Ovulation is the few hours in each cycle when an ovary releases an egg — and it is the only time in the month a pregnancy can begin. Because sperm survive far longer than the egg does, the days that matter for conception start well before ovulation and end almost as soon as it happens. This calculator estimates that window from your last period and your usual cycle length.
- What it estimates
- Ovulation day and the fertile window
- Method
- Calendar (cycle length minus luteal phase)
- Fertile window
- 5 days before ovulation to 1 day after
- Egg survival
- 12–24 hours after release
- Sperm survival
- Up to 5 days in fertile cervical mucus
Formula
Calendar method, anchored on the luteal phaseOvulation = LMP + (cycle length − luteal phase) Fertile window = ovulation − 5 days to ovulation + 1 day
- LMP
- — First day of the last menstrual period
- cycle length
- — Day 1 of one period to day 1 of the next
- luteal phase
- — Days from ovulation to the next period, usually 12–16
Anchoring on the luteal phase rather than halving the cycle is what makes this work for cycles that are not 28 days. The follicular phase — period to ovulation — is the half that stretches and shrinks; the luteal phase stays close to 14 days whatever the cycle length. A 35-day cycle therefore ovulates around day 22, not day 17.
Ovulation Calculator examples
| Input | Result | Category |
|---|---|---|
| LMP Jan 5, 2026 · 28-day cycle | Jan 19, 2026 | Cycle day 15 |
| LMP Jan 5, 2026 · 24-day cycle | Jan 15, 2026 | Cycle day 11 |
| LMP Jan 5, 2026 · 32-day cycle | Jan 23, 2026 | Cycle day 19 |
| LMP Feb 14, 2026 · 28-day cycle | Feb 28, 2026 | Cycle day 15 |
| LMP Mar 1, 2026 · 35-day cycle | Mar 22, 2026 | Cycle day 22 |
| LMP Apr 20, 2026 · 28-day cycle, 12-day luteal | May 6, 2026 | Cycle day 17 |
When do you ovulate?
In a textbook 28-day cycle, ovulation falls about 14 days after the first day of your period — and about 14 days before the next one starts. The second of those framings is the reliable one. If your cycles run 32 days, you ovulate around 18 days after your period rather than 16, because the extra days are added before ovulation, not after it.
| Cycle length | Days after your period starts | Fertile window |
|---|---|---|
| 21 days | Day 7 | Days 2–8 |
| 24 days | Day 10 | Days 5–11 |
| 28 days | Day 14 | Days 9–15 |
| 32 days | Day 18 | Days 13–19 |
| 35 days | Day 21 | Days 16–22 |
Counting the first day of bleeding as day 1, a 28-day cycle puts ovulation on cycle day 15. Many sources round this to "day 14", a one-day difference that comes purely from whether the first day of the cycle is counted as day 0 or day 1. It makes no practical difference to when you should be trying.
What is the fertile window?
The fertile window is the span of days on which intercourse can result in pregnancy. It is asymmetrical, and understanding why is the single most useful thing to know about conception timing.
- Sperm can survive up to five days inside the reproductive tract when fertile cervical mucus is present, so intercourse days before ovulation can still lead to pregnancy.
- The egg survives only 12 to 24 hours after release. Once that passes, the cycle is over for conception purposes.
- That asymmetry means the window opens five days before ovulation and closes about a day after it.
- The highest chance of conception comes from intercourse on the two days before ovulation and the day itself.
This is why waiting for a positive ovulation test before trying can be too late. A test detects the luteinising hormone surge roughly 24 to 36 hours before the egg is released, which catches the tail of the window rather than the peak of it.
How accurate is a calendar estimate of ovulation?
Moderately — it is a good starting point and a poor final answer. Research tracking hormone levels has found that even among women who describe their cycles as regular, the day of ovulation varies by several days from cycle to cycle. Only a minority ovulate on the day a calendar calculation predicts.
The practical consequence is not to abandon the calendar but to widen the window it suggests. Having sex every one to two days across the estimated fertile window covers the uncertainty without requiring precision timing, and works out better than trying to hit a single day.
What if your cycles are irregular?
A calendar method needs a predictable cycle length to work. If your cycles vary by more than about seven or eight days, or fall outside the 21-to-35-day range, this calculator will give a wide and not very useful window. Tracking cervical mucus or using ovulation predictor kits across several weeks is a better approach.
Persistently irregular cycles are worth investigating rather than working around. Polycystic ovary syndrome, thyroid disease, high prolactin, significant weight change, and heavy exercise all disrupt ovulation, and several of them are straightforward to treat once identified.
When should you see a doctor about fertility?
Standard guidance is to seek assessment after 12 months of regular unprotected sex without conception if you are under 35, and after 6 months if you are 35 or over. The shorter interval reflects the fact that both egg quantity and quality decline more steeply after the mid-thirties, so waiting costs more.
- Sooner than that if your periods are absent, very irregular, or unusually painful.
- Sooner if you have had pelvic surgery, pelvic inflammatory disease, or endometriosis.
- Sooner if you have had two or more miscarriages.
- Assessment covers both partners — male factors account for roughly a third to a half of cases.
Frequently asked questions
How many days after your period are you most fertile?
In a 28-day cycle, the most fertile days fall roughly 12 to 15 days after your period starts. Subtract 14 from your own cycle length to find your ovulation day, then count the five days before it: those six days plus ovulation day itself are your window.
Can you get pregnant right after your period?
Yes, if your cycles are short. With a 21-day cycle you ovulate around day 7, so the fertile window opens on day 2 — while you may still be bleeding. Sperm surviving five days makes late-period intercourse a genuine possibility for short cycles.
How long after ovulation can you get pregnant?
About 24 hours. The egg survives 12 to 24 hours after release and cannot be fertilised after that, so the window closes quickly once ovulation happens. Anything you do to improve your odds needs to happen in the days before ovulation, not after.
Does a longer cycle mean later ovulation?
Yes. The luteal phase — ovulation to the next period — stays near 14 days whatever your cycle length. Extra days are added to the first half, before ovulation. Someone with a 35-day cycle ovulates around day 21, a full week later than someone with a 28-day cycle.
Is the day of ovulation the same as the day of conception?
Effectively yes, but not the same as the day of intercourse. Fertilisation happens within 24 hours of the egg being released. Because sperm survive up to five days, the sex that caused a pregnancy may have happened several days before the conception date itself.
How often should you have sex to conceive?
Every one to two days through the fertile window gives the best odds without needing precise timing. Daily is fine and does not meaningfully reduce sperm quality; abstaining to "save up" does not help. Outside the window, timing has no effect on your chances.
Sources
- How to get pregnant — Mayo Clinic
- Menstruation and Menstrual Problems — Eunice Kennedy Shriver National Institute of Child Health and Human Development
- Fertility Awareness-Based Methods of Family Planning — American College of Obstetricians and Gynecologists
- Your menstrual cycle — Office on Women's Health, U.S. Department of Health and Human Services