Fertility Calculator
Estimate your most fertile days and predicted ovulation date based on the start of your last period, your average cycle length, and your luteal phase.
Calculate Your Fertile Window
Enter the first day of your last period along with your average cycle and luteal phase length to get an instant, personalised estimate of your fertile days and ovulation date.
Cycle Details
Enter your last period and cycle information
Fertility Estimate
Your predicted fertile window and ovulation date
Typical Cycle Phase Lengths
General ranges for the phases of a menstrual cycle. Individual cycles can vary, and these figures are for reference only.
| Cycle Phase | Typical Range | Average |
|---|---|---|
| Full Menstrual Cycle | 21 – 35 days | 28 days |
| Follicular Phase (period start to ovulation) | 11 – 21 days | 14 days |
| Fertile Window (before + day of ovulation) | 5 – 6 days | 6 days |
| Luteal Phase (ovulation to next period) | 10 – 16 days | 14 days |
| Egg Viability After Ovulation | 12 – 24 hours | ~24 hours |
| Sperm Viability in Reproductive Tract | Up to 5 days | ~3 days |
Fertility Calculator FAQ
Answers to the most frequently asked questions about estimating ovulation and your fertile window.
Your fertile window is estimated as the six days ending on your predicted ovulation day. Ovulation is calculated by counting back the length of your luteal phase (typically 14 days) from the end of your cycle, since the luteal phase is far more consistent in length than the first half of the cycle.
The luteal phase is the time between ovulation and the start of your next period. It is usually between 10 and 16 days, with 14 days being the average. Because it varies less than the pre-ovulation phase, working backward from your next expected period using the luteal phase length gives a more accurate ovulation estimate than simply assuming ovulation falls on day 14 of every cycle.
This calculator uses your average cycle length, so irregular cycles will reduce accuracy. If your cycles vary by more than a few days, consider tracking basal body temperature, cervical mucus, or ovulation predictor kits alongside this estimate, or speak with a doctor about irregular cycles.
No. This tool provides a statistical estimate for general planning purposes only and should not be relied upon as a method of contraception or as a substitute for medical advice. Ovulation timing can shift due to stress, illness, travel, and many other factors.