A period calculator answers the most practical cycle question — when will the next one start? — and a few related ones: which day of the cycle is today, what phase is your body in, and when are the next several periods likely to fall. Give it the start date of your most recent period and your typical cycle length, and it lays everything out on one page.
How to use the period calculator
- Enter the start date of your most recent period — the first day of real bleeding, not spotting.
- Enter your average cycle length and how many days your period usually lasts.
- Leave Today’s date as is, or change it to look ahead — for example, to check whether a trip overlaps your period.
- Read the next expected period and today’s cycle day on the tape. The calendar shades this cycle, and the table lists the next six.
How the next period is predicted
The calculation assumes each cycle matches your average:
If more than one full cycle has passed since the date you entered, the calculator assumes the periods in between arrived on schedule and continues from the latest one. Ovulation is placed 14 days before each expected period, with the fertile window covering the five days before it plus ovulation day.
Worked example
Last period began March 2, 2026; cycles average 28 days and periods last 5 days.
On March 20, 18 days have passed, so it is cycle day 19 — in the luteal phase, after the estimated ovulation on March 16.
Next period: March 2 + 28 = March 30, expected to run through April 3. The one after starts around April 27.
With a 30-day cycle, on April 15 the calculator assumes a period began on April 1, making April 15 cycle day 15 and the next period due May 1.
The four phases of the cycle
| Phase | Typical timing (28-day cycle) | What is happening |
|---|---|---|
| Menstrual | Days 1–5 | The uterine lining sheds; hormone levels are low |
| Follicular | Day 1 to ovulation | Rising estrogen matures a follicle and rebuilds the lining |
| Ovulation | Around day 15 | An LH surge releases an egg, which lives about a day |
| Luteal | After ovulation, 12–16 days | Progesterone prepares the lining; if no pregnancy, levels drop and a period follows |
The follicular phase is the part that varies most between people and between months. The luteal phase is steadier, which is why a longer cycle mostly means later ovulation rather than a longer wait after it.
What counts as a normal cycle
Clinicians generally consider adult cycles of about 21 to 35 days typical, with bleeding that lasts up to about seven days; international FIGO criteria put the normal range slightly wider, at 24 to 38 days. Variation of a few days from month to month is normal. It is worth checking in with a healthcare professional if periods stop for three months or more, regularly come more often than every 21 days or less often than every 35, last longer than seven days, or are heavy enough to soak through protection every hour or two.
Getting better predictions
Log the start date of every period. After a few months you will know your true average and how much it varies, and you can widen your expectations accordingly — a cycle that ranges from 27 to 33 days means any predicted date carries roughly a three-day margin either side. To plan around fertility rather than bleeding, the ovulation calculator lets you set your luteal phase length and shows a fertility-focused calendar.
This calculator is informational only and is not medical advice. It cannot diagnose cycle problems or confirm pregnancy, and it should not be used as birth control. Consult a healthcare professional with any concerns about your cycle.
Frequently asked questions
How do I work out when my next period is due?
Add your average cycle length to the first day of your last period. With a 28-day cycle and a period that started on March 2, the next one is due around March 30. The calculator repeats this to project six cycles ahead.
What is a normal menstrual cycle length?
For adults, cycles of about 21 to 35 days with bleeding that lasts up to about seven days are generally considered typical. In the first few years after periods begin, cycles of 21 to 45 days are common. Your own average matters more than the textbook 28 days.
Why is my period late?
Common reasons include pregnancy, stress, illness, travel, significant weight change, intense exercise, breastfeeding, some medications and the hormonal shifts of perimenopause. If you might be pregnant, a home test on or after the expected period day is a sensible first step.
How do I measure my cycle length?
Count from the first day of bleeding in one period up to, but not including, the first day of the next. Averaging three to six cycles gives a better figure for predictions than any single month.
Does the calculator work with irregular periods?
It uses your average, so it will be less precise if your cycles vary a lot. Treat each predicted date as the middle of a window of several days, and update the last-period date each time a new period starts.