Estimate your next period date and simple future cycle forecast from your average cycle.
This calculator estimates future periods from past cycle averages. Real cycles can vary from month to month, so treat the dates as a forecast rather than a guarantee.
Tracking your menstrual cycle is one of the most valuable health habits you can develop. A period calculator helps you predict your next period, identify patterns in your cycle, and flag irregularities that might warrant medical attention. This complete guide covers the menstrual cycle in detail, what "normal" looks like, the causes of irregular periods, NHS guidance on menstrual health, and the statistics and context that matter for UK and US readers.
The menstrual cycle is the monthly hormonal cycle that governs the reproductive system. It begins on the first day of bleeding (day 1) and ends the day before the next period starts. The average cycle length is 28 days, but the normal range is 21–35 days. Cycle length can vary by several days each month even in women who consider themselves regular — a variation of up to 7–9 days between the shortest and longest cycle in a year is considered normal by the NHS.
The menstrual cycle is divided into four phases:
The period calculator uses a simple formula to predict your next period:
Next Period Start = Last Period Start + Average Cycle Length
For example, if your last period started on 1 March and your average cycle length is 28 days, your next period is predicted to start on 29 March. If your cycle averages 32 days, your next period would be on 2 April. The calculator also projects subsequent periods by continuing to add the cycle length.
For the most accurate predictions, track several cycles before relying on the calculator. Averaging 3–6 months of cycle data reduces the impact of individual cycle variations and provides a more reliable prediction window.
A typical period lasts 3–7 days, with 5 days being average. Bleeding is usually heaviest in the first 1–2 days and gradually lightens. Normal period blood volume is 20–80 ml (roughly 1–5 tablespoons) over the entire period. The NHS considers heavy menstrual bleeding (HMB) when blood loss exceeds 80 ml per period or is heavy enough to significantly impact quality of life — this affects approximately 1 in 3 women in the UK at some point in their reproductive years.
In the US, heavy menstrual bleeding affects an estimated 10–35% of women of reproductive age, according to the American College of Obstetricians and Gynecologists (ACOG). Both the NHS and ACOG advise women with suspected HMB to consult their doctor, as causes range from hormonal imbalances to uterine fibroids, polyps, and bleeding disorders. Effective treatments exist, from the Mirena IUS (intrauterine system, levonorgestrel-releasing) to tranexamic acid and, when necessary, surgical options.
Irregular periods — defined as cycles consistently shorter than 21 days, longer than 35 days, or highly variable — have numerous potential causes:
The NHS advises that any woman experiencing significant changes in period regularity, very heavy or very light periods, bleeding between periods, or periods that have stopped for more than 3 months (without pregnancy) should consult their GP. In the US, ACOG provides similar guidance, recommending evaluation for amenorrhoea lasting more than 3 months.
Premenstrual syndrome (PMS) affects an estimated 20–40% of women of reproductive age in both the UK and US. It is characterised by physical and emotional symptoms occurring in the 1–2 weeks before menstruation (the luteal phase) that resolve within a few days of the period starting. Common symptoms include mood swings, irritability, anxiety, bloating, breast tenderness, fatigue, headaches, and food cravings.
Severe PMS, known as premenstrual dysphoric disorder (PMDD), affects approximately 3–8% of women and can significantly impair daily functioning. The NHS and ACOG both recognise PMDD as a distinct clinical condition. Treatment options include lifestyle changes (exercise, dietary modification), cognitive behavioural therapy (CBT), hormonal treatments (combined pill, progesterone), and selective serotonin reuptake inhibitors (SSRIs). In the UK, NICE guidance CG63 provides recommendations on managing PMS; in the US, ACOG Practice Bulletin 155 covers PMDD.
Exercise can affect menstrual cycles in complex ways. Moderate regular exercise generally supports cycle regularity and reduces PMS symptoms. However, excessive training at high intensity — particularly when combined with low calorie intake — can suppress the hypothalamic-pituitary-ovarian axis, leading to functional hypothalamic amenorrhoea (FHA). This is particularly common in female athletes, dancers, and women with eating disorders. FHA is associated with low bone density (due to oestrogen deficiency) and increased fracture risk, making it a serious concern beyond reproductive health. The UK Sports and Exercise Medicine community (and the British Journal of Sports Medicine) has highlighted the Female Athlete Triad (energy deficiency, menstrual dysfunction, low bone density) as a significant health issue in active women.
Period poverty — the inability to afford menstrual products — affects hundreds of thousands of people in both countries. In the UK, a BBC survey in 2019 found that 1 in 10 girls aged 14–21 had been unable to afford menstrual products. Scotland became the first country in the world to make period products free to all who need them, through the Period Products (Free Provision) (Scotland) Act 2021. England, Wales, and Northern Ireland have introduced period product schemes in schools, but universal provision has not been legislated at a national level. In the US, menstrual equity legislation has been introduced in multiple states; as of 2024, 24 states and Washington D.C. have eliminated the "tampon tax" (sales tax on menstrual products), though full federal legislation has not passed.
Perimenopause marks the transition to menopause and typically begins in a woman's mid-to-late 40s, though it can start earlier. During perimenopause, oestrogen levels fluctuate unpredictably. Cycles may become shorter or longer; periods may become heavier, lighter, or less frequent. Spotting between periods is common. Perimenopause typically lasts 4–8 years before menopause (defined as 12 consecutive months without a period) is reached. The average age of menopause in the UK is 51 and in the US is 51–52 years.
The NHS advises women experiencing perimenopausal symptoms to speak to their GP, who can discuss hormone replacement therapy (HRT) and other management options. NICE guideline NG23 covers the management of menopause in the UK. In the US, the Menopause Society (formerly NAMS) provides clinical guidelines on perimenopause management.
Menstrual tracking has evolved significantly with smartphone technology. Popular apps including Clue, Flo, Ovia, and Period Tracker collect cycle data to provide increasingly accurate predictions over time. These apps use statistical algorithms that improve with more months of data. Some, like Natural Cycles, are regulated as contraceptive devices (CE-marked in the UK, FDA-cleared in the US) and incorporate temperature tracking alongside cycle data.
Manual calculation — as this calculator performs — is straightforward and reliable for women with relatively regular cycles. The NHS recommends keeping a period diary as a first step in identifying irregularities before any GP consultation. Period diaries are particularly useful in documenting heavy menstrual bleeding, as they provide objective evidence that helps GPs decide on appropriate investigations and treatment.
There is minor terminological variation between the UK and US. In the UK, the clinical term "menstruation" is used alongside the everyday "period." Heavy menstrual bleeding (HMB) replaces the older term "menorrhagia" in current NHS and NICE guidance. In the US, "menstruation" and "period" are equally used clinically. "Dysmenorrhoea" (menstrual cramps) is the clinical term used in both countries; the US spelling is "dysmenorrhea." Period products are more commonly called "sanitary towels" or "sanitary pads" in the UK and "maxi pads" or "pads" in the US. "Tampons" are universal.
To predict your next period, add your average cycle length to the first day of your last period. For example, if your last period started on 5 March and your cycle averages 28 days, your next period is expected around 2 April. For the most accurate prediction, average your cycle length from the past 3–6 months, as cycles can vary by several days each month.
The NHS defines a normal cycle as 21–35 days. Most people average 28 days, but significant variation exists. A cycle is considered irregular if it falls consistently outside the 21–35 day range or varies by more than 7–9 days between cycles over several months. Occasional variations of a few days are entirely normal.
Common causes of irregular periods include stress, significant weight change, polycystic ovary syndrome (PCOS), thyroid dysfunction, hyperprolactinaemia, perimenopause, and excessive exercise combined with low calorie intake. Pregnancy is always the first thing to rule out if a period is late. If irregularity persists, the NHS recommends seeing your GP for blood tests to investigate hormonal causes.
A typical period lasts 3–7 days. Periods shorter than 2 days or longer than 7–8 days may warrant investigation, though both can be normal for some individuals. If your period length has changed significantly from your usual pattern, particularly becoming much heavier or longer, it is worth discussing with your GP.
Heavy menstrual bleeding (HMB), formerly called menorrhagia, is defined clinically as blood loss exceeding 80 ml per period or bleeding that significantly interferes with daily life. In practice, signs include soaking through a pad or tampon every hour for several hours, passing large clots (larger than a 50p or quarter coin), or needing to use double protection. You should see your GP if this is happening regularly. Effective NHS treatments include the Mirena IUS, tranexamic acid, and hormonal medications.
Moderate exercise generally helps regulate cycles and can reduce PMS symptoms. However, very intense training combined with low calorie intake — common in elite athletes, dancers, and those with eating disorders — can suppress ovulation and cause irregular or absent periods (functional hypothalamic amenorrhoea). This is associated with low bone density and is a serious health concern beyond reproductive health.
Period tracking apps become more accurate the more cycle data they have. They use statistical algorithms to identify your average cycle length and predict future periods. Apps typically have a ±2–3 day margin of error for well-tracked cycles. Regulated contraceptive apps like Natural Cycles (which incorporates temperature data) have higher evidence standards than general tracking apps. None should be relied on as sole contraception unless specifically certified for that purpose.
PMS is caused by hormonal changes in the luteal phase — particularly the rise and fall of progesterone and oestrogen. Symptoms typically begin in the 1–2 weeks before a period and resolve within a few days of it starting. Lifestyle measures (regular aerobic exercise, reducing caffeine and salt, adequate sleep) help many women. If symptoms are severe, your GP can discuss hormonal treatments (combined oral contraceptive pill, progesterone) or SSRIs for emotional symptoms. Severe PMS (PMDD) is recognised as a clinical diagnosis by both NICE (UK) and ACOG (US).