Ovulation & Fertile Window Calculator

Enter the first day of your last period and your average cycle length to get a general estimate of your ovulation date, fertile window, and next predicted period. This is a simple educational tool based on average cycle physiology – it is not a medical or diagnostic device.

How this is calculated

This calculator uses standard menstrual cycle physiology: the luteal phase – the time from ovulation to the start of the next period – is consistently around 14 days, regardless of a person's total cycle length. Working back from this:

• Estimated ovulation date = first day of your last period + (your average cycle length − 14 days)
• Estimated fertile window = the 5 days before your estimated ovulation date, through to 1 day after it – because sperm can survive in the body for up to around 5 days, and the egg is viable for approximately 24 hours after release
• Next predicted period = first day of your last period + your average cycle length

This is a general estimate based on average cycle physiology, not a diagnosis. It becomes less reliable if your cycles are irregular, which is common with PCOS/PMOS. It does not diagnose or treat fertility issues or PCOS/PMOS. If you are actively trying to conceive, or have any concerns about your fertility, please speak to your GP, gynaecologist, or a fertility specialist. All dates you enter are calculated instantly in your browser – nothing you type into this calculator is stored, saved, or sent to any server or third party.

Related reading

• PCOS (PMOS) Nutrition – my approach to supporting hormonal health, insulin sensitivity, and fertility-related nutrition concerns
• Fertility Nutrition
• Seed Cycling for Hormone Balance

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