Ovulation Calculator
Instant results as you type.
days
days
Most fertile days
2026-08-10 to 2026-08-16
Ovulation likely on2026-08-15
Which day of your cycle that is14
Next period expected2026-08-29
Due date if you conceive this cycle2027-05-08
Please readThese dates are an estimate from your cycle length, not an observation — cycles vary month to month, and stress, illness and travel move ovulation. Do not use this as contraception: the window shown is when conception is most likely, not the only time it is possible.
For education only — this is not medical advice. The figures are population averages: real energy use, appetite and rate of loss vary from person to person, and weight loss slows as you get lighter. If you are pregnant, managing diabetes, heart or joint conditions, or taking medication, talk to a doctor before changing your diet or activity.
This estimates when you are most likely to ovulate, and the handful of days around it when conception is most likely. It counts backwards from your next expected period rather than forwards from your last one, which matters: the stretch between ovulation and the next period stays roughly fixed at about fourteen days whatever your cycle length, while the part before ovulation is what stretches and shrinks. A calculator that simply halves the cycle points at the wrong week for anyone whose cycle is not 28 days. These are estimates from a calendar, not observations of your body, and they are not a method of contraception.
How to use it
- Enter the first day of your last period — the first day of bleeding, not the last.
- Enter your cycle length: first day of one period to first day of the next. If it varies, use your average and read the result as a range rather than a date.
- Leave the luteal phase at 14 unless a clinician has told you otherwise. It is the part of the cycle that stays put.
- The fertile window covers the five days before ovulation and the day after, because sperm survive several days and the egg does not.
Examples
- A 28-day cycle starting 1 August: fertile from 10 to 16 August, with ovulation around the 15th
- A 35-day cycle from the same date: ovulation near day 21, not day 17 as halving the cycle would suggest
Frequently asked questions
- How is the ovulation date worked out?
- By counting back from your next expected period, not forward from your last one. Your next period is the last period plus your cycle length; ovulation is roughly fourteen days before that. This ordering matters because the luteal phase — ovulation to period — is fairly consistent between people and between cycles, while the follicular phase before ovulation is the variable part. Counting forwards from the last period, or simply halving the cycle, gets the right answer only for a 28-day cycle and drifts further wrong the further from 28 you are.
- How long is the fertile window?
- About six days: the five days before ovulation, plus the day of ovulation itself. The asymmetry is biological rather than arbitrary. Sperm can survive in the reproductive tract for up to five days waiting for an egg, so intercourse several days before ovulation can still result in conception. The egg itself survives only around twelve to twenty-four hours after release, which is why the window closes almost immediately afterwards. In practice the two or three days immediately before ovulation are the most likely of the six.
- Can I use this as contraception?
- No. Please do not. This is a calendar estimate built from an average, and it cannot know whether you ovulated, whether you ovulated early, or whether this cycle is like your last one. Illness, stress, travel and sleep disruption all move ovulation, sometimes by several days, and a window that is wrong by three days is worse than no window at all if you were relying on it. Fertility-awareness methods that do work use daily basal temperature, cervical mucus observation and often urine tests together, with training — not a date arithmetic page. If you are avoiding pregnancy, talk to a clinician about a method designed for that.
- What if my cycle is irregular?
- Then treat the output as a rough centre rather than a date. Enter your average cycle length and note that your real ovulation could fall several days either side; if your cycles range from 26 to 34 days, run it at both and take the whole span as the window to watch. Irregular cycles are common and usually unremarkable, but a cycle consistently shorter than 21 days or longer than 35, or one that changes suddenly, is worth mentioning to a doctor — not because this page can tell you anything, but because the underlying cause is worth knowing.
- Why does a longer cycle not just mean ovulating later in the middle?
- Because the two halves of the cycle do not stretch equally. The luteal phase after ovulation is governed by the corpus luteum, which lives a fairly fixed number of days — near fourteen for most people — and then breaks down, which is what triggers the period. So a longer cycle is almost always a longer follicular phase before ovulation, not a longer second half. On a 35-day cycle, ovulation lands around day 21 rather than day 17 or 18. This is the single most common error in ovulation calculators, and it points people at the wrong week.