How to use it· 4 steps
- Enter the first day of your last period — the day bleeding started, which counts as cycle day 1.
- Enter your usual cycle length, from the first day of one period to the first day of the next. If it varies, use the average of the last few.
- Leave the luteal phase at 14 days unless you have measured yours with ovulation tests or temperature charting.
- Read your fertile window, estimated ovulation day and next period, then check the calendar and the six-cycle forecast.
Step by step
How to use the ovulation calculator
- 1Enter the first day of your last period — the day bleeding started, which counts as cycle day 1.
- 2Enter your usual cycle length, from the first day of one period to the first day of the next. If it varies, use the average of the last few.
- 3Leave the luteal phase at 14 days unless you have measured yours with ovulation tests or temperature charting.
- 4Read your fertile window, estimated ovulation day and next period, then check the calendar and the six-cycle forecast.
How the estimate works
A menstrual cycle has two halves. The first, the follicular phase, runs from your period until ovulation, and it is the part that varies from person to person and from cycle to cycle. The second, the luteal phase, runs from ovulation to the next period and is far more stable — usually around 12 to 14 days, with 14 the textbook figure.
That stability is why calculators count backwards. This one takes the date your next period is due — the last period plus your cycle length — and places ovulation one luteal phase before it. For a 28-day cycle that is 14 days after your period started; for a 35-day cycle, 21 days.
A worked example: a period that started on 1 March 2026, a 28-day cycle and a 14-day luteal phase put the next period on 29 March and ovulation on 15 March, with the fertile window running from 10 to 15 March. You may read that ovulation happens 'on day 14'; counting the first day of bleeding as day 1, the same estimate is day 15. The difference is in the counting, not the biology. It is also the assumption pregnancy dating makes, so this calculator and the due date calculator agree with each other.
The fertile window, and why it is six days
An egg survives for roughly 12 to 24 hours after ovulation, while sperm can survive in the reproductive tract for up to five days. Together, that makes the six-day fertile window: the five days before ovulation, and the day itself.
The defining study is Wilcox, Weinberg and Baird's, published in the New England Journal of Medicine in 1995. It tracked ovulation with daily hormone measurements in 221 women trying to conceive: every pregnancy came from intercourse in the six days ending on ovulation day, and the chance of conception rose from about 10% five days before ovulation to about 33% on the day itself.
The practical reading is that the days just before ovulation are nearly as fertile as ovulation day, and waiting for signs that ovulation has already happened can mean missing most of the window. Couples trying to conceive are usually advised to have sex every one to two days through it, rather than aiming for a single day.
Why calendar estimates miss — and what to use instead
The calendar assumes this cycle will behave like your average one, and even regular cycles often do not. In a 2000 BMJ analysis of the same women, only about 30% had their fertile window entirely within days 10 to 17 — the days clinical guidelines then named — and on every day from day 6 to day 21 there was at least a 10% chance of being in the fertile window.
Irregular cycles make the estimate weaker still. If your cycle length varies by more than a few days, treat these dates as a rough guide. Cycles regularly longer than 35 days — outside the usual adult range of 21 to 35 — are worth mentioning to a doctor, because they often mean ovulation is irregular.
Better signals exist, and they measure this cycle instead of predicting it. Urine ovulation tests detect the surge of luteinising hormone (LH) that comes a day or so before ovulation. Basal body temperature rises by a few tenths of a degree after ovulation, confirming it in hindsight. Cervical mucus becomes clear, slippery and stretchy on the most fertile days. Used together, they show what is actually happening rather than what a calendar expects.
Not contraception
A calendar estimate is not a method of birth control. Fertility-awareness methods do exist, and some are effective when taught properly and followed precisely — but they rely on daily tracking of temperature and mucus, abstinence or barrier methods across the whole fertile window, and safety margins on both sides of it. A date from a website is none of those things.
Ovulation can also come early, which matters at the start of a cycle. Because sperm can survive for five days, sex during or just after a period can lead to pregnancy in a short cycle: with a 21-day cycle, the fertile window can begin while the period is still going.
This tool gives estimates, not medical advice. If you are trying to conceive and it has not happened after 12 months of regular unprotected sex — or 6 months if you are 35 or older — that is the point at which fertility specialists recommend an assessment.
Answers
Frequently asked questions
When am I most fertile?
In the six days ending on ovulation day, especially the two or three days just before it. In a regular 28-day cycle that is usually around days 10 to 15, counting the first day of your period as day 1 — but ovulation tests are the way to find it in a particular cycle.
Can I get pregnant just after my period?
Yes, particularly with a short cycle. Sperm can survive for up to five days, so in a 21- to 24-day cycle the fertile window can start during or straight after your period.
How accurate is an ovulation calculator?
It is an estimate built on averages. Ovulation shifts from cycle to cycle even in regular cycles: one study found only about 30% of women had their whole fertile window within days 10 to 17. LH tests and temperature charting measure the cycle you are actually in.
What is the luteal phase, and should I change it?
It is the time from ovulation to your next period, usually 12–14 days. Leave it at 14 unless you have measured yours — for example, by counting the days from a positive LH test to the start of your next period.
Can I use this to avoid pregnancy?
No. Calendar estimates are not contraception. Ovulation can come earlier or later than predicted, and a calendar alone gives no reliable safe days.
Is my cycle information stored?
No. Everything is worked out in your browser, and your dates are never uploaded, saved or shared.