Ovulation & Fertility Calculator
Estimate your ovulation day and fertile window from your last period.
Results update as you type.
About this calculator
An ovulation calculator estimates your most fertile days from the first day of your last period and your average cycle length. Ovulation typically occurs about 14 days before your next period, and the fertile window spans the five days before ovulation plus the day itself, because sperm can survive several days. Cycle-based estimates are a guide, not a guarantee.
The tool counts forward from your last period. On a 28-day cycle, ovulation lands near day 14 (28 − 14) and your next period around day 28. Change the cycle length and it moves accordingly: a 30-day cycle puts ovulation near day 16, since it always sits about 14 days before the next expected period. The fertile window it shows runs from five days before ovulation through ovulation day.
Use it to time intercourse when trying to conceive — the days leading up to ovulation are the most fertile — or simply to understand your cycle. It also projects the next few cycles so you can plan ahead. Because ovulation timing varies month to month, especially with irregular cycles, treat the dates as an estimate rather than reliable contraception or medical advice.
Frequently asked questions
When do I ovulate?
Ovulation usually happens about 14 days before your next period starts, so on a 28-day cycle that is roughly day 14. This tool counts forward from your last period using your cycle length.
What is the fertile window?
It is the six-day span ending on ovulation day — the five days before plus the day itself — because sperm can survive up to five days and the egg lasts about a day. Intercourse in this window gives the best chance of conception.
How reliable is a cycle-based estimate?
It is only an estimate and assumes regular cycles. Ovulation predictor kits or basal body temperature tracking pinpoint the day more precisely, especially for irregular cycles.
How many days after my period do I ovulate?
Ovulation is about 14 days before your next period, so on a 28-day cycle it falls near day 14 — roughly a week after a typical period ends. A longer cycle pushes it later, to about day 16 on a 30-day cycle.
What are the best days to conceive?
The two to three days leading up to and including ovulation are the most fertile. The tool marks the six-day fertile window so you can time intercourse within it for the best chance of conception.
Can I get pregnant outside the fertile window?
It is much less likely but not impossible, because ovulation timing shifts and sperm survive several days. Cycle-based estimates are a guide for conceiving, not reliable contraception.
How do I confirm that I ovulated?
Ovulation predictor kits detect the LH surge about a day before ovulation, and basal body temperature rises slightly afterward. These confirm the timing more precisely than a calendar estimate, which helps with irregular cycles.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/ovulation/curl
curl "https://calculator.free/api/v1/ovulation/?cycle=28"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/ovulation/?" + new URLSearchParams({
"cycle": "28"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.