Pregnancy calculator

Ovulation & Fertile Window Calculator

Five methods to find your most fertile days. Start with what you know — your last period, a BBT chart, a positive OPK, an irregular-cycle history, or a Clomid date — and we'll give you the rest.

Last reviewed 18 May 2026

Method

Typical: 28. Use yours if you know it.

Ovulation day

Thursday, 4 June 2026

Most fertile single day

Fertile window

30 May – 5 Jun

6 days · sperm + egg viability

Next period (if no conception)

Thursday, 18 June 2026

Cycle 28 days

Your fertile days

Sat

30

May

Sun

31

May

Mon

1

Jun

Tue

2

Jun

Peak

Wed

3

Jun

Peak

Thu

4

Jun

Ovulation

Fri

5

Jun

FertilePeak fertilityOvulation day

Calendar-based ovulation estimates are guides, not guarantees. Real ovulation varies with stress, illness, sleep, and many other factors. For higher accuracy combine methods (BBT + OPK + cervical mucus). Read our medical disclaimer.

How to use this calculator

  1. Choose the method that matches what you can measure today.
  2. Enter your dates and numbers. Results update live.
  3. Combine methods when you can — OPK plus BBT plus mucus together is more reliable than any single signal.

Background: the science of ovulation

The two phases of a menstrual cycle

Every cycle has two phases. The follicular phase runs from the first day of your period until ovulation — this is where cycle-length variation lives. The luteal phase runs from ovulation until your next period and is remarkably stable at 12–16 days regardless of total cycle length. That stability is why the most reliable calendar-based ovulation rule works backwards from cycle length: ovulation = LMP + (cycle_length − 14).

The hormonal cascade

  • FSH (follicle-stimulating hormone) rises in the early follicular phase, recruiting a cohort of follicles.
  • Estrogen rises as the dominant follicle matures. This drives the cervical-mucus changes you can observe.
  • LH (luteinizing hormone) surges sharply when estrogen peaks. This is what OPKs detect. Ovulation follows 12–36 hours later.
  • Progesterone rises after ovulation from the corpus luteum, causing the BBT shift and maintaining the uterine lining until either pregnancy implants or the cycle ends.

Why each method captures something different

  • Calendar (LMP): easy, no equipment, but assumes regular cycles.
  • BBT: retrospective — confirms ovulation HAS happened, not predicts it. Useful for learning your own cycle length.
  • Cervical mucus: peri-ovulatory — tracks estrogen rise. Subjective but free.
  • OPK: predicts — catches the LH surge 12–36 hours before ovulation. Best single tool for timed intercourse.
  • Clomid: deterministic timing — the medication itself dictates when ovulation occurs.

How to interpret your results

The fertile window

The fertile window is the 6-day span ending one day after ovulation. It accounts for sperm survival (up to 5 days) and egg viability (12–24 hours post-ovulation). The two days before ovulation are the highest-probability days for conception because viable sperm are already in place when the egg releases.

Confidence by method

  • OPK + BBT confirmation: very high confidence in timing.
  • BBT alone (charted ≥3 cycles): high confidence in YOUR pattern, but retrospective.
  • Calendar (regular cycles): ±2–3 days typically.
  • Calendar (irregular cycles): wide window — pair with OPKs.
  • Single positive OPK with no BBT data: good predictive value (12–36 h window).

Best try-to-conceive practice

Have intercourse every 1–2 days during the fertile window. This maximises the chance of viable sperm being present when the egg releases. Daily intercourse is slightly better than every-other-day; longer abstinence does NOT improve fertility and may modestly worsen sperm quality.

Limitations — what this calculator does NOT do

  • It does not diagnose fertility problems. If you've been trying for 12+ months (or 6+ if you're 35+), see your provider.
  • It does not work as contraception. Calendar and even fertility-awareness methods together are less reliable than hormonal or barrier contraception.
  • It does not predict cycles in PCOS, perimenopause, post-pill amenorrhea, breastfeeding cycles, or other anovulatory states.
  • It does not replace medical evaluation if you have a suspected luteal-phase defect (short luteal phase <10 days), recurrent miscarriage, or other ovulation concerns.

Sources

  • Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy, and sex of the baby. New England Journal of Medicine. 1995;333(23):1517-1521.
  • Stanford JB, White GL, Hatasaka H. Timing intercourse to achieve pregnancy: current evidence. Obstetrics & Gynecology. 2002;100(6):1333-1341.
  • American College of Obstetricians and Gynecologists / American Society for Reproductive Medicine. Optimizing natural fertility: a committee opinion. Fertility and Sterility. 2017.
  • Practice Committee of the American Society for Reproductive Medicine. Use of clomiphene citrate in infertile women: a committee opinion. Fertility and Sterility. 2013;100(2):341-348.
  • Ecochard R, et al. Self-identification of the clinical fertile window and the ovulation period. Fertility and Sterility. 2015.
  • World Health Organization. Family planning: a global handbook for providers — chapter on fertility-awareness-based methods.

Our editorial process is described in our methodology. This calculator is not a substitute for medical advice — read our medical disclaimer.

Frequently asked questions

Which method is most accurate?
Multiple methods used together are the most reliable. OPK (LH testing) catches the LH surge that precedes ovulation by 12–36 hours. BBT confirms ovulation has happened (the rise occurs after, not before). Cervical mucus changes track estrogen levels around ovulation. Calendar methods are easiest but least precise. Pair OPK with BBT for confidence in both predicting and confirming.
How does the standard (LMP) method work?
It uses the luteal-phase rule: the luteal phase (ovulation to next period) is fairly stable at ~14 days, so ovulation = LMP + (cycle_length − 14). For a 28-day cycle that's day 14; for a 32-day cycle that's day 18. We then build the 6-day fertile window backwards from there.
Why is the fertile window 6 days?
Sperm can survive up to 5 days in the reproductive tract under favorable cervical mucus. The egg is viable only 12–24 hours after ovulation. The overlap is roughly 5 days before ovulation + the day of + ~1 day after = 6 days. The two days immediately before ovulation are statistically the highest-probability days for conception because viable sperm are already in place when the egg releases.
What does a BBT shift actually mean?
After ovulation, progesterone produced by the corpus luteum raises basal body temperature by about 0.3°F (0.17°C). The shift is small but reliable when measured under consistent conditions — first thing on waking, before getting out of bed, same time each day. A sustained rise of ≥3 days is convincing evidence that ovulation has happened. By convention we date ovulation to one day BEFORE the first high reading.
What does cervical mucus tell me?
Cervical mucus mirrors estrogen levels. Early cycle: dry or scanty. Mid-cycle (as estrogen rises): sticky → creamy → watery → egg-white (clear, stretchy, slippery). Egg-white mucus is the most fertile texture — it lubricates and supports sperm survival. After ovulation, mucus thickens or dries up again.
How do I interpret an OPK (LH test)?
A positive OPK shows the test line as dark or darker than the control line (line tests) or shows a smiley face / 'peak' (digital