Period calculator
Your next period is estimated by adding your average cycle length to the first day of your last period. With a 28-day cycle, a period that started on 3 November 2026 is next expected on 1 December 2026. Each later period is one more cycle length ahead and lasts as many days as your periods usually do. Cycles vary by a few days, so treat each date as the middle of a window, not a promise.
Nothing you enter leaves your browser: the calculation runs on your device and nothing is stored.
Worked example
Your last period started on 3 November 2026, your cycles are usually 28 days long and your period lasts 5 days.
- Period 1: 3 November 2026 + 28 days (1 × 28) = 1 December 2026, lasting until 5 December 2026 (5 days, counting the first day).
- Period 2: 3 November 2026 + 56 days (2 × 28) = 29 December 2026, lasting until 2 January 2027 (5 days, counting the first day).
- Period 3: 3 November 2026 + 84 days (3 × 28) = 26 January 2027, lasting until 30 January 2027 (5 days, counting the first day).
Because cycles vary, plan for each period to arrive a few days either side of these dates.
How the calculation works
Day 1 of a cycle is the first day of bleeding, and your cycle length is the number of days from one period start to the next. The calculator adds that number to the first day of your last period to find the next start, then keeps adding it for each period after. The end date is the start date plus your period length, minus one, because the first day counts.
If the date you enter is more than one cycle ago, the calculator projects forward by whole cycles from it, the same way the Lunation app does, and lists the periods still ahead of today. The further back the date, the less reliable that projection becomes, because small differences in cycle length add up.
How accurate is this?
A calendar prediction assumes every cycle is the same length, and real cycles are not. In Bull and colleagues’ 2019 analysis of 612,613 cycles logged in a cycle-tracking app, the average cycle was 29.3 days, only 13% of cycles were exactly 28 days, and one person’s cycle length typically varied with a standard deviation of about 2.6 days. Most of that variation comes from the time before ovulation, which can shift with stress, illness or travel.
For most adults, cycles between 21 and 35 days long are typical (ACOG, NHS). The first prediction is the most reliable one; the sixth carries the error of six cycles. Using the average of your last few cycles, rather than a textbook 28, is the single biggest improvement you can make.
Frequently asked questions
How accurate is a period calculator?
It is as accurate as your cycles are regular. In a study of more than 600,000 cycles, only 13% were exactly 28 days long, and a typical person’s cycle length varied by a few days, so expect each period within a few days of the predicted date.
What if my cycles are irregular?
Use the average of several recent cycles, which the cycle length calculator works out from your start dates, and read each prediction as a window rather than a single day. If your cycles are often shorter than 21 days or longer than 35, or your usual pattern changes, it is worth mentioning to a doctor.
Why did my period not start on the predicted day?
Stress, illness, travel, changes in weight or exercise, and hormonal contraception can all shift when you ovulate, and your period follows. A difference of a few days from one cycle to the next is common.
Does the period length change the predicted start dates?
No. The start dates depend only on your last period and your cycle length. Period length sets how many days each predicted period lasts.
Are my dates saved or sent anywhere?
No. The calculation runs in your browser and nothing you enter is sent to a server or stored. Closing the page clears it.
Sources
- Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (Bull et al., 2019). npj Digital Medicine.
- Abnormal Uterine Bleeding (FAQ): what is a normal menstrual cycle?. American College of Obstetricians and Gynecologists (ACOG).
- Irregular periods. NHS.
- Periods and fertility in the menstrual cycle. NHS.
Written by the Lunation editorial team.
Related guides
- How to Predict Your Next PeriodA simple method to estimate your next period: average your recent cycles, add it to your last start date, and plan for a window of a few days.
- How Long Is a Normal Menstrual Cycle?Most cycles last 21 to 35 days, and few are exactly 28. What large studies show about cycle length by age, how much variation is typical, and when to ask.
- What Are the Phases of the Menstrual Cycle?The four phases of the menstrual cycle explained: typical days, the hormones behind each one, and why real cycles rarely follow the 28-day textbook.
More calculators
- Ovulation calculatorEstimate your ovulation day (about 14 days before your next period) and six-day fertile window for the next three cycles.
- Due date calculatorEstimate a due date, current pregnancy week and trimester from your last period (280 days) or conception date (266 days).
- Cycle length calculatorWork out your average cycle length, shortest and longest cycle, and how much it varies from your period start dates.
Medical disclaimer. This calculator gives general wellness estimates, not medical advice. It does not diagnose or treat any condition and is not a method of contraception. If something about your cycle or your health worries you, talk to a doctor or midwife.