Ovulation Predictor from Cycle History

A tool for individuals tracking menstrual cycles to estimate fertile windows and ovulation day based on historical cycle lengths. Supports both calendar days and cycle-length averaging. All calculations run instantly and privately in your browser—no data leaves your device.

Educational tool only, not medical advice. For family planning, consult a healthcare provider.

Enter Your Cycle Data

Calculation Method

Past Cycle Lengths (Days) Enter 3-12 cycles

Enter the length (in days) of your most recent menstrual cycles, from first day of period to day before next period.

Luteal Phase (Days)

Typical: 10-16 days

Fertile Window (Days)

Understanding Your Fertile Window

Tracking your menstrual cycle can help identify your fertile window—the days when you're most likely to conceive. This tool uses your historical cycle data to estimate when ovulation typically occurs for you.

How to Use This Tool

Record the length of your last 3-12 menstrual cycles (day 1 of bleeding to day before next period).
Enter each cycle length in the input fields. Add more cycles for better accuracy.
Choose your calculation method (standard is recommended for most).
Adjust luteal phase if you know yours (default 14 days works for many).
Click "Calculate" to see your estimated ovulation day and fertile window.

Realistic Examples

Cycle Lengths	Avg Cycle	Ovulation Day	Fertile Window
28, 28, 29, 27	28 days	Day 14	Days 10-15
32, 30, 31, 33	31.5 days	Day 17.5	Days 13.5-19.5
26, 35, 29, 31	30.25 days	Day 16.25	Days 12.25-18.25

Typical Cycle Characteristics

Phase	Duration	Description
Follicular	Variable (7-21 days)	Before ovulation; length varies most
Ovulation	12-24 hours	Egg release; peak fertility
Luteal	10-16 days (avg 14)	After ovulation; more consistent
Fertile Window	5-6 days	Includes days sperm can survive

Frequently Asked Questions

How accurate is this prediction?

It's an estimate based on historical patterns. Actual ovulation can vary due to stress, illness, travel, or hormonal changes. Tracking basal body temperature or using ovulation predictor kits provides more precise timing.

What if my cycles are irregular?

Irregular cycles make prediction less reliable. If your cycle length varies by more than 7-9 days regularly, consider discussing this with a healthcare provider to rule out underlying conditions.

When should I seek professional help?

Consult a provider if: cycles are consistently <21 or >35 days, you have severe pain, no period for 90+ days (not pregnant), or have been trying to conceive for 12+ months (6+ months if over 35).

How do I measure cycle length correctly?

Day 1 is the first day of full menstrual flow (spotting doesn't count). Count days until the day before your next period starts. Track for 3+ months for reliable patterns.

Formulas & Assumptions

Standard Method: Ovulation Day = Average Cycle Length − 14 (assumes 14-day luteal phase)

Alternate Method: Ovulation Day = Average Cycle Length − [Your Luteal Phase Input]

Fertile Window: Ovulation Day ± [Fertile Days/2] (centered on ovulation)

Regularity Calculation: (Max Cycle − Min Cycle) ≤ 7 days = "Regular"; ≤ 14 days = "Moderate"; >14 days = "Irregular"

Rounding: Results rounded to nearest 0.5 days for display, full precision used internally

Assumes relatively consistent luteal phase length across cycles

Does not account for anovulatory cycles or hormonal variations

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Ovulation Predictor from Cycle History

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