Period Cycle Calculator
Predict your next twelve periods, with the fertile window and ovulation day for each cycle.
Your cycle
Normal range is 21–35 days
Usually 3–7 days
This is a calendar projection, not a tracker: it assumes every cycle is the same length as the one you entered. Do not rely on it for contraception: calendar methods are among the least effective there are, and ovulation timing shifts more than most people expect.
This calculator predicts when your next periods are due. Enter the date your last one started, your usual cycle length and how long you normally bleed for.
It returns the expected start date of your next period with a countdown, the phase of your cycle you are in today, your estimated ovulation day and fertile window, and a calendar of the next twelve cycles you can export to your phone.
What is a menstrual cycle?
Your menstrual cycle is the repeating pattern your body goes through each month to prepare for a possible pregnancy. It is counted from the first day of one period to the first day of the next, not from the day bleeding stops.
Most people learn that a cycle is 28 days, but that is only the average. Anything from 21 to 35 days is considered normal for adults, and cycles that vary by a few days month to month are common and usually nothing to worry about.
This calculator takes the date your last period started and how long your cycle usually runs, then counts forward to estimate when the next few are due. It also marks your likely fertile days, since those are set by the same timing.
The luteal phase, from ovulation to your next period, stays close to 14 days even when your cycle length changes. The first half is what stretches or shortens. That is why counting back from your next period finds ovulation more reliably than counting forward to day 14.
What to enter
- First day of last period
- The date bleeding actually started, even if it was light at first. This is day 1 of your cycle.
- Cycle length
- The number of days from the start of one period to the start of the next. If it varies, use your usual average. 28 is a reasonable starting point if you are unsure.
- Period lasts
- How many days you normally bleed for. Between 2 and 7 days is typical. This only shades the calendar and does not change the predicted dates.
- Show my next twelve cycles
- Extends the forecast to a full year, which is useful for planning around holidays, exams, or travel.
What this assumes
Your cycles are fairly regular. If they swing widely from month to month, treat the dates as a rough guide.
Ovulation is estimated as roughly 14 days before your next period is due. That gap is the most consistent part of the cycle for most people, but it is still an estimate.
This is a prediction, not a test. It cannot detect pregnancy, and it is not a method of contraception.
How to calculate your next period
Predicting the next period is simple addition. The fertile window is the part worth understanding properly.
- last start date
- Day 1 of your most recent period
- cycle length
- Your usual number of days between periods
Start from day 1. Take the first day of your last period. Bleeding day one is the start of the cycle, not the end of the previous one.
Add your cycle length. Count forward that many days to get the likely start of your next period.
Find ovulation. Count back about 14 days from that predicted date. The time between ovulation and your period stays fairly fixed even when cycle length changes, which is why counting backwards works better than counting forwards.
Mark the fertile window. Pregnancy is possible from about 5 days before ovulation up to the day itself, because sperm can survive that long. That gives roughly a six-day window, which the ovulation calculator looks at in more detail.
See a worked example: a period starting 1 March with a 30-day cycle
- Last period started
- 1 March
- Usual cycle length
- 30 days
Add 30 days to 1 March. That lands on 31 March, so that is when the next period is likely to start.
Count back 14 days from 31 March to find ovulation: around 17 March.
The fertile window runs from about 12 March to 17 March.
Notice that ovulation is not on day 14 here. On a 30-day cycle it falls around day 16, which is why counting back from the next period is more reliable than assuming day 14.
Next period around 31 March, most fertile 12 to 17 March
Frequently asked questions
For adults, anywhere from 21 to 35 days counts as normal. The often quoted 28 days is just the middle of that range, not a target.
Teenagers in the first few years after starting their periods often have longer and less predictable cycles, which usually settle over time.
The calculator assumes your cycle repeats at a steady length. Real cycles move around. Stress, illness, travel, hard training, weight change, and poor sleep can all delay ovulation, and a later ovulation means a later period.
A few days either side of the predicted date is ordinary. If a period is more than a week late and pregnancy is possible, take a test.
No. Please do not rely on it for that. Predicted dates are based on past averages, and ovulation can shift without warning, which is exactly when calendar methods fail.
If you want to avoid pregnancy, speak to a doctor or pharmacist about a proper contraceptive method.
Note the first day of bleeding each month for three or four months, then work out the gaps between those dates. Use your shortest and longest gap as a range rather than a single number.
If your cycles are consistently shorter than 21 days or longer than 35, or the length swings wildly, that is worth mentioning to a doctor.
Four, and they overlap. The period is the bleed itself. The follicular phase follows, when an egg matures. Ovulation is the release of that egg. The luteal phase is the roughly two weeks after, ending when the next period begins.
The luteal phase is the steady one, usually 12 to 16 days. Most of the variation between people, and between your own months, happens before ovulation rather than after it.
Problems people actually run into
Assuming ovulation always lands on day 14
Day 14 only holds on a textbook 28-day cycle. On a 32-day cycle ovulation is nearer day 18; on a 25-day cycle it is nearer day 11.
People trying to conceive often time things around day 14 regardless of their own cycle length and miss the window entirely. Counting backwards from the next expected period is the more reliable approach.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Sources
- Your menstrual cycle · Office on Women's Health, U.S. Department of Health and Human Services
- Menstruation in girls and adolescents: using the menstrual cycle as a vital sign · American College of Obstetricians and Gynecologists
Last updated: August 29, 2026