A menstrual cycle is counted from the first day of one period to the day before the next begins. Once you know your usual cycle length, predicting the next several periods is simple arithmetic — and useful for far more than planning a holiday. Cycle length and regularity are among the more informative vital signs you can track without any equipment.
- What it predicts
- Your next six period start and end dates
- Normal cycle length
- 21 to 35 days in adults
- Normal period length
- 2 to 7 days of bleeding
- Typical blood loss
- About 30–80 mL across a period
- Cycle day 1
- The first day of real bleeding, not spotting
Formula
Calendar projection from a fixed cycle lengthPeriod n starts = LMP + (n × cycle length) Period ends = start + (period length − 1)
- LMP
- — First day of your last menstrual period
- cycle length
- — Day 1 of one period to day 1 of the next
- n
- — How many cycles ahead you are projecting
The end date subtracts one because day 1 of bleeding counts as the first day: a five-day period that starts on the 21st finishes on the 25th, not the 26th. The projection assumes every cycle is exactly the same length, which is the main reason predictions drift the further out they go.
Period Calculator examples
| Input | Result | Category |
|---|---|---|
| LMP Jan 5, 2026 · 28-day cycle, 5-day period | Feb 2, 2026 | In 13 days |
| LMP Jan 5, 2026 · 21-day cycle, 4-day period | Jan 26, 2026 | In 6 days |
| LMP Jan 5, 2026 · 35-day cycle, 7-day period | Feb 9, 2026 | In 20 days |
| LMP Feb 14, 2026 · 30-day cycle, 6-day period | Mar 16, 2026 | In 15 days |
| LMP Jun 1, 2026 · 26-day cycle, 3-day period | Jun 27, 2026 | In 9 days |
| LMP Sep 10, 2026 · 28-day cycle, 5-day period | Oct 8, 2026 | In 13 days |
How do you calculate your next period?
Add your average cycle length to the first day of your last period. If your last period began on 24 July and your cycles run 28 days, the next starts around 21 August, the one after around 18 September, and so on.
To find your average, note the first day of each period for three or four months, count the days between consecutive start days, and take the mean. Do not count from the end of one period to the start of the next — that measures something else, and will make every prediction too early.
What is a normal menstrual cycle?
In adults, anything from 21 to 35 days is normal, and the average is around 29. Teenagers in the first few years after their first period often run longer and less predictably — up to 45 days — as the cycle establishes itself.
| Measure | Usual range | Worth investigating |
|---|---|---|
| Cycle length (adults) | 21–35 days | Under 21 or over 35 days |
| Cycle-to-cycle variation | Within about 7–9 days | Swings wider than that |
| Days of bleeding | 2–7 days | More than 8 days |
| Blood loss | About 30–80 mL | Soaking a pad or tampon hourly |
| Missed periods | None outside pregnancy | Three or more in a row |
Variation of a few days between cycles is normal and expected. It is a persistent pattern — consistently short, consistently long, or wildly unpredictable — that is worth raising, not a single unusual month.
Why is my period late?
Pregnancy is the first thing to rule out if there is any chance of it, and a home test taken from the day of the missed period is reliable. Beyond that, the cycle is sensitive to almost anything that disturbs the body's baseline.
- Stress, poor sleep, illness, and long-haul travel can all delay ovulation, which delays the period by the same amount.
- Significant weight loss or gain, and heavy training loads, can suppress ovulation entirely.
- Polycystic ovary syndrome, thyroid disorders, and high prolactin lengthen or stop cycles.
- Hormonal contraception, and the months after stopping it, change the pattern — some methods stop periods altogether, which is not harmful.
- Perimenopause, typically from the mid-forties, makes cycles shorter first and then increasingly erratic.
What happens across the menstrual cycle?
The cycle has two halves separated by ovulation. The first half varies in length between people and between cycles; the second is far more consistent, which is why ovulation is best located by counting backwards from the next period rather than forwards from the last one.
| Phase | Days | What happens |
|---|---|---|
| Menstruation | 1–5 | The uterine lining is shed |
| Follicular | 1–13 | Follicles mature; the lining rebuilds |
| Ovulation | About day 14 | An egg is released; the fertile window closes |
| Luteal | 15–28 | Progesterone rises, then falls if there is no pregnancy |
When a cycle runs long or short, it is almost always the follicular phase that has stretched or shortened. The luteal phase stays close to 14 days, which is the assumption the estimated ovulation date above rests on.
How far ahead can period predictions be trusted?
Errors accumulate. If your cycles vary by three days either side of the average, the first prediction is within three days, but the sixth could be well over a week out because each cycle's variation compounds on the last.
The practical approach is to trust the next one or two dates for planning and treat the rest as approximate. Re-entering your actual start date each month resets the drift and keeps the near-term predictions sharp.
Frequently asked questions
How do I calculate my menstrual cycle length?
Count from the first day of one period to the day before the next period starts. That total is your cycle length. Do it over three or four months and take the average, because a single cycle is a poor guide to your usual pattern.
Is a 35-day cycle normal?
Yes. Adult cycles from 21 to 35 days are all within the normal range. A consistent 35-day cycle simply means you ovulate later — around day 21 rather than day 14 — so your fertile window sits later in the month than the standard advice assumes.
How late can a period be before I should worry?
A period up to about a week late, occasionally, is common and rarely significant. Take a pregnancy test if there is any chance of pregnancy. Three or more consecutive missed periods without pregnancy warrants an appointment, as does a sudden change in an otherwise regular pattern.
Can stress delay my period?
Yes. Stress acts on the hypothalamus, which can postpone ovulation. Because the luteal phase stays about 14 days, delaying ovulation by a week delays the period by a week. Illness, travel across time zones, and disrupted sleep have the same effect.
How long should a period last?
Two to seven days of bleeding is normal, with most falling between three and five. Bleeding for more than eight days, or heavy enough to soak a pad or tampon every hour for several hours, is worth investigating for causes such as fibroids or a bleeding disorder.
Do periods sync up between people who live together?
The evidence does not support it. The original 1971 study has not held up to replication, and the apparent effect is well explained by chance: with cycles of different lengths, start dates drift in and out of alignment naturally over a few months.
Sources
- Menstruation and Menstrual Problems — Eunice Kennedy Shriver National Institute of Child Health and Human Development
- Menstrual cycle: What's normal, what's not — Mayo Clinic
- Your menstrual cycle — Office on Women's Health, U.S. Department of Health and Human Services
- Menstruation — MedlinePlus, U.S. National Library of Medicine