Predict your next period date, ovulation day, and cycle phases based on your last period and cycle length
This calculator provides estimates based on average cycle patterns. Individual cycles can vary significantly. This tool is for informational purposes only and does not constitute medical advice. Consult your GP for any concerns about your menstrual cycle.
The menstrual cycle is a monthly series of hormonal changes that prepare the body for pregnancy. Understanding your cycle can help you predict your next period, identify your most fertile days, and recognise any irregularities that may warrant medical attention. The NHS defines a normal menstrual cycle as lasting between 21 and 35 days, with 28 days being the most commonly cited average. However, cycle length can vary from person to person and even from month to month. Each menstrual cycle consists of four phases: menstruation (your period), the follicular phase (when the body prepares to release an egg), ovulation (when the egg is released from the ovary), and the luteal phase (after ovulation, when the body either supports a potential pregnancy or prepares to shed the uterine lining). This calculator helps you map these phases based on your last period date and average cycle length, giving you a clearer picture of where you are in your cycle and when to expect your next period. Tracking your menstrual cycle is recommended by the NHS as a useful tool for understanding your body. It can help identify patterns that may indicate conditions such as polycystic ovary syndrome (PCOS), endometriosis, thyroid disorders, or perimenopause. If your cycles are consistently outside the 21-35 day range, or if you experience unusually heavy bleeding, severe pain, or bleeding between periods, the NHS recommends consulting your GP.
To predict your menstrual cycle: 1. Enter the start date of your last period. This is the first day of full bleeding (not spotting). Enter the day, month, and year separately. 2. Enter your average cycle length in days. This is the number of days from the first day of one period to the first day of the next. If you are not sure, 28 days is a reasonable starting point. Most cycles fall between 21 and 35 days. 3. Enter your typical period length in days. This is how many days your period usually lasts, typically between 2 and 8 days. 4. View your results. The calculator shows your predicted next period date, estimated ovulation date, current cycle phase, and the next three upcoming period dates. It also displays a chart showing the duration of each cycle phase. 5. Track over several months for better accuracy. The more cycles you track, the better you will understand your personal pattern and average cycle length.
Menstrual cycle prediction is based on the assumption that cycles repeat at roughly regular intervals. The calculator uses the following logic: Next period date = last period start date + cycle length (in days) Ovulation is estimated to occur approximately 14 days before the next period. This is based on the observation that the luteal phase (the phase after ovulation) is relatively consistent at around 14 days for most women, even when overall cycle length varies. So: Ovulation day = cycle length - 14 (counting from the first day of the period). The cycle phases are calculated as follows: - Menstruation: Day 1 to the end of your period length - Follicular phase: Day 1 to the day before ovulation (includes menstruation) - Ovulation: The estimated ovulation day - Luteal phase: The day after ovulation to the end of the cycle The calculator also determines your current phase based on today's date relative to your cycle. If your last period was recent, you may be in menstruation or the follicular phase. If ovulation has passed, you are in the luteal phase. These are estimates based on averages. Real cycles can vary due to stress, illness, travel, weight changes, exercise patterns, and hormonal fluctuations. For precise ovulation detection, the NHS suggests using ovulation predictor kits or basal body temperature charting.
The NHS recommends seeing your GP about your periods if they suddenly become irregular, if you have periods more often than every 21 days or less often than every 35 days, if your periods last longer than 7 days, if you experience very heavy bleeding (soaking through a pad or tampon every hour), or if you have severe pain that affects your daily life. These could be signs of conditions that benefit from medical investigation and treatment. Keeping a record of your cycle dates, symptoms, and flow can be very helpful information to bring to a GP appointment.