Sperm survive up to five days
In fertile, slippery cervical mucus, sperm are protected and nourished for as long as five days. Intercourse well before release still counts, which is why the window opens early.
Free ovulation calculator
Find the six days that matter
Conception is only possible on about six days of a cycle. This calculator works backwards from your next period — the stable half of the cycle — so the dates hold up whether your cycle is 24 days or 38.
- Your ovulation day and six-day fertile window
- The days to start testing, so you don't miss the surge
- Implantation window, test dates and a projected due date
Enter your last period to begin
You will get your ovulation date, the six-day fertile window, the day to start testing, the implantation window and a projected due date — for the next three cycles.
- Why it asks for cycle length. Ovulation is not on day 14 unless your cycle is 28 days long.
- Why it asks for the luteal phase. It is the fixed half of the cycle, so it is the reliable thing to count back from.
- What it cannot do. Predict a cycle that varies by more than a few days. For those, track mucus and test.
The mechanism
Why the window is six days long and not one
The egg is the short-lived half of the equation. Sperm are the reason the window opens days earlier.
The egg lasts 12–24 hours
Once released, an egg is fertilisable for less than a day. This is the hard edge of the window — by the day after ovulation, the chance for that cycle is essentially gone.
The luteal phase is the fixed half
From ovulation to your next period is reliably 12 to 14 days. The first half is what varies. That is why counting backwards from the next period beats counting forwards from the last one.
LH surges 24–36 hours ahead
A positive ovulation test means release is likely within a day and a half. Enough warning to act on, not enough to plan a week around — which is why mucus matters too.
The library
Three hubs, seventeen guides
Everything on this site sits under one of three questions: when do I ovulate, what improves the odds, and what happens once I am pregnant.
Ovulation
Ovulation: How It Works and How to Track It
One egg, one release, roughly twelve to twenty-four hours of viability. Everything else about timing conception follows from knowing when that happens in your cycle.
- Signs of Ovulation: The Six Your Body Actually Gives You
- How Ovulation Tests Work — and How to Read a Confusing One
- Clearblue Ovulation Test Instructions, Explained Properly
- Basal Body Temperature Charting: Reading the Shift
Fertility
Fertility and Conception: What Moves the Odds
Timing sets the ceiling. Nutrition, weight, sleep, thyroid and iron decide how close you get to it — and which changes are worth your effort.
- Natural Fertility Boosters: What the Evidence Supports
- Vitamin D and Fertility: Sorting Signal From Hype
- Food to Avoid for Fertility — and What Is Overstated
- Anaemia and Infertility: The Link People Miss
Early Pregnancy
Early Pregnancy, Dating and Pregnancy Loss
From the first positive test to the end of the first trimester — how dating works, what the risk numbers really say, and what happens after a loss.
- How to Calculate Your Pregnancy Due Date
- Healthy Natural Pregnancy Tips That Are Worth the Effort
- Chances of Miscarriage at 6 Weeks — and Every Week After
- Miscarriage or Period? How to Tell the Difference
Common questions
About the calculator
How do I calculate my ovulation day?
Subtract your luteal phase length — usually 12 to 14 days — from the date your next period is due. That projects ovulation backwards from the stable half of the cycle instead of assuming day 14, which is what makes it work for a cycle of any length.
How accurate is an ovulation calculator?
It is as accurate as your cycle is regular. If your cycles land within two or three days of each other, the projection is usually within a day or two of the real event. If they swing by a week or more, treat the dates as a testing window rather than an answer, and confirm with cervical mucus or an LH test.
What if I do not know my luteal phase length?
Leave it at 14 — that is the population average and a reasonable default. If you want your own number, chart your waking temperature for one cycle and count from the first high reading to the day before your period.
Can I still conceive outside the fertile window?
Not from intercourse well outside it. What does happen is that ovulation itself arrives earlier or later than projected, which moves the window rather than widening it. That is the main reason a calendar estimate should be confirmed by a real signal.
Does this calculator store my data?
No. The dates are calculated on the fly and appear in the page address so you can bookmark or share them. Nothing is written to a database and there are no tracking cookies attached to your inputs.
Ovulation.guide provides general information, not medical advice. See our medical disclaimer.