Menstrual Cycle Calculator – Cycle Day, Phases, Next Two Periods

What this estimates

This calculator shows your current cycle day and phase, estimates ovulation with a confidence band, highlights a five day fertile window, and places three day windows for the next two period starts. Dates are displayed as 01 Jan 2025 to avoid format confusion.

Counting rules we use

• Cycle day = 1 on the first day of bleeding that needs pads or tampons. Day count increases by one each calendar day.
• Cycle length is your usual number of days from one period start to the next. If entries are far from typical population ranges, windows are widened.
• Luteal phase is the days from ovulation to the next period. If you do not enter a number, we use 14 as a common default. If you enter a number, we clamp to 10 to 17 for calculations.
• Variability adds a plus or minus band to ovulation and period windows. We clamp this input to 0 to 7 days. Higher variability means wider windows.

How the calculator maps inputs to outputs

• Cycle anchor: We locate the start of the current cycle by stepping forward or backward from your last period date in blocks of your average cycle length until the anchor encloses today.
• Phase label: Menstruation phase through your entered usual flow length. Follicular phase up to the peri ovulatory band. Peri ovulatory phase for a small band around the estimated ovulation day. Luteal phase after that until the next expected period.
• Estimated ovulation: Day index = cycle length minus luteal length within the current cycle. The confidence band widens by your variability setting and widens further when cycle length is outside common ranges.
• Fertile window: Five days centered on the estimated ovulation day. If cycle length is flagged as unusual, we extend this by about one day on each side. For precise timing, use an ovulation tool with OPK or BBT data.
• Next two period windows: Centered on the next two projected cycle starts. Each is widened to a three day window plus your variability and widened further if your cycle length is outside the usual range.
• Pregnancy test timing: We show the earliest reliable day as the day after the end of the first projected period window. Testing earlier may give false negatives.

Edge cases and guards

• Cycle length under 21 or over 45: We widen ovulation and period windows and display a caution note. Many adults cycle between 21 and 35 days, though wider patterns occur across the population and at younger ages.
• Future dates: Future last period dates are allowed. The tool still places phases and future windows relative to that anchor.
• Unusual luteal entries: Values outside 10 to 17 are allowed visually but clamped for math to avoid misleading results.
• Time zones: All dates are computed on your device to avoid time zone mismatches.

Limitations

These are estimates for general education. Actual ovulation varies cycle to cycle, and can shift with stress, illness, travel, or medication. If your periods are very irregular, windows will be broad and clinical evaluation may be helpful. This is not medical advice.

Sources

• ACOG — Your menstrual cycle: common cycle lengths and phase basics.
• Wilcox et al., NEJM 1995 — Timing of the fertile window: fertile window spans about six days ending on ovulation.
• Wilcox et al., BMJ 2000 — Timing of the fertile window revisited: day specific conception probabilities and variability.
• Hsu and Li 2013 — Luteal phase length variation: luteal commonly near 14 days but can range roughly 11 to 17.
• NICE Guidance — Fertility awareness methods: practical use of cycle tracking with OPK and BBT.