Conception Calculator
Find the days you are most likely to conceive from your last period and cycle length — plus when to test, when your next period is due, and the due date if you are pregnant.
Count from the first day of full bleeding to the day before your next period starts. If the length moves around, average three or four months rather than using the most recent one.
Ovulation is estimated as your cycle length minus fourteen days, so every date here moves with the cycle length you enter. Treat them as the middle of a range, not as fixed appointments.
| Period Start | Conception Window | Due Date |
|---|
Each row assumes your cycle keeps the same length. The further down the table you read, the more a few days of natural variation compound — the first row or two are the ones worth planning around.
The six days that matter
A cycle is roughly a month long, but the part of it in which pregnancy is possible is not. An egg survives about twelve to twenty-four hours after it is released. Sperm last far longer — commonly three days, sometimes five, occasionally longer still in favourable conditions. Put those two facts together and the fertile stretch is about six days wide, and it sits almost entirely before ovulation rather than after it.
That shape surprises people. The instinct is to aim for the day of ovulation itself, but by then the window is nearly closed. Aiming a few days earlier means sperm are already waiting when the egg arrives, which is why the two days immediately before ovulation carry the highest chance of any single day in the cycle. The calculator marks this as the best stretch for intercourse, opening five days ahead of the estimated release.
How the dates are worked out
Everything on this page follows from one assumption: the second half of the cycle is more stable than the first.
After ovulation, the corpus luteum sustains the uterine lining for a fairly fixed period — around fourteen days — before hormone levels fall and a period begins. The first half, from bleeding to ovulation, is the part that stretches and shrinks. So instead of counting forward to a fixed day 14, the calculator counts backwards from the next expected period.
first day of last period + (cycle length − 14)
Fertile window
ovulation − 5 days → ovulation + 2 days
Most probable conception days
ovulation − 2 days → ovulation + 3 days
Due date if pregnant
first day of last period + 280 days + (cycle length − 28)
On a 28-day cycle those sums collapse to the familiar ones: ovulation on day 14, a due date 40 weeks out. On any other cycle length they do not, which is the whole reason the cycle-length box is there.
Reading each date the calculator gives you
Ovulation window is a five-day band around the estimated release day, not a promise. It is drawn wide deliberately, because the underlying estimate carries a few days of uncertainty on either side even for someone with very regular cycles.
If you already know the conception date rather than the cycle, our pregnancy conception calculator works from that instead, and the pregnancy weight gain calculator tracks the range recommended for your starting BMI.
Best days for intercourse is the practical answer to "when should we try". It runs from five days before ovulation to two days after, which covers sperm survival on one side and the egg's short life on the other.
Most probable conception days is a narrower band centred on release — the stretch in which fertilisation is most likely to actually occur, as opposed to the stretch in which intercourse is worth having.
Pregnancy test is set at roughly nine days after ovulation, which is early. A negative on that date means very little; a positive means a lot. Next period is simply one cycle length on from the date you entered, and due date if pregnant is the 280-day count adjusted for your cycle.
Why cycle length changes everything
The single most consequential number on the form is the cycle length, and it is the one most often left at the default.
Consider the same starting date with three different cycles. At 26 days, ovulation lands on day 12. At 28, on day 14. At 35, on day 21 — more than a week later than the shortest case, from the same first day of bleeding. Someone with a 35-day cycle following generic day-14 advice would be aiming a full week early and could easily miss the window entirely, month after month, while doing everything the internet told them to.
Measure your cycle from the first day of full bleeding to the day before the next one starts. If it moves around, average three or four months rather than using the most recent one, and treat the output as a range.
Signs worth tracking alongside the calendar
Calendar prediction is a hypothesis. These three methods test it against what your body is actually doing, and each has a different strength.
Cervical mucus is the only one that warns you in advance and costs nothing. In the days approaching ovulation it becomes clear, slippery and stretchy, closer to raw egg white than to the tacky or dry texture of the rest of the cycle. That change is oestrogen-driven and is a live signal that the window is opening.
Ovulation predictor kits detect the surge in luteinising hormone that triggers release, usually giving twenty-four to thirty-six hours of notice. They are the most precise advance warning available at home, though they can mislead in polycystic ovary syndrome, where the hormone can be elevated without ovulation following.
Basal body temperature works the other way round: it confirms after the fact. Resting temperature rises by around a third to half a degree Celsius once progesterone takes over, and stays raised until the next period. It cannot tell you to try today, but a few months of charts will tell you which day of your cycle you actually ovulate on, which then makes every future prediction sharper.
What the odds actually look like
Even with the timing right, conception is a per-cycle probability rather than a certainty. For a couple in their twenties or early thirties with no fertility problems, a well-timed cycle carries roughly a one-in-four to one-in-three chance. That is high compared with a random month and low compared with most people's expectations.
