Period Calculator
Predict your next period and track your menstrual cycle. Enter your last period date and cycle length to see what to expect next.
The Menstrual Cycle
Menstrual phase
Period begins
Follicular phase
Rising estrogen
Ovulation
Egg released
Luteal phase
Progesterone rises
Next period
Cycle resets
Average cycle: 28 days (range 21–35)
Enter Your Cycle Details
Range: 21–35 days
Range: 3–8 days
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How Menstrual Cycles Work
The menstrual cycle is a monthly hormonal process that prepares the female body for potential pregnancy. It is divided into four overlapping phases, each driven by changing levels of oestrogen, progesterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH).
The menstrual phase begins on day 1 of the cycle when the uterine lining (endometrium) is shed if pregnancy did not occur in the previous cycle. This phase typically lasts 3–7 days. Simultaneously, the follicular phase begins as FSH stimulates follicles in the ovaries to develop — one of which will eventually release a mature egg.
Rising oestrogen during the follicular phase rebuilds the uterine lining and triggers an LH surge around the middle of the cycle. This surge causes ovulation: the release of a mature egg from the ovary. After ovulation, the luteal phase begins as the collapsed follicle (now called the corpus luteum) produces progesterone to maintain the uterine lining. If fertilisation does not occur, progesterone levels drop, the lining is shed, and the cycle begins again.
How to Predict Your Next Period
The simplest way to predict your next period is to add your average cycle length to the first day of your last period. If your cycle is consistently 28 days and your last period started on the 1st of the month, your next period is expected to start around the 29th. This method works well for people with regular cycles.
For people with variable cycles, tracking several months of cycle data and calculating the average provides a more reliable estimate. Cycle-tracking apps can record this data automatically and adjust predictions over time. Period prediction becomes more accurate the more cycle data is available.
It is important to remember that a prediction is an estimate, not a guarantee. Ovulation — and therefore the next period — can shift by several days in response to physical and emotional stress, illness, changes in sleep, travel across time zones, or starting and stopping hormonal contraception. A period arriving 1–3 days earlier or later than predicted is normal variation.
Why Period Dates Can Change
Even in people with generally regular cycles, the precise date of ovulation — and consequently the date of the next period — can shift from month to month. The luteal phase (the time between ovulation and the next period) is relatively consistent at around 14 days, but the follicular phase (from the period to ovulation) is more variable and responds to external influences.
Stress is one of the most common factors that can delay ovulation. The hypothalamus, which regulates the hormonal cascade leading to ovulation, is sensitive to physical and psychological stress signals. Under significant stress, it may delay or suppress ovulation entirely, pushing the next period back. Illness, significant changes in body weight, intensive exercise, and disrupted sleep can have similar effects.
Starting or stopping hormonal contraception, including the pill, hormonal IUDs, injections, or implants, can also cause irregular periods as the body adjusts. In some cases, it may take several months for regular cycles to resume after stopping hormonal contraception. If periods remain significantly irregular after this adjustment period, speaking with a healthcare provider is advisable.
Frequently Asked Questions
How do I calculate my next period?
To estimate your next period date, add your average cycle length (measured from the first day of one period to the first day of the next) to the first day of your last period. For example: if your last period started on June 1 and your average cycle is 28 days, your next period is estimated to start on June 29. This calculator performs this calculation automatically, and also shows your ovulation date and fertile window within the same cycle.
What is a normal cycle length?
A typical menstrual cycle ranges from 21 to 35 days, with 28 days being the most commonly cited average. However, there is considerable natural variation between individuals, and cycle length can also vary from month to month within the same person. Cycles shorter than 21 days or longer than 35 days, or cycles that vary by more than 7–9 days regularly, may warrant discussion with a healthcare provider.
Why is my period late?
Menstrual periods can be late for many reasons beyond pregnancy. Common causes include stress (both physical and emotional), significant changes in weight or exercise habits, illness, travel and disruption to sleep patterns, thyroid dysfunction, polycystic ovary syndrome (PCOS), and hormonal fluctuations. A single late or missed period is not always a cause for concern, particularly if you have had a negative pregnancy test. If periods are frequently irregular or absent, speaking with a healthcare provider is advisable.
When should I take a pregnancy test if my period is late?
If your period is more than a few days late and you have had unprotected sex, taking a pregnancy test is a reasonable step. Modern home pregnancy tests are very accurate from the first day of a missed period. For the most reliable result, test with the first urine of the morning (when hCG concentration is highest) and follow the instructions carefully. If the result is negative but your period still has not arrived, repeat the test 2–3 days later or consult a healthcare provider.
