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Period Calculator — Free Online (Tolyo.app)

Track cycle dates and next period prediction. Fast • Private • Client-side.

Period Calculator
Popular

Typical range 21–35 days.

Most periods last 3–7 days.

Adds a realistic window (accounts for stress, travel, etc.).

Cycle day (today)
1
Next period (est.)
Tue, Oct 21, 2025
Window 2025-10-19 → 2025-10-23
Ovulation (est.)
Tue, Oct 07, 2025
≈ next period − 14 days
Upcoming periods (next 3)
  • Start
    Tue, Oct 21, 2025
    Ends Sat, Oct 25, 2025
  • Start
    Tue, Nov 18, 2025
    Ends Sat, Nov 22, 2025
  • Start
    Tue, Dec 16, 2025
    Ends Sat, Dec 20, 2025

Estimates only; cycles vary. For irregular or missed periods, heavy bleeding, or pain, seek medical advice.

How the Period Calculator Works

This Tolyo.app free online tool estimates your next period by adding your average cycle length to the first day of your last period (LMP). You can also include your usual period length and optional variability to see a practical range.

Scientific background (easy to understand)

  • Cycle length varies person to person. Typical cycles are ~21–35 days. The follicular phase (period to ovulation) is the most variable part of the cycle.
  • The luteal phase (ovulation to next period) is more stable for many people, often around ~14 days. That’s why ovulation is often estimated as next period − 14 days.
  • Stress, illness, travel, and age can shift timing. Small month-to-month changes are normal. Use the variability option to reflect this.
  • Tracking several months improves accuracy: update your average cycle length if patterns change.

Example

If LMP is 2025-09-01, average cycle length 29, and period length 5 days, then your next period is around 2025-09-30 (range depends on variability). Ovulation estimate is about 14 days before the next period.

Estimates only; not a diagnostic tool. For irregular cycles, pregnancy, or symptoms, talk to a clinician.

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