Period Cycle Calculator

Determine your exact cycle day and biological phase for any specific date on the calendar.

Cycle Day Lookup & Timeline

Enter your last period date and the target date you want to check.

Calculated on your device. Nothing is saved.

Date bleeding began

Select any date to look up

days
days
Key Takeaway: Looking up future cycle days allows you to schedule medical appointments, vacations, or athletic competitions when you are in your optimal phase.

Cycle Day Formula

Days Elapsed = Target Date − LMP
Cycle Day = (Days Elapsed mod Cycle Length) + 1

When to See a Doctor

Online calculators provide general estimates based on biological averages. Consult a healthcare provider (such as an OB/GYN or general practitioner) if you notice:

  • Your periods suddenly become irregular after being consistent
  • Your cycle is consistently shorter than 21 days or longer than 35 days
  • Your period bleeding lasts longer than 7 days or is unusually heavy (soaking through a pad/tampon every hour)
  • You experience severe, debilitating pelvic pain or cramping that disrupts daily life
  • You experience bleeding between periods or after intercourse
  • You are under 35 and have been trying to conceive for 12 months (or 6 months if 35+)
  • Your period has stopped completely for 90 days or more (and you are not pregnant)
Next Logical Step

Explore All Four Phases

Learn the biology and hormonal characteristics of each phase.

Open Period Cycle Phases Calculator

Frequently Asked Questions

Evidence-based answers to common questions about your cycle and fertility.

How do I calculate what cycle day a future date will be?

The calculator takes the number of days between your last period start (LMP) and your target date, calculates the modulo of your cycle length, and adds 1 to determine the exact projected cycle day and corresponding phase.

Explore other free, private tools for tracking your menstrual cycle and fertility.

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Medical & Scientific Sources

  1. American College of Obstetricians and Gynecologists (ACOG)(2022). "Your Menstrual Cycle"
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