Health · 26 calculators
Ovulation Calculator
Ovulation cannot be felt reliably, so it is estimated from the cycle. The standard method counts backwards from the next expected period, because the interval after ovulation is far more consistent than the interval before it.
Estimated ovulation day
14
- Fertile window opens
- 9
- Fertile window closes
- 15
- Length of the fertile window
- 7
- Estimated implantation day
- 23
Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.
The formula this calculator uses
Ovulation day = cycle length - luteal phase length
- O
- The estimated day of ovulation, counted from the first day of the period
- C
- The total length of the cycle
- P
- The length of the luteal phase, the interval from ovulation to the next period
How to check the result by hand
- 1
Establish the usual cycle length
Count from the first day of one period to the day before the next begins, and average over several cycles. Twenty eight days is the textbook average but individual cycles vary widely.
- 2
Take the luteal phase length
Fourteen days is the standard assumption and is close to fixed for most people. Unless you have a measured reason to change it, leave it at fourteen.
- 3
Subtract to get the ovulation day
Twenty eight minus fourteen puts ovulation on day 14. Counting always starts at day one on the first day of bleeding.
- 4
Work out the fertile window around it
Five days before through one day after gives day 9 to day 15, a seven day window. Sperm survival is why the days before ovulation count.
- 5
Treat it as an estimate, not a measurement
The calendar method predicts from cycle length alone. If the timing matters, confirm with temperature tracking or luteinising hormone tests rather than relying on the count.
For worked examples, common mistakes and the limits of this formula, read the full How To Calculate Ovulation page.