily life (NHS). ACOG says to tell your doctor if you worry that your bleeding is too heavy. It can be a sign of an underlying health problem (ACOG). The blood loss can also lead to iron-deficiency anaemia, which means too little iron in the blood (ACOG, WHO).
When to see a doctor
- Your period lasts longer than 7 days (NHS, ACOG).
- Your periods are heavy by the signs above, affect your life, or have been heavy for some time (NHS).
- You bleed between periods or after sex: the NHS says this should always be checked (NHS).
- You have severe pain during your periods (NHS).
- You are a teenager and your period comes more often than every 21 days, less often than every 45 days, or more than 90 days apart (ACOG).
Get emergency medical care right away if both of these apply (ACOG):
- you are changing pads or tampons every hour for more than 2 hours in a row; and
- you also have chest pain, have shortness of breath, and are lightheaded or dizzy.
On heavy bleeding, ACOG’s page for teens also says to see your doctor right away if you are light-headed, dizzy, or have a racing pulse (ACOG). If your period has stopped or is late, read why a period can be late.
Questions about period and cycle length
Is a 3-day period normal?
Three days is inside the 2 to 7 days that both the NHS and ACOG give as the usual length of a period (NHS, ACOG).
Is a 35-day cycle normal?
It is at the top of the usual range of 21 to 35 days (NHS, ACOG). The NHS calls periods irregular when the gap between them is more than 35 days. It says to speak to a doctor if your periods are irregular (NHS). For more, see irregular periods.
How many pads or tampons a day is too many?
The NHS and ACOG give no number per day; they look at how often you change. The NHS says every 1 to 2 hours (NHS), and ACOG’s page for teens says more than once every 1 to 2 hours (ACOG). ACOG’s page on heavy bleeding says soaking through one or more pads or tampons every hour for several hours in a row (ACOG).
Can my cycle length change from month to month?
Yes. The NHS says the gap between periods can sometimes be a bit shorter or longer than the average of about 28 days (NHS). ACOG counts cycles as irregular when their length varies by more than 7 to 9 days (ACOG).
What is a normal cycle for a teenager?
ACOG says cycles of 21 to 45 days are normal for teens (ACOG). It may take 6 years or more after the first period for the cycle to become regular (ACOG).
How this page was made
Hossam Hamouda, the developer of Period Calc, wrote this page from the guidance of the NHS, ACOG and WHO pages listed under Sources, and checked it against them on the date shown at the top. Those organisations have not reviewed it, and neither has a doctor. Where they give different numbers, this page names who says what. It cannot tell you what is usual for your own body. If something worries you, see when to see a doctor.
Sources
- Periods and fertility in the menstrual cycle (NHS)
- Periods (NHS)
- Heavy periods (NHS)
- Irregular periods (NHS)
- Vaginal bleeding between periods or after sex (NHS)
- Abnormal Uterine Bleeding (ACOG)
- Amenorrhea: Absence of Periods (ACOG)
- Heavy Menstrual Bleeding (ACOG)
- Heavy and Abnormal Periods (ACOG)
- Your First Period (ACOG)
- Menstrual health (WHO)