What it calculates
Ovulation is when an ovary releases an egg. The days just before and on ovulation are the most fertile days of the cycle.
How it works
The next period is estimated from your cycle length; ovulation is placed 14 days before it, and the fertile window runs from 5 days before to 1 day after.
Formula
Ovulation ≈ Next period − 14 days
- Fertile window = ovulation − 5 days to ovulation + 1 day
How to use it
- Enter the first day of your last period.
- Enter your average cycle length.
- Read the dates for the next three cycles.
For a period that began a week ago with a 28-day cycle:
Why it helps
- Plans three cycles ahead.
- Simple and private.
Limitations
- Assumes regular cycles; stress, illness and other factors shift ovulation.
- Not a reliable method of contraception.
Frequently asked questions
Can I use this to avoid pregnancy?
No. Calendar estimates are not reliable enough for contraception.
Do you store what I enter?
No. This calculator runs entirely in your browser. Your measurements and dates are not sent to or stored on our servers.
Sources
These calculators provide estimates only and are not a substitute for medical advice. Dates and ranges vary between individuals; follow the guidance of your doctor or midwife. Your inputs stay in your browser and are not stored by UrbanFiling.