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 August 31, 2026, from https://calcvita.com/en/calculators/period
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Understanding Your Menstrual Cycle: A Complete Guide
Learn how your menstrual cycle works, what each phase means, and how to track your period effectively for better health awareness.
Read the full article →How does this calculator work?
This period calculator uses the calendar method to estimate your upcoming menstrual cycles. It takes the first day of your last period, your average cycle length, and period duration to predict future periods, fertile windows, and ovulation dates. Ovulation is estimated as cycle length minus 14 days (Wilcox et al., NEJM 1995), and PMS symptoms typically begin 7–10 days before the next period (ACOG Practice Bulletin).
Understanding cycle phases
The menstrual cycle consists of several phases: the menstrual phase (bleeding), the follicular phase (preparation for ovulation), the ovulation/fertile phase (when conception is most likely), and the luteal phase (after ovulation). PMS symptoms typically occur in the late luteal phase, 7–10 days before the next period begins (Reed & Carr, Endotext 2018).
What is PMS?
Premenstrual syndrome (PMS) affects up to 75% of menstruating women. Symptoms include mood changes, bloating, breast tenderness, and fatigue. They typically start 7–10 days before your period and resolve once bleeding begins. If symptoms severely impact daily life, consult your doctor about premenstrual dysphoric disorder (PMDD) (ACOG Practice Bulletin No. 15).
Irregular cycles and when to see a doctor
Cycles shorter than 21 days or longer than 45 days may indicate underlying conditions such as PCOS, thyroid disorders, or hormonal imbalances. If you experience very heavy bleeding, severe pain, or cycles that vary by more than 7–9 days, consult your healthcare provider. This calculator works best with regular cycles and provides estimates only.
Scientific Sources
- ACOG Practice Bulletin — Management of Abnormal Uterine Bleeding
- 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 — Periods and 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?
- A normal menstrual cycle ranges from 21 to 45 days, with the average being 28 days. However, a study by Bull et al. (2019) analyzing over 600,000 cycles found that only 13% of women have exactly 28-day cycles. Cycles between 24 and 38 days are considered clinically normal by ACOG. Variations of up to 7-9 days between cycles are common and generally not a cause for concern.
- How does the calculator predict my next period?
- The calculator uses the calendar method: it takes the first day of your last period and adds your average cycle length to predict the next period start date. Ovulation is estimated at cycle length minus 14 days, based on research by Wilcox et al. (NEJM 1995), since the luteal phase is consistently ~14 days. The fertile window spans 6 days (5 days before ovulation + ovulation day), and PMS onset is estimated at 10 days before the next period.
- Can I use this calculator with irregular cycles?
- This calculator provides the most accurate results for regular cycles. If your cycles vary by more than 7-9 days, the predictions will be less reliable. For irregular cycles, consider tracking over several months and using the average. Conditions like PCOS, thyroid disorders, stress, or significant weight changes can cause cycle irregularity. Consult your doctor if your cycles are consistently irregular.
- What is PMS and when does it start?
- Premenstrual syndrome (PMS) is a set of physical and emotional symptoms that occur in the days leading up to menstruation. According to ACOG Practice Bulletin No. 15, PMS affects up to 75% of menstruating women. Symptoms typically begin 7-10 days before your period and include mood changes, bloating, breast tenderness, fatigue, and food cravings. They usually resolve within 1-2 days after bleeding starts.
- When is my fertile window?
- Your fertile window spans approximately 6 days each cycle: the 5 days before ovulation and ovulation day itself. Research by Dunson et al. (2002) shows that conception probability peaks on the 2 days before ovulation. Sperm can survive up to 5 days in the reproductive tract, while the egg remains viable for only 12-24 hours after release. This calculator estimates your fertile window using the calendar method.
- When should I see a doctor about my period?
- See a doctor if: your cycles are shorter than 21 days or longer than 45 days, you bleed for more than 8 days, you experience very heavy bleeding (soaking through a pad/tampon every hour), you have severe menstrual cramps that interfere with daily activities, you miss 3 or more consecutive periods (without being pregnant), or you experience bleeding between periods. These may indicate conditions requiring medical evaluation (ACOG, Reed & Carr, Endotext 2018).
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