Menstrual Cycle Calculator

When your next six periods are due, which day of your cycle you are on today, and why a regular long cycle matters less than an irregular short one.

📊 What Counts as Normal

Cycle length, adults21 to 35 days
Cycle length, teenagers21 to 45 days
Bleeding days2 to 7 days
Luteal phaseusually 12 to 14 days
Variation between cyclesunder 7 to 9 days
Typical blood loss5 to 80 ml total

Reading These Dates Honestly

Regularity matters more than length. A steady 35-day cycle is normal; one that swings between 24 and 35 is worth mentioning to a doctor, even though its average sits in range.
This is arithmetic, not biology. It projects your usual cycle forward — it cannot know about the stress, illness, travel or sleep change that will shift the next one.
Ovulation here is estimated backwards from the next period, because the luteal phase is the stable part of the cycle. Counting forwards from the last period is the classic mistake.
Do not use these dates as contraception. Sperm survive up to five days, ovulation moves, and calendar methods fail often enough to be a poor choice on their own.

How a Cycle Is Counted, and Where People Go Wrong

Day one is the first day of proper bleeding, not spotting and not the day the period ends. The cycle runs to the day before the next period starts, so a twenty-eight-day cycle means twenty-eight days from one first day to the next first day. That definition is where most confusion begins: people count from the end of bleeding, or from the middle, and end up with numbers that will not line up with anything a clinician asks them. The cycle has two halves. The follicular phase runs from day one to ovulation and is the part that varies, stretching or shortening with stress, illness, travel and weight change. The luteal phase runs from ovulation to the next period and is remarkably consistent, usually twelve to fourteen days in the same person. That asymmetry is why this page projects ovulation backwards from the expected next period rather than forwards from the last one.

Why Regularity Beats Length

A great deal of period anxiety comes from the twenty-eight-day figure, which is an average and not a target. Anywhere from twenty-one to thirty-five days is normal in adults, and up to forty-five in the first years after menarche. Someone with a steady thirty-five-day cycle has a perfectly ordinary cycle that simply happens to be long. What actually carries information is variation: if your cycles swing by more than seven to nine days from one to the next, that is the pattern worth raising with a doctor, even when the average sits comfortably inside the normal range. It is also the pattern that makes any calculator, this one included, much less useful — projecting a stable cycle forward is arithmetic, but projecting an unstable one forward is guesswork with a date attached, and this page will not pretend otherwise.

When the Pattern Is Worth a Conversation

Cycles consistently shorter than 21 days or longer than 35 in an adult, once the first couple of years after menarche have passed.
Variation of more than seven to nine days between consecutive cycles, even if the average looks normal.
Bleeding that lasts more than seven days, or that soaks through a pad or tampon every hour for several hours.
No period for three months without pregnancy, or no period by age fifteen.
Bleeding between periods, or after sex, or after menopause.
Pain severe enough to keep you from work, school or ordinary activity — that is not something to put up with.

What This Calculator Cannot Do

It is not contraception. Sperm survive up to five days, ovulation shifts, and calendar-only methods have a high real-world failure rate.
It cannot detect pregnancy. A late period has many causes, and only a test answers that question.
It assumes your cycle repeats. Hormonal contraception, PCOS, thyroid disease, perimenopause and breastfeeding all break that assumption.
It projects from a single cycle length. Averaging your last three to six cycles gives a far better input than guessing.
The 14-day luteal phase is a default, not a measurement. If you have tracked yours and it differs, enter your own.
Teenagers in the first years after menarche have cycles that are genuinely irregular by design, and projections mean little there.
If the date you enter is several cycles old, the page rolls it forward to the cycle you are actually in rather than showing you a date that has already passed.

Worked Examples

The same last period, two cycle lengths

Last period 1 March, 28-day cycle

Next period = 1 March + 28 days
Ovulation = next period − 14 days of luteal phase
Period ends = start + 5 bleeding days − 1

Next period 29 March, ovulation about 15 March

The same 1 March with a 35-day cycle

Next period = 1 March + 35 days = 5 April
Ovulation = 5 April − 14 = 22 March
The luteal phase is unchanged; the follicular one absorbed the extra week

Ovulation a week later, period a week later

Frequently Asked Questions

How do I calculate my menstrual cycle?

Count from the first day of proper bleeding to the day before your next period starts. That total is your cycle length. Do not count from the end of your period, which is the most common mistake and produces a number that will not match what a clinician expects.

Is a 35-day cycle normal?

Yes. Anywhere from 21 to 35 days is normal in adults, and up to 45 days in the first years after periods begin. A steady 35-day cycle is an ordinary cycle that happens to be long. What matters more than the length is whether it stays roughly the same from month to month.

When should I worry about an irregular cycle?

When consecutive cycles vary by more than seven to nine days, when they are consistently under 21 or over 35 days in an adult, when bleeding lasts more than seven days, or when three months pass without a period and you are not pregnant. Any of those is worth raising with a doctor.

How is this different from an ovulation calculator?

They share most of the arithmetic but answer different questions. This page is about your periods: when the next six are due and which cycle day you are on. An ovulation calculator is about conception: the fertile window, the best days to try, and what follows. If you are trying to conceive, that is the page you want.

Can I use this to avoid pregnancy?

No. Sperm survive up to five days in the reproductive tract, ovulation shifts with stress and illness, and calendar-based methods used alone fail often in real use. If you are avoiding pregnancy, use a method designed for it and talk to a clinician about which one.

Why does the calculator work ovulation out backwards?

Because the luteal phase — ovulation to period — is the stable half of the cycle, usually 12 to 14 days in the same person, while the follicular half stretches and shrinks. Counting forwards from your last period assumes the variable half is fixed, which is exactly backwards, so this page counts back from the expected next period instead.