Free ovulation calculatorCalculate ovulation: your fertile days at a glance
First day of your last period + cycle length — that's all it takes for an honest estimate of your ovulation date, fertile window and next period. No account, no app, right here.
Calculator
When do I ovulate?
Cycle length = from the first day of your period to the day before the next one starts. If you don't know it, 28 is the average.
Your inputs stay with you: everything is calculated right in your browser — nothing is sent, nothing is stored.
Important: an ovulation calculator is not a birth-control method. Ovulation easily shifts by days, sperm survive for up to five days — the “infertile” days a calendar calculation promises often aren't. If you want reliable contraception, talk to your gynecologist.
The method
How does the calculation work?
The calculator uses the most important rule of thumb in cycle math: the luteal phase — the stretch from ovulation to the next period — is fairly fixed at around 14 days. From that follows ovulation ≈ cycle length minus 14. Not “always day 14”, as often assumed: on a 32-day cycle, ovulation lands around day 18.
Just as important is the honest framing: the result is an estimate based on averages. Stress, poor sleep, illness, travel or weight changes can shift ovulation by days — in either direction. If you want more precision, combine basal body temperature with cervical mucus observation, or use ovulation tests. And if you want to see how the phases of your cycle fit together, we walk through them step by step in the 4 cycle phases, explained simply.
“Six days of opportunity per cycle: five before ovulation, one after — the best ones come right before it.”
Biology
Understanding the fertile window
The egg itself is only viable for about 12 to 24 hours after ovulation. The reason the fertile window still spans roughly six days is sperm: they survive in the body for up to five days, waiting there for ovulation. That's why the calculator counts five days before ovulation through one day after.
The highest probability of pregnancy falls on the two days immediately before ovulation — not on ovulation day itself. And because ovulation moves around, the calculated window is a frame of reference for trying to conceive, not a guarantee — and certainly not a basis for contraception.
The difference
And what should I eat around ovulation?
The question other ovulation calculators don't ask. Yet your metabolism changes along with your hormones: around ovulation your body runs at full speed — antioxidants from berries, colorful vegetables and nuts, plus fiber to support clearing estrogen afterwards, are an especially good fit right now. In the luteal phase that follows, your metabolic rate rises by 100–300 kcal, and cravings are biochemistry, not weak willpower.
What that means for your plate: What to Eat During Ovulation covers this phase in depth, and our cycle phase calculator guide tells you which phase you're in on any given day. To understand all four phases, start with the cycle phases at a glance.
FAQ
Ovulation calculator FAQ
How accurate are ovulation calculators?
An ovulation calculator gives you an estimate based on averages — nothing more. The rule of thumb, ovulation ≈ cycle length minus 14, holds on average, but your actual ovulation can shift by several days: stress, poor sleep, illness, travel or weight changes all move it. If you want more precision, combine basal body temperature charting with cervical mucus observation, or use ovulation tests.
Can I use an ovulation calculator for birth control?
No — absolutely not. An ovulation calculator is not a method of contraception. Sperm survive for up to five days, ovulation easily shifts by days, and a single misjudged cycle is enough for an unintended pregnancy. If you want reliable birth control, talk to your gynecologist or OB-GYN about methods that actually work.
When do I ovulate?
As a rule of thumb: about 14 days before your next period starts. The luteal phase — the stretch from ovulation to the next period — is fairly fixed at around 14 days, so ovulation ≈ cycle length minus 14. Not "always day 14", as often assumed: on a 32-day cycle, ovulation lands around day 18. Enter your last period and cycle length above for a personalized estimate.
What exactly is the fertile window?
The days on which sex can lead to pregnancy: roughly five days before ovulation through one day after. The reason it's that long: sperm survive in the body for up to five days, while the egg itself is only viable for about 12 to 24 hours after ovulation. The highest probability falls on the two days immediately before ovulation.
What if my cycle is irregular?
Then the calculation is less reliable as a result — it assumes a reasonably constant cycle length. Use the average of your last three to six cycles as the input. If your cycle varies a lot, or is consistently shorter than 21 or longer than 35 days, that belongs in a gynecologist's office — especially if you're trying to conceive.
When should I take an ovulation test?
Ovulation tests measure luteinizing hormone (LH) in urine, which surges about 24 to 36 hours before ovulation. They're useful when you're trying to conceive and want to pin down the fertile window more precisely than any calculator can. Start testing a few days before your calculated ovulation date — the calculator above gives you that starting point.
We've been trying for a while — when should we see a doctor?
The standard guidance: if you're under 35 and haven't conceived after twelve months of regular, unprotected sex, get it checked out — from 35 on, after six months. And before that: if your period stays away, your cycles are very irregular, or you have known pre-existing conditions, see a gynecologist sooner rather than later.
Does what I eat affect my fertility?
Nutrition can support a healthy cycle — it's not a miracle cure. A balanced foundation with enough energy, iron, folate and omega-3 fatty acids is the baseline; severe calorie deficits, on the other hand, can suppress ovulation. For what to actually put on your plate during the ovulation phase, read our article on what to eat during ovulation.
Never do the math again: the app knows your cycle — and your plate
Bauchgefühl tracks your cycle and suggests recipes that match your phase. Start with the 2-minute quiz.
Get your recipe plan – freeOr keep reading in our guide to the 4 phases of your menstrual cycle.
Disclaimer: This page is for general information only and does not replace medical advice. The calculator provides an estimate based on averages and is explicitly not a method of contraception. If you're struggling to conceive, please see your gynecologist.