Z

Ovulation & Fertility Calculator

Estimate your ovulation date and fertile window from your cycle.

Runs in your browser — files never leave your device

Ovulation
2024-01-15
Fertile from
2024-01-10
Fertile until
2024-01-16

An estimate based on cycle averages — not a contraception method.

How it works

This calculator estimates your next ovulation date and fertile window from two inputs: the first day of your last period and your average cycle length. It relies on a well-established pattern in the menstrual cycle: the luteal phase — the stretch from ovulation to the next period — is relatively constant at about 14 days, while the phase before ovulation is what varies between people and between cycles. Ovulation therefore lands about 14 days before your next period, whatever your cycle length.

The formulas are: ovulation date = last period start + (cycle length − 14) days, and fertile window = 5 days before ovulation through 1 day after. The window is built that way because sperm can survive in the reproductive tract for up to about five days, while the egg is viable for roughly 12–24 hours after release — so intercourse in the days before ovulation can still result in conception.

Worked example: your last period started June 1, 2026, and your cycles average 28 days. Ovulation = June 1 + (28 − 14) = June 1 + 14 days = June 15, 2026. The fertile window runs June 10 through June 16, and the next period would be expected around June 29. With a 32-day cycle and the same start date, the estimate shifts to June 19.

This is a calendar estimate, not a detection method. It assumes your cycles are regular and your luteal phase is exactly 14 days — both vary in real life, and ovulation can shift even in otherwise regular cycles because of stress, illness, travel, or conditions such as PCOS. Ovulation predictor kits (which detect the LH surge), basal body temperature, and cervical-mucus tracking observe what your body is actually doing and time the window more reliably. This tool is general guidance only — not medical advice — and it is not a contraception method; do not rely on it to prevent pregnancy.

Frequently asked questions

How is the ovulation date estimated?
The calculator adds (cycle length − 14) days to the first day of your last period. That works because the luteal phase — the stretch from ovulation to the next period — stays close to 14 days for most people, while it’s the phase before ovulation that varies. So ovulation is estimated at about 14 days before your next expected period, whatever your cycle length.
What counts as a normal cycle length?
Cycles of roughly 21 to 35 days are generally considered normal for adults, and modest variation from cycle to cycle is common. Count from the first day of bleeding in one period to the first day of the next. If your cycles are consistently outside that range or highly irregular, it’s worth mentioning to a clinician.
Why is the fertile window about a week long?
Sperm can survive in the reproductive tract for up to about five days, while the released egg is viable for roughly 12–24 hours. Intercourse in the days before ovulation can therefore still lead to conception, which is why the tool marks the window from five days before the estimated ovulation date through one day after it.
What if my cycle length varies?
Enter your average, but read the result as a range rather than a precise date — cycles that run anywhere from 26 to 30 days move the estimated ovulation day across a five-day span by themselves. The more irregular your cycles, the less useful any calendar estimate becomes. Ovulation predictor kits, which detect the LH surge in urine, give a direct signal in exactly that situation.
Can I use this to avoid pregnancy?
No. Sperm survival plus natural variation in ovulation timing leaves calendar estimates with far too much room for error, and this tool makes no claim of contraceptive reliability. If you want to prevent pregnancy, use an actual contraception method and discuss options with a clinician.
Does this confirm that I ovulate?
No — it assumes ovulation happens and estimates when it would fall, but some cycles are anovulatory (no egg is released), which a calendar cannot detect. An LH surge on a test strip, a sustained rise in basal body temperature, or ultrasound monitoring are used to confirm ovulation. See a clinician if you have concerns about ovulation or fertility.
Are my dates uploaded anywhere?
No. The estimate is computed entirely in your browser, and your cycle data never leaves your device.

Related tools