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LunWise

Period calculator

Find out when your next period is likely to start, plus your estimated fertile window and ovulation day for the next three cycles.

Calculated on your device. Nothing you enter is sent or stored.

Enter the first day of your last period to see your dates.

How this calculator works

  • Your next period is estimated by adding your usual cycle length to the first day of your last period.
  • Ovulation is estimated about 14 days before each predicted period.
  • The fertile window is the five days before ovulation, the day of ovulation and the day after.

Also try the ovulation calculator, or read how to tell when you're ovulating.

Questions and answers

How accurate is a period calculator?

It's a good estimate if your cycles are regular. Most cycles vary by a few days, and only about 13% are exactly 28 days, so a tracker that learns your real cycle will be more accurate over time.

How do I count my cycle length?

Count from the first day of one period (the first day of real bleeding, not spotting) to the day before your next period starts. A typical cycle is 24 to 38 days.

What if my periods are irregular?

A calculator that assumes a fixed length won't be reliable. LunWise weights your recent cycles, shows a confidence range, and has modes for PCOS and perimenopause.

Sources

  1. Bull JR, Rowland SP, Scherwitzl EB, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine, 2019.
  2. Munro MG, Critchley HOD, Fraser IS. The two FIGO systems for normal and abnormal uterine bleeding symptoms (2018 revisions). Int J Gynaecol Obstet.

Estimates only. Not medical advice and not a method of contraception.