About this calculator
A period calculator estimates next period dates, ovulation, and fertile vs safer-day ranges from LMP and cycle length.
Who should use it: People tracking menstrual cycles for planning and awareness.
When to use it: Use it for calendar estimates — not as contraception or medical diagnosis.
Next period and cycle phases
Next period is last menstrual period (LMP) plus cycle length. Ovulation uses a default 14-day luteal phase. The fertile window is modeled as about 5 days before ovulation through 1 day after. Safer days are cycle days outside period bleeding and outside that fertile window. Not for contraception.
Step-by-step
Enter the first day of your last period
Cycle day 1 anchors next-period, ovulation, and calendar estimates.
Set cycle and period lengths
Typical cycle 21–45 days (often ~28); period length usually 2–10 days.
Review next period and cycle map
See next period, ovulation, fertile window, safer-day ranges, and cycle day.
Worked example: June 1 LMP, 28-day cycle, 5-day period
LMP June 1, 28-day cycle, 5-day period — next period, ovulation, and fertile window.
- Next period = June 1 + 28 → June 29
- Period ends June 5; ovulation ≈ June 15
- Fertile window June 10–16; safer days June 6–9 and June 17–28
Next period ≈ June 29; ovulation ≈ June 15.
How to interpret the result
Predictions degrade when cycles are irregular; logged averages beat a single assumed 28-day cycle.
Key definitions
- Cycle length
- Days from the start of one period to the start of the next.
- Safer days
- Estimated days outside period bleeding and outside the fertile window — not a birth-control method.
- Luteal phase
- Days from ovulation to the next period — often about 12–14 days; this tool uses 14.
Common use cases
- Planning around an upcoming period
- Cycle awareness alongside ovulation timing
- Understanding fertile vs safer day estimates
Tips
- Irregular cycles reduce prediction accuracy.
- This is not a birth-control method.
- Pair with clinical advice when timing or symptoms matter.
Common mistakes
Assuming every cycle is 28 days
Fix: Enter your average cycle length from several recent cycles.
Using safer-day estimates as contraception
Fix: Use medically approved birth control methods for pregnancy prevention.
Limitations
- Not a birth-control method or medical device.
- Symptoms and clinical advice override calendar estimates.