A little help from MamaSkin
Ovulation & fertile window calculator
Find a rough fertile window from your last period and usual cycle length. A starting point, rather than a prediction.
One less date to work out
Your estimate will appear here
A starting point for understanding your cycle. Your body may follow a different timetable.
A window, with room for real life
Knowing roughly where you are in a cycle can help when you’re trying to conceive. But a neat calendar can make biology look more predictable than it is. We show you the assumption alongside the dates, so you can decide how useful the estimate is for you.
How we calculate it
Next period = first day of last period + usual cycle length. Modelled ovulation = next period − 14 days. The fertile window runs from five days before that modelled date through the date itself. These are six calendar dates, not six guaranteed fertile days.
Actual ovulation may happen earlier or later, and outside the displayed window. Use this as a rough planning aid only. It is not suitable for contraception.
Sources & a note from us
Sources checked 28 October 2026. These organisations have not reviewed or endorsed the tool. General information, not a diagnosis or personal medical advice.
A worked example
For a period starting on 1 October 2026 and a 28-day cycle, the next period estimate is 29 October. The modelled ovulation date is 15 October, and the displayed window is 10–15 October.
Example dates only. Your own result uses the dates you enter.
Your questions, answered
How does this ovulation calculator work?
It estimates the next period from your cycle length, then places ovulation 14 days before that. The displayed six-day window includes the five preceding days and that modelled ovulation day.
Does everyone ovulate 14 days before a period?
No. The NHS describes a range of around 10–16 days. A calendar cannot identify your own ovulation date, even when your periods are regular.
Can I use it with irregular periods or while breastfeeding?
The regular-cycle assumption may not fit. Irregular periods, recent birth, breastfeeding, PCOS and fertility treatment can make this calendar estimate less useful. Your clinician can help you choose a more appropriate approach.
Can I use the window to avoid pregnancy?
No. Dates outside this estimate are not “safe days”. This calculator is not contraception and cannot confirm whether you have ovulated.
A skincare question, too?
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