Cumulatively it looks better. Around eight in ten couples conceive within a year of trying, and most of those within the first six months. Age is the strongest single modifier: the monthly chance declines gradually through the thirties and more steeply after forty, as both the number and quality of eggs fall.
The usual guidance is to seek advice after twelve months of trying under 35, and after six months at 35 or over — sooner if periods are absent, very irregular, or there is a known reason to expect difficulty.
What this calculator must not be used for
It is a tool for trying to conceive, not for avoiding it. Calendar-based methods are among the least reliable forms of contraception in typical use, and the reason is built into the arithmetic on this page: every date depends on ovulation happening when the average says it will. Illness, stress, travel, disrupted sleep and simple natural variation all move it. A window that looked safe can turn out not to have been.
It also cannot detect whether ovulation happened at all. Cycles without release occur even in people who bleed regularly, and nothing on a calendar will reveal one.
What this calculator does not cover
The estimate assumes regular, ovulatory cycles and a typical luteal phase. It does not adjust for polycystic ovary syndrome, thyroid disorders, endometriosis, breastfeeding, recent hormonal contraception or perimenopause, all of which change cycle behaviour in ways an average cannot capture. It knows nothing about sperm health, which is a factor in roughly a third to a half of cases where a couple struggles.
It also assumes a natural cycle. Pregnancies conceived through IVF are dated from the transfer and the embryo's age rather than from a period, so the due date here will not apply — the due date calculator handles that case, and the pregnancy calculator covers dating once pregnancy is confirmed. If you are already tracking a cycle in progress, the period calculator projects the bleeding dates rather than the fertile ones.
Frequently asked questions
When am I most likely to conceive?
In the five or six days that end on the day you ovulate. Sperm can survive several days in the reproductive tract, so intercourse before ovulation still leads to pregnancy, while the egg itself only lives about a day after release. That asymmetry is why the window shown here opens well before ovulation and closes soon after it, and why the two or three days immediately before ovulation carry the highest chance of all.
How does this calculator know when I ovulate?
It works backwards from your next period rather than forwards from your last one. The stretch between ovulation and the following period is fairly consistent at around fourteen days for most people, while the first half of the cycle varies. So the estimate is your cycle length minus fourteen, counted from the first day of your last period. That is an average, not a measurement, and a cycle that varies by a few days will move ovulation with it.
Is ovulation always on day 14?
No, and that is one of the most persistent myths about cycles. Day 14 is only correct for a 28-day cycle. On a 32-day cycle ovulation lands nearer day 18, and on a 24-day cycle nearer day 10. Studies tracking hormone levels have found that only a minority of cycles ovulate on day 14 exactly, and that the day varies between cycles in the same person. Treat any single predicted day as the middle of a range.
How accurate is a conception date worked out this way?
It is a reasonable starting point and no more. Calendar prediction assumes your cycle length is stable and that your luteal phase is exactly fourteen days, and neither is guaranteed. If you want something firmer, ovulation predictor kits detect the hormone surge a day or two before release, and basal body temperature confirms afterwards that ovulation happened. Used together with these dates, they narrow the estimate considerably.
Why does the calculator suggest a pregnancy test date?
Because testing too early is the most common reason for a false negative. Implantation usually happens six to ten days after fertilisation, and the hormone a home test detects only starts rising afterwards. The date suggested here is roughly nine days past ovulation, which is about the earliest a sensitive test has a chance. A negative before your period is due is worth repeating a few days later.
Can I get pregnant outside the fertile window?
It is unlikely but not impossible, because the window depends on ovulation actually happening when predicted. If you ovulate several days later than the calendar suggests, intercourse that looked safely outside the window was not. This is exactly why calendar methods are among the least reliable forms of contraception, and this tool should never be used to avoid pregnancy.
What if my cycles are irregular?
Enter your average length and treat the result as a broad range rather than a set of dates. If your cycles vary by more than a few days, the useful approach is to track several months and note the shortest and longest, which gives you a window instead of a point. Cycles that are consistently very short, very long, or wildly unpredictable are worth raising with a doctor, since they can point to an underlying cause.
How is the due date on this page worked out?
It counts 280 days from the first day of your last period, then adjusts for how far your cycle differs from 28 days. A 33-day cycle pushes the date five days later, because ovulation happened five days later. It is the same arithmetic a midwife uses at a first appointment, and it is normally revised after a dating scan, which measures the baby directly rather than inferring from the calendar.
This calculator gives a general estimate for people with regular menstrual cycles and is not medical advice. Ovulation can happen earlier or later than any calendar predicts, so these dates must never be relied on to avoid pregnancy. If you have been trying to conceive for twelve months, or six months from age 35, or if your periods are absent or very irregular, speak to a doctor. More about how we build and check these tools is on our About page, and our Privacy Policy explains what we do and do not collect.