Ovulation Calculator
Last updated: June 2026 · Free · No sign-up required
Enter values above and click Calculate to see your result instantly.
Quick reference
Ovulation day by cycle length:
| Cycle length | Approx. ovulation day |
|---|---|
| 26 days | Day 12 |
| 28 days | Day 14 |
| 30 days | Day 16 |
| 32 days | Day 18 |
monitor_weight A starting point, not a diagnosis
This calculator gives you the same number a clinician would compute by hand from your inputs. It is informational, never a substitute for a doctor. If a result surprises you, write it down and discuss it at your next appointment — that conversation is the goal, not a verdict from a webpage.
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Every health calculator on CalcaTools uses an equation from the WHO, NIH, ACOG, CDC, or a published peer-reviewed paper. The exact citation is in the "Formula & methodology" section. We re-check sources quarterly and update the page when guidance changes — the "Last reviewed" date is at the top.
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Health calculations only help if you can actually use them. We use plain-English labels (no jargon), accept both metric and US units, and show what a "normal" range looks like alongside your result — so you understand what the number means, not just what it is.
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Your weight, height, age, pregnancy details, and any other health input never leave your browser. Calculations run client-side; nothing is logged or stored on our side. We don't want your medical data — we just want the formula to work for you.
Interpretation guide
Fertility window:
| Phase | Detail |
|---|---|
| Fertile window | ~5 days before + ovulation day |
| Most fertile | 1–2 days before ovulation |
| Egg lifespan | About 12–24 hours |
| Sperm lifespan | Up to ~5 days |
Formula & methodology
Formula: Ovulation day = cycle length - 14; fertile window = ovulation day - 5 through ovulation day
Ovulation typically occurs about 14 days before the next period, so the calculator subtracts 14 from your cycle length and counts from your last period's first day. The fertile window spans roughly the five days before ovulation plus the day itself, because sperm can survive up to five days while the egg lasts about a day. This estimate suits regular cycles; ovulation tests or charting give more precision for irregular ones.