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Ovulation calculator
The ovulation calculator estimates your most fertile days from the first day of your last period and the average length of your cycle. The logic is simple: ovulation tends to happen about 14 days before the next period, and the fertile window runs from a few days before until the day after ovulation. Enter the LMP and your cycle length to see the dates.
Fertile window by cycle length
| Cycle | Ovulation (cycle day) | Fertile window (cycle days) |
|---|---|---|
| 26 days | Day 12 | Day 7 to day 13 |
| 28 days | Day 14 | Day 9 to day 15 |
| 30 days | Day 16 | Day 11 to day 17 |
| 32 days | Day 18 | Day 13 to day 19 |
| 35 days | Day 21 | Day 16 to day 22 |
How the fertile window is estimated
Ovulation = (LMP + cycle length) − 14 days
The phase between ovulation and the next period, called the luteal phase, is the most stable part of the cycle and lasts about 14 days in most women. That is why the calculation starts from the next predicted period and subtracts 14 days. For an LMP on 1 January with a 28 day cycle, the next period is estimated for 29 January, ovulation for 15 January and the fertile window from 10 to 16 January. For a 30 day cycle with an LMP on 1 June, ovulation falls around 17 June. The fertile window accounts for the roughly five days sperm survive before ovulation plus the day after it.
Why it is only an estimate
Many cycles are not perfectly regular, and ovulation can come earlier or later due to stress, travel, sleep, intense exercise and various health conditions. That is why this tool is for planning and body awareness, not a contraceptive method: it does not protect against pregnancy. Observation methods such as basal body temperature, cervical mucus and ovulation tests raise the accuracy. If you are tracking a pregnancy, use the due date calculator as well. Sources: American College of Obstetricians and Gynecologists (ACOG) and public health services such as the NHS.
Disclaimer: an educational estimate of the cycle. It is not a contraceptive method and must not be used to avoid pregnancy. For contraception or fertility, consult a gynecologist.
Last updated: · Methodology and sources