Free Online Calculators
Calculate fertile days & ovulation from cycle length
This ovulation calculator uses the start date of your last period and your average cycle length to estimate when you'll ovulate and which days you're most fertile. It applies the standard rule that ovulation typically occurs about 14 days before your next period. Use it to better understand your cycle and plan accordingly.
With a 28-day cycle, ovulation falls on cycle day 14 and the fertile window runs from day 9 to day 15. The table below covers cycles from 21 to 35 days.
| Cycle length | Ovulation on cycle day | Fertile from day | Fertile until day |
|---|---|---|---|
| 21 days | 7 | 2 | 8 |
| 22 days | 8 | 3 | 9 |
| 23 days | 9 | 4 | 10 |
| 24 days | 10 | 5 | 11 |
| 25 days | 11 | 6 | 12 |
| 26 days | 12 | 7 | 13 |
| 27 days | 13 | 8 | 14 |
| 28 days | 14 | 9 | 15 |
| 29 days | 15 | 10 | 16 |
| 30 days | 16 | 11 | 17 |
| 31 days | 17 | 12 | 18 |
| 32 days | 18 | 13 | 19 |
| 33 days | 19 | 14 | 20 |
| 34 days | 20 | 15 | 21 |
| 35 days | 21 | 16 | 22 |
Typical signs are clear, stretchy cervical mucus like raw egg white, a slight rise in basal body temperature after ovulation, mild one-sided pelvic pain, breast tenderness, a softer and more open cervix, increased sex drive, and light spotting. Most women notice only some of them.
Around ovulation many women report more energy, a higher sex drive and feeling more sociable, driven by peaking oestrogen and the LH surge. Others notice nothing at all, so behaviour is not a reliable way to time the fertile window.
Regular cycles of similar length, a clear shift to slippery egg-white cervical mucus mid-cycle, a positive LH test each month and a temperature rise afterwards all suggest you are ovulating reliably. These signs indicate ovulation but say nothing about egg quality or tubal health.
Young age, regular ovulatory cycles, open fallopian tubes, a healthy body weight, no smoking and no untreated thyroid or hormonal disorders. Fertility also depends on the partner's sperm quality and on well-timed intercourse in the fertile window.
No, not from symptoms alone. Cervical mucus changes, LH tests and basal body temperature give strong evidence, but only ultrasound monitoring or a progesterone blood test after the expected ovulation can confirm that an egg was actually released.
The ovulation calculator is a free, easy-to-use tool that helps you identify your most fertile days based on your menstrual cycle. Whether you are trying to conceive or simply want to understand your body better, this calculator provides accurate estimates of when you are most likely to ovulate.
By entering the first day of your last period and your average cycle length, you will instantly receive a personalized overview of your fertile window – the days during which conception is most likely to occur.
The calculator uses a well-established biological principle: the luteal phase – the time between ovulation and the next period – is relatively constant at approximately 14 days for most women. This means ovulation typically occurs about 14 days before your next expected period.
Here is how to use the calculator in three simple steps:
Example: If your cycle is 28 days long and your last period started on May 1st, ovulation is expected around May 14th. Your fertile window would span roughly May 9th to May 16th – the days with the highest chance of conception.
Understanding your fertile window can significantly improve your chances of conceiving. A female egg is only viable for 12 to 24 hours after ovulation. However, sperm can survive in the female reproductive tract for up to 5 days. This means that having intercourse in the days leading up to ovulation can still result in fertilization.
Research consistently shows that couples who time intercourse within the fertile window have significantly higher conception rates than those who do not track their cycle at all.
This tool is especially useful for:
Important note: The ovulation calculator is not a contraceptive method and should not be used as a means of birth control. If you are trying to avoid pregnancy, please use a reliable contraception method.
For the most accurate picture of your fertility, consider combining the calculator with:
The calculator provides a good estimate for women with regular cycles. For women with irregular cycles, the prediction may be less precise. Combining it with physical signs of ovulation improves accuracy.
Use the average length of your last three to six cycles. If your cycle varies by more than 7 days regularly, consult a healthcare provider to rule out underlying conditions.
No. This calculator is not a contraceptive tool. Always use a reliable birth control method if you wish to avoid pregnancy.
The fertile window typically lasts about 6 days – the five days before ovulation and the day of ovulation itself. The highest probability of conception occurs in the two to three days before ovulation.
If you have been trying to conceive for more than 12 months (or 6 months if you are over 35) without success, it is advisable to consult a fertility specialist or gynecologist.