If you want to know when your next period will start — to plan a trip, pack supplies, or simply feel prepared — you only need two pieces of information: the first day of your last period and the usual length of your cycle. Add one to the other, and you have your predicted date. For the fastest answer, use our period calculator — or keep reading to learn the method, see worked examples, and understand what to do when your cycles are irregular.
Period prediction is not about precision to the hour. It is about giving you a reliable window so you are not caught off guard. Most cycles fall between 21 and 35 days, with 28 days often used as the textbook average. Your own average — calculated from your last few cycles — will always beat a generic 28-day guess.
Next period start = first day of your last period + your cycle length in days. If your last period started January 1 and your cycle is 28 days, your next period is predicted for January 29.
Predict Your Next PeriodDay 1 is the first day of full menstrual flow — the day you need a pad, tampon, period underwear, or menstrual cup because bleeding has genuinely started. Light spotting in the day or two before, where you only see a little pink or brown on tissue, does not count as Day 1.
Getting Day 1 right matters because every calculation starts from it. If you count spotting as Day 1, your cycle will look one to two days longer than it really is, and your prediction will drift early over time. A simple rule: if you are unsure whether it is spotting or flow, wait until you see a clear need for protection. That day is Day 1.
Your cycle length is the number of days from Day 1 of one period to the day before Day 1 of the next period. In other words, if your last period started on January 1 and your next period started on January 29, your cycle length was 28 days. You count the days between the two start dates. You do not count the days of bleeding itself — bleeding typically lasts 3 to 7 days, but the cycle is the full span from one start to the next.
The method is simple addition. You need:
Then apply the formula:
Next period = first day of last period + cycle length
For example, if your last period started on January 1 and your cycle is 28 days, you add 28 days to January 1. January has 31 days, so January 1 + 28 days lands on January 29. That is your predicted start date.
If you prefer to think in weeks, a 28-day cycle is exactly four weeks, so your next period falls on the same weekday four weeks later. A 30-day cycle is four weeks plus two days, so it falls two weekdays later. A 26-day cycle is three weeks plus five days. The calendar math is the same; the weeks framing just makes it easier to picture.
Let us work through three examples with different cycle lengths so you can match the one closest to yours.
Last period started: January 1. Cycle length: 28 days. Calculation: January 1 + 28 days = January 29. Predicted next period: January 29. To check: January has 31 days. From January 1, adding 28 days moves you 28 days forward on the calendar: January 1 + 28 = January 29. If you then want the period after that, add another 28 days: January 29 + 28 days = February 26 (January has 31 days, so January 29 + 2 days = January 31, plus 26 more days = February 26).
Last period started: March 5. Cycle length: 30 days. Calculation: March 5 + 30 days = April 4. Predicted next period: April 4. To check: March has 31 days. March 5 + 26 days = March 31, plus 4 more days = April 4. A 30-day cycle is common, and it means your period shifts a few days later each month relative to the calendar.
Last period started: May 10. Cycle length: 26 days. Calculation: May 10 + 26 days = June 5. Predicted next period: June 5. To check: May has 31 days. May 10 + 21 days = May 31, plus 5 more days = June 5. A 26-day cycle is on the shorter side of normal, and it means you may sometimes have two periods in one calendar month.
Notice that none of these predictions claim your period will start at a specific hour. Treat the predicted date as the center of a window. For regular cycles, most periods arrive within one to two days of the predicted date. If you are planning travel, pack supplies a day or two before the predicted date so you are covered even if it arrives a little early.
If your cycles vary in length, a single cycle is not a reliable guide. The better approach is to average your last three cycles. That average smooths out one short or long month and gives you a more stable prediction.
Here is a worked example. Suppose your last three cycle lengths were 26 days, 30 days, and 28 days. Add them together and divide by three:
You would then use 28 days as your cycle length for the prediction. If your last period started on January 1, your predicted next period is January 1 + 28 days = January 29.
For irregular cycles, it also helps to track a range rather than a single date. Using the same example, your shortest recent cycle was 26 days and your longest was 30 days. From a January 1 start, that gives a window of January 27 (January 1 + 26 days) to January 31 (January 1 + 30 days). Planning around the whole window — January 27 to January 31 — is more realistic than expecting a single day.
To find your own average, write down the Day 1 date of your last four periods. That gives you three cycle lengths: the gaps between each pair of consecutive Day 1 dates. Average those three gaps. If you want a deeper explanation of how to measure cycle length itself, see our guide on how to calculate menstrual cycle length.
| Your Last Period Started | Your Average Cycle | Predicted Next Period |
|---|---|---|
| January 1 | 26 days | January 27 |
| January 1 | 28 days | January 29 |
| January 1 | 30 days | January 31 |
| January 1 | 32 days | February 2 |
| January 1 | 35 days | February 5 |
Check the table: January 1 + 26 days = January 27. January 1 + 28 days = January 29. January 1 + 30 days = January 31. January 1 + 32 days = February 2 (January 1 + 30 days = January 31, plus 2 more days = February 2). January 1 + 35 days = February 5 (January 1 + 30 days = January 31, plus 5 more days = February 5). Use the same addition for your own start date.
You can project several months ahead by repeatedly adding your cycle length, but accuracy falls the further you go. For regular cycles, predicting one to three months ahead is usually reasonable. Beyond that, small variations add up — a cycle that is one day longer than average, repeated over six months, shifts the prediction by six days.
A practical approach is to predict one cycle at a time and update the prediction each time a period starts. Keep a simple log: Day 1 date, cycle length, and any notes about stress, travel, illness, or medication that month. Over three to six months, that log becomes your most reliable predictor, because it reflects your actual pattern rather than a textbook average.
Our period calculator does the calendar addition for you and can show several future dates at once. It is still an estimate, not a guarantee — think of it as a planning window, not a scheduled appointment.
Even regular cycles shift from time to time. Common, non-worrying reasons include:
A period that is a few days early or late now and then is usually normal. What matters more is the pattern over time. If your cycles are consistently much shorter, much longer, or highly variable, or if you notice a sudden change from your usual pattern, that is worth discussing with your doctor.
Your period dates and your fertile window are connected. Ovulation typically happens about 14 days before your next period starts — not 14 days after your last period. So if your cycle is 28 days, ovulation is usually around Day 14. If your cycle is 32 days, ovulation is usually around Day 18 (32 − 14 = 18). If your cycle is 26 days, ovulation is usually around Day 12 (26 − 14 = 12).
That relationship is why an accurate period prediction helps with fertility awareness. If you are trying to conceive, use our ovulation calculator to estimate your fertile window. If you are tracking for other reasons, our safe days calculator can help you understand the calendar-based fertile window — but do not rely on calendar methods alone for contraception. They have a high failure rate in real-world use, especially with irregular cycles. Talk to your doctor about reliable contraception if pregnancy is not planned.
Prediction tools are for planning, not diagnosis. See your doctor if you notice any of the following:
If you are ever unsure, it is always reasonable to ask your doctor. Bring your cycle log — Day 1 dates, cycle lengths, and any symptoms — because that record helps your doctor understand your pattern much faster than memory alone.
Disclaimer: This article is for general information only and is not medical advice. If you have concerns about your menstrual cycle, missed periods, heavy bleeding, or severe pain, talk to your doctor or a registered dietitian before changing your diet or exercise routine.
Written by the Live Fitness Check editorial team · Reviewed against our calculator methodology · Last updated October 2026
