Women's Health
Period Calculator
Work out when your next periods are due, and which day of your cycle you are on today.
Period Calculator
These dates are estimates from an average cycle. Real cycles vary month to month, so treat every result as approximate and confirm anything you are relying on with a midwife or doctor.
About the Period Calculator
This period predictor and menstrual cycle calculator is simple to use. Enter the first day of your last period and your usual cycle length, and this calculator predicts when your next several periods are due. It also tells you which day of your cycle you are on today, which is the figure most apps ask for and most people have to count on their fingers.
It projects six cycles ahead, showing the expected start and end date of each, so you can see the pattern rather than just the next one. That is genuinely useful for planning a holiday, an event, or simply knowing what to expect.
Everything runs from an average. If your cycles vary — and most do, at least a little — treat every date as a best guess with a few days of give either side.
How to use the Period Calculator
- Pick the first day of your last period — the first day of bleeding, not the last.
- Enter your average cycle length, counted from the first day of one period to the first day of the next.
- Enter how many days your period lasts.
- Select Predict my periods.
- The table projects six cycles ahead, with the ovulation day marked in each.
How the prediction works
Next period start = first day of last period + cycle length
Period end = start + duration − 1
Cycle day today = days since the first day of your last period + 1
Worked example
Last period started 3 September, cycle length 28 days, periods last 5 days.
Next period: 3 September + 28 days = 1 October, ending 5 October.
The one after: 29 October. Then 26 November, 24 December, and so on.
Six cycles projected, each 28 days after the last
Notice the dates drift earlier through the calendar month. A 28-day cycle is shorter than most months, so the start date moves back by two or three days each time.
Menstrual cycle length calculator: counting correctly
Cycle length is measured from the first day of one period to the day before the next one starts. It is not the gap between the end of one and the start of the next, and it is not the length of the bleed itself.
- Period starts 3 September, next period starts 1 October → cycle length is 28 days.
- Period starts 3 September, next starts 4 October → cycle length is 31 days.
Day 1 is the first day of proper bleeding, not spotting beforehand. If you track for three or four months and average the results, you will get a far better input than a single reading.
What counts as a normal cycle
| Cycle length | How it is usually described |
|---|---|
| Under 21 days | Short — worth mentioning to a doctor |
| 21 – 35 days | Within the normal range for adults |
| 21 – 45 days | Normal in the first years after periods begin |
| Over 35 days | Long — worth mentioning to a doctor |
| Varies by more than 7–9 days | Irregular — common, but worth discussing |
The average is often quoted as 28 days, which has given that number more authority than it deserves. Plenty of people have a perfectly healthy 25-day or 33-day cycle. Consistency for you matters more than matching an average.
If your cycles are irregular
Calendar prediction assumes a repeating pattern, so it becomes much less useful when cycles vary widely. Some practical adjustments:
- Use your average across the last three to six cycles rather than the most recent one.
- Expect a wider margin. If your cycles range 26 to 34 days, your prediction carries a week of uncertainty.
- Track the actual dates for a few months. A record is worth far more than a prediction, and it is exactly what a doctor will ask for.
Cycles are commonly disrupted by stress, illness, travel, significant weight change, intense exercise, hormonal contraception, perimenopause, and conditions such as PCOS or thyroid disorders. Persistent irregularity, very heavy bleeding, bleeding between periods, or periods stopping without an obvious reason are all worth a GP appointment — not because they are necessarily serious, but because they are treatable and worth understanding.
The phases of a cycle
| Phase | Roughly when | What happens |
|---|---|---|
| Menstrual | Days 1–5 | The uterine lining sheds |
| Follicular | Day 1 to ovulation | A follicle matures; this phase varies most in length |
| Ovulation | About 14 days before the next period | An egg is released |
| Luteal | Ovulation to next period | Fairly constant, usually 12–14 days |
The important detail is that the luteal phase is the stable one. When a cycle runs long or short, it is almost always the follicular phase that changed. This is why ovulation is estimated by counting backwards from the expected next period rather than forwards from the last one — and it is exactly what the ovulation calculator does.
Do not use this as contraception. Calendar-based prediction is one of the least reliable methods of avoiding pregnancy — cycles shift with stress, illness, travel, weight change and age, and ovulation can happen earlier or later than any average predicts. If you are trying not to conceive, talk to a doctor or sexual health service about a method with a proper effectiveness rate.
This is an estimate, not medical advice. It works from average cycle figures, and real bodies vary a great deal month to month. Nothing here replaces a consultation. For anything you are relying on — a pregnancy, fertility treatment, a health concern, or a decision about contraception — speak to a midwife, doctor or sexual health service who can assess your own situation.
How to calculate next period date
Add your average cycle length to the first day of your last period. That is all the calculator does — and it does it for the next several cycles at once.
Worked example
Your last period started on 3 October. Your cycles are usually 29 days.
Next period: 1 November, then 30 November, then 29 December
Missed period calculator: is my period late?
- A few days early or late is normal. Cycles vary by up to about 7 days for many people.
- 5 or more days past the expected date is usually called a late period. If you could be pregnant, take a pregnancy test.
- No period for 3 months (when you usually have them) is a reason to see a doctor.
Stress, travel, illness, weight change and new medicines can all delay a period.
Period calculator for irregular periods
If your cycles change a lot, use the average of your last 3 to 6 cycles as the cycle length. Treat each prediction as a window, not a date — say, three days either side. A normal adult cycle is 21 to 35 days. In the first few years after periods start, 21 to 45 days is common.
First period estimator: when will it start?
Most girls get their first period between 10 and 15, with 12 a common age. It usually arrives about 2 to 3 years after breasts start to develop, and often around 6 to 12 months after white discharge starts. A doctor visit is a good idea if there is no period by 15, or none within 3 years of breast development.
Other ways to use this calculator
Predicting periods and cycle days. It works as a menses period calculator, a period cycle calculator and a menstrual cycle calendar.
- Period cycle and menstrual calculator — enter the first day of your last period and your cycle length.
- Irregular cycle calculator — use the average of your last 3–6 cycles and treat dates as windows.
- “Am I late?” period calculator — 5 or more days past the predicted date usually counts as late.
- Average cycle length calculator — add your recent cycle lengths and divide by how many.
- Next period calculator and next period predictor — your upcoming periods show in the table.
- Period tracker — note each start date and update your average cycle length.
- First period calculator — see the first period section above.
Related guides
Frequently asked questions
Count from the first day of one period to the day before the next one starts. If your period began on 3 September and the next began on 1 October, your cycle is 28 days. Day 1 is the first day of real bleeding, not spotting beforehand. Track three or four cycles and average them — a single measurement tells you much less than a short record does.
Use your average across the last three to six cycles rather than the most recent one, and expect a wider margin of error — if your cycles range from 26 to 34 days, the prediction carries about a week of uncertainty. Keeping a record of actual dates is more valuable than any prediction, and it is the first thing a doctor will ask about.
For adults, 21 to 35 days is generally considered normal, and in the first years after periods begin the range extends to about 45 days. The commonly quoted 28-day average has more authority than it deserves — a steady 25-day or 33-day cycle is perfectly healthy. Cycles regularly under 21 days, over 35, or varying by more than a week are worth mentioning to a doctor.
No. Calendar-based prediction is among the least reliable ways to avoid pregnancy. Cycles shift with stress, illness, travel and weight change, and ovulation can occur several days earlier or later than any average suggests — sperm can also survive around five days, widening the risk period considerably. If you want to avoid pregnancy, speak to a doctor or sexual health service about a method with a proper effectiveness rate.
Add your usual cycle length to the first day of your last period. If your last period started on 3 October and your cycle is 29 days, your next period is due around 1 November.
A period that is 5 or more days later than expected is usually called late. If you might be pregnant, take a test. If you miss periods for 3 months in a row, see a doctor.
Sources and further reading
- NHS — Periods and the menstrual cycle — normal cycle length, irregularity and when to seek help
- ACOG — Your first period and menstrual cycles — cycle ranges for adolescents and adults
- NICHD — Menstruation and menstrual problems — the four phases of the cycle
- NHS — Contraception guide — why calendar methods are unreliable for preventing pregnancy