Period Calculator — Menstrual Cycle
Predict your next period, fertile window, ovulation and PMS dates. Track 6 cycles ahead and know your current cycle phase. Free, instant.
CalcVita. (2026). Period Calculator — Menstrual Cycle. CalcVita. Retrieved October 16, 2026, from https://calcvita.com/en/calculators/period
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Read the full article →How does this calculator work?
This period calculator uses the calendar method. From the first day of your last period, your average cycle length and how many days you bleed, it predicts your next six periods, places ovulation 14 days before each period (cycle length minus 14), marks the fertile window as the 5 days before ovulation plus ovulation day (Wilcox et al., NEJM 1995) and shades the last 10 days of each cycle as the premenstrual phase. Real cycles vary, so the dates are estimates.
Understanding cycle phases
The menstrual cycle has a menstrual phase (bleeding), a follicular phase (an egg matures; this is the phase whose length varies most), ovulation, and a luteal phase from ovulation to the next period (Reed & Carr, Endotext 2018). The fertile window covers the days just before ovulation, and premenstrual symptoms, when they occur, appear at the end of the luteal phase.
What is PMS?
Premenstrual syndrome (PMS) is a set of physical and emotional symptoms, such as mood changes, bloating, breast tenderness and tiredness, that recur in the days before the period. ACOG diagnoses it when symptoms appear in the 5 days before the period for at least three consecutive cycles, end within 4 days after bleeding starts and interfere with daily life. If symptoms severely affect your daily life, ask your doctor about premenstrual dysphoric disorder (PMDD).
Irregular cycles and when to see a doctor
In adults, cycles shorter than 24 days or longer than 38 days, or that differ from one another by more than 7-9 days, fall outside the FIGO normal range (Munro et al., 2018) and may reflect conditions such as PCOS, thyroid disorders or other hormonal imbalances. In teenagers, cycles of 21 to 45 days are still typical (ACOG). If you have very heavy bleeding, severe pain or irregular cycles, see your healthcare provider. This calculator works best with regular cycles and provides estimates only.
Scientific Sources
- Munro MG et al. (2018) — The two FIGO systems for normal and abnormal uterine bleeding symptoms: 2018 revisions. Int J Gynaecol Obstet. PMID 30198563
- ACOG Committee Opinion No. 651 — Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign
- ACOG — Premenstrual Syndrome (PMS), patient FAQ
- Wilcox AJ et al. (1995) — Timing of sexual intercourse in relation to ovulation. NEJM. PMID 7477165
- Reed BG & Carr BR (2018) — The Normal Menstrual Cycle and the Control of Ovulation. Endotext.
- NHS — Fertility in the menstrual cycle
- Bull JR et al. (2019) — Real-world menstrual cycle characteristics of more than 600,000 cycles. NPJ Digital Medicine. 2:83
Frequently asked questions
- What is a normal menstrual cycle length?
- In adults, FIGO (the International Federation of Gynecology and Obstetrics) considers a cycle normal when it lasts 24 to 38 days, bleeding lasts up to 8 days and the shortest and longest cycles differ by no more than 7-9 days (Munro et al., 2018). In adolescents during the first years after their first period, cycles of 21 to 45 days are typical (ACOG). A perfect 28-day cycle is the exception: in Bull et al. (2019), an analysis of more than 600,000 cycles, only 13% of cycles lasted exactly 28 days.
- How does the calculator predict my next period?
- It uses the calendar method: it adds your average cycle length to the first day of your last period to predict the next one, and repeats this for six cycles. It places ovulation 14 days before the next period (cycle length minus 14), marks the fertile window as the 5 days before ovulation plus ovulation day (Wilcox et al., NEJM 1995) and shades the last 10 days of the cycle as the premenstrual phase. The luteal phase is not always 14 days (it averaged 12.4 days in Bull et al. 2019), so all dates are estimates.
- Can I use this calculator with irregular cycles?
- The predictions are most reliable with regular cycles. If your cycles differ from one another by more than 7-9 days, they will be less accurate. With irregular cycles, track several months and enter the average. PCOS, thyroid disorders, stress or major weight changes can make cycles irregular; see your doctor if your cycles are consistently irregular.
- What is PMS and when does it start?
- Premenstrual syndrome (PMS) is a pattern of physical and emotional symptoms, such as mood changes, bloating, breast tenderness and tiredness, in the days before the period. ACOG diagnoses it when symptoms appear in the 5 days before the period for at least three consecutive cycles, end within 4 days after bleeding starts and interfere with normal activities. The calculator shades the last 10 days of the cycle, a wider margin for these symptoms. If they severely affect your daily life, ask your doctor about premenstrual dysphoric disorder (PMDD).
- When is my fertile window?
- About 6 days each cycle: the 5 days before ovulation and ovulation day itself. In Wilcox et al. (1995), nearly all pregnancies came from intercourse within that window, and the probability of conception rose from 10% five days before ovulation to 33% on the day of ovulation. Sperm can survive up to 5 days in the reproductive tract, while the egg remains viable for only 12-24 hours. The calculator estimates the window with the calendar method, so it is approximate.
- When should I see a doctor about my period?
- See a doctor if your cycles are repeatedly shorter than 24 days or longer than 38 days (in teenagers, shorter than 21 or longer than 45), you bleed for more than 8 days, you have very heavy bleeding (soaking through a pad or tampon every hour for several hours in a row), period pain that stops your daily activities, more than 3 months without a period when you are not pregnant, or bleeding between periods. These warning signs follow FIGO (Munro et al., 2018) and ACOG.
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