onlinecalculator.me

Free online period tracker and calculator

A period calculator projects your upcoming cycles from your last period date, showing when each period starts and ends, your ovulation date, and the fertile window for each cycle.

About this calculator

How to use

  1. Enter the first day of your most recent period.
  2. Set your cycle length (typically 21–35 days, default 28).
  3. Set your period duration (typically 3–7 days, default 5).
  4. Choose how many future cycles to display (1–6).
  5. Each cycle card shows the period window, ovulation date, and fertile window.

The formulas

For cycle number N (starting at 1):

startDate      = lastPeriodDate + (N − 1) × cycleLength
endDate        = startDate + periodDuration − 1
ovulationDate  = startDate + (cycleLength − 14)
fertileStart   = ovulationDate − 5 days
fertileEnd     = ovulationDate + 1 day

Source: Treloar AE et al. “Variation of the human menstrual cycle through reproductive life.” Int J Fertil. 1967;12(1):77–126.

Worked example

Last period: April 14, 2026. Cycle: 28 days. Duration: 5 days. Show 2 cycles.

Cycle 1:

  • Period: April 14 – April 18
  • Ovulation: April 28 (day 14 of cycle)
  • Fertile window: April 23 – April 29

Cycle 2:

  • Period: May 12 – May 16
  • Ovulation: May 26 (day 14 of cycle)
  • Fertile window: May 21 – May 27

Notes

  • These dates are estimates based on a regular cycle length.
  • Actual ovulation can vary by several days due to stress, illness, or other factors.
  • For irregular cycles, consider tracking basal body temperature or using LH predictor tests.
  • This calculator is for informational planning. Consult a healthcare provider for clinical guidance.

Frequently asked

How are future period dates calculated?
Each cycle starts exactly cycleLength days after the previous one. For cycle N, startDate = lastPeriodDate + (N−1) × cycleLength. The end date is startDate + periodDuration − 1.
How is the ovulation date estimated?
Ovulation is estimated at cycleStart + (cycleLength − 14). For example, with a 28-day cycle, ovulation falls on day 14. The −14 represents the roughly constant luteal phase — the time from ovulation to the next period.
What is the fertile window?
The fertile window is the 6 days ending on the day after ovulation — 5 days before ovulation plus ovulation day itself. Sperm can survive up to 5 days in the reproductive tract.
How many future cycles can I see?
The calculator shows 1 to 6 future cycles. Select the number of cycles using the drop-down.
Can I share my predictions?
Yes. Press "Share with my numbers" to copy a URL containing your inputs that can be opened on any device.