To calculate your next period, count the number of days from the first day of one period to the first day of the next. That number is your cycle length. Add that many days to the first day of your most recent period, and you have your estimated start date. For most women, a normal cycle falls between 21 and 35 days, with 28 days being the average.
The more cycles you track, the more accurate your prediction becomes. Here’s how to do it step by step and what can throw your estimate off.
The Basic Calculation
Day 1 of your cycle is always the first day of actual bleeding, not spotting. Mark that date. When your next period starts, mark that date too. The number of days between those two marks is one cycle length. Repeat this for at least three to six cycles, then average the results. If your last three cycles were 29, 31, and 28 days, your average cycle length is about 29 days.
To predict your next period, take the start date of your most recent period and add your average cycle length. If your last period started on June 1 and your average cycle is 29 days, your next period is expected around June 30.
If you want a range instead of a single date, note your shortest and longest cycles from the past six months. Add your shortest cycle length to your last period start date for the earliest expected date, and your longest for the latest. This gives you a realistic window rather than a single day that may be off.
Why Your Cycle Has a Predictable Half
Your menstrual cycle has two main phases. The first half (before ovulation) can vary quite a bit, ranging from 11 to 27 days depending on the cycle. The second half (after ovulation until your period arrives) is much more consistent, typically lasting 11 to 17 days with an average of 14 days. This is why the time between ovulation and your period is the most reliable part of the cycle for prediction.
This matters because if you can pinpoint when you ovulate, you can predict your period with more confidence than the calendar alone. Ovulation happens roughly 14 days before your next period starts, not 14 days after your last period, which is a common misunderstanding. In a 35-day cycle, for example, ovulation likely happens around day 21, not day 14.
Tracking Physical Signs for Better Accuracy
A calendar gives you a rough estimate. Adding physical signs narrows the window. Two methods are especially useful.
Basal body temperature (BBT): Your resting temperature rises slightly (about 0.5 to 1°F) after ovulation and stays elevated until your period starts. If you take your temperature every morning before getting out of bed, you’ll see the shift. Once you confirm ovulation through that temperature rise, expect your period in roughly 10 to 16 days.
Cervical mucus: In the days leading up to ovulation, cervical mucus becomes clear, slippery, and stretchy, similar to raw egg whites. After ovulation, it dries up or becomes sticky. Once you notice that shift from wet to dry, your period is typically about two weeks away.
Combining both methods (sometimes called the symptothermal method) gives you the most reliable picture of where you are in your cycle. Some electronic fertility monitors can also detect hormone changes in urine to confirm ovulation timing, which takes the guesswork out of reading physical signs.
What Can Throw Off Your Prediction
Even with careful tracking, your cycle can shift. Stress is one of the most common disruptors. Emotional, physical, or nutritional stress triggers your body to release cortisol and endorphins at levels that interfere with the hormones driving your cycle. A particularly stressful month can delay ovulation, which pushes your entire cycle later.
Other factors that commonly delay or change your period:
- Significant weight changes: Rapid weight loss or gain can disrupt hormone production enough to shift your cycle by days or even cause you to skip a period entirely.
- Intense exercise: Heavy training loads, especially endurance sports, can delay or suppress ovulation.
- Illness: Being sick around the time you’d normally ovulate can push ovulation back, delaying your period.
- Thyroid or pituitary conditions: These glands regulate the hormones that control your cycle. If they’re over- or underactive, your cycle length can become unpredictable.
- Polycystic ovary syndrome (PCOS): This condition causes irregular or absent ovulation, making calendar-based predictions unreliable without additional tracking.
When any of these factors are at play, your prediction might be off by several days or more. That doesn’t necessarily mean something is wrong. It means the variable half of your cycle (before ovulation) stretched longer than usual.
What Counts as Irregular
Some variation from month to month is completely normal. A cycle that’s 28 days one month and 30 the next is not irregular. According to ACOG, a cycle is considered abnormal if it consistently comes more often than every 21 days or less often than every 45 days, if your periods go more than 90 days apart even once, or if bleeding lasts longer than 7 days. If your periods were previously regular and then became unpredictable, that pattern shift is also worth noting.
For teens, a wider range is expected. Cycles anywhere from 21 to 45 days are considered normal in the first few years after menstruation begins, because the hormonal system is still maturing. Predictions will be less precise during this stage, and that’s typical.
How Perimenopause Changes the Math
If you’re in your 40s and noticing your predictions are increasingly off, perimenopause is a likely explanation. Early on, cycles tend to get slightly shorter, and the number of bleeding days may change. As perimenopause progresses, cycle length can vary by 7 days or more from one month to the next. Later in the transition, it’s common to skip periods entirely, sometimes going 60 days or longer between cycles.
These shifts happen because the ovaries produce hormones less consistently and release eggs less frequently. Tracking on a calendar or app is still useful during this stage, not so much for precise prediction, but for spotting patterns and having a record to review. If cycles become dramatically irregular or bleeding becomes very heavy, that record helps you and your doctor identify what’s changed.
Putting It All Together
For a quick estimate, the calendar method works: track at least three cycles, average the length, and count forward from your last period’s start date. For a more accurate prediction, layer in physical signs like temperature and cervical mucus to confirm when ovulation actually happened. Give yourself a window of a few days rather than expecting a single exact date, especially if your cycles vary by more than a couple of days month to month.
Period tracking apps automate this math and can incorporate additional data points, but they’re only as good as the information you give them. Consistently logging your start dates, and ideally your physical signs, is what makes any method more accurate over time.