Safe Period (Ogino-Knaus) Calculator
The Safe Period refers to the days in the menstrual cycle when conception is least likely. For women with varying cycle lengths, the standard rhythm method can be risky.
This calculator uses the Ogino-Knaus Method, which considers your Shortest and Longest cycles from the last 6 months to calculate a safer, more personalized fertile window.
Shortest Cycle (Days)
Longest Cycle (Days)
Risk Assessment
OGINO-KNAUS
Today’s Status
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Fertile Window Opens
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Fertile Window Closes
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Cycle View
Safe 🛡️
Unsafe ⚠️
Period 🩸
Sun
Mon
Tue
Wed
Thu
Fri
Sat
The Math Behind Ogino-Knaus
Unlike simple calculators that assume a 28-day cycle, this method accounts for natural variation.
- First Fertile Day: Calculated as [Shortest Cycle Length] minus 18.
- Last Fertile Day: Calculated as [Longest Cycle Length] minus 11.
- Result: This creates a wider “Unsafe” window to account for the possibility of early or late ovulation.
How to Use
1
Enter LMP
Select the first day of your most recent period.
2
Cycle History
Review your last 6 months. Enter the number of days in your shortest cycle and your longest cycle.
3
Check Status
Look at the “Today’s Status” card. Red means high risk. Green means lower risk.
Important: Natural family planning methods have a failure rate of up to 24% with typical use. This calculator estimates risk but cannot guarantee prevention of pregnancy. It does not protect against STIs.