Ovulation Calculator
Enter the first day of your last period and your average cycle length. You get an estimated ovulation day, your six fertile days with their published conception odds, your next period date, and a due date estimate if this is the cycle — drawn on a calendar, with the next three cycles mapped too.
Your fertile days, day by day
Per-day figures are the published estimates of Wilcox, Weinberg & Baird (New England Journal of Medicine, 1995), from 221 women whose ovulation day was confirmed by hormone testing: roughly 10% five days before ovulation, 16% four days before, 14% three days before, 27% two days before, 31% one day before and 33% on ovulation day itself — and effectively 0% the day after. They are population averages from a modest sample, not a personal prediction.
This cycle on a calendar
Your next cycles at a glance
Each later cycle assumes the same average length. Real cycles drift by a few days, so treat dates further out as rougher estimates.
DueDateCalc works out your due date, week-by-week timeline and milestones.
How the estimate is worked out
A cycle has two halves that behave differently. The second half — from ovulation to the next period, called the luteal phase — is fairly steady at about 14 days for most people. The first half, the follicular phase, is the part that stretches and shrinks, which is why longer cycles ovulate later rather than having a longer wait after ovulation. So the calculator counts backwards from your expected next period: ovulation ≈ last period + (cycle length − luteal phase). On a 28-day cycle that lands on day 14; on a 32-day cycle it lands on day 18, not day 14 — a common place where the simple "add 14 days" rule goes wrong.
The fertile window is the six days ending on ovulation day. Sperm can survive in the reproductive tract for up to five days, while an egg survives only about 12–24 hours after release — so the window opens five days early and closes the day after ovulation. The per-day percentages shown above come from the most-cited study on this (Wilcox et al., NEJM 1995), in which every recorded pregnancy came from intercourse during those six days, and none from the day after ovulation.
What this calculator cannot do
It cannot confirm that you ovulated, or exactly when — it projects averages from past cycles, and real cycles vary by several days even in people who are usually regular. If timing matters, combine the estimate with ovulation predictor kits or basal body temperature tracking (both explained in how ovulation works), and speak to a clinician if you have been trying for 12 months without success (6 months if you are over 35), or sooner if your cycles are very irregular. These estimates must not be used to try to avoid pregnancy: calendar methods have a high typical-use failure rate precisely because ovulation day moves.
Fertile window calculator
Just the window: the six days, the odds on each, and why the window is shaped the way it is.
Period calculator
When is your next period due — and the three after that? Includes each cycle's fertile days.
Pregnancy test calculator
Testing too early is the top cause of false negatives. See the earliest sensible day to test, by DPO.
Conception calculator
Work backwards from a due date to the likely conception window, or forwards from conception to a due date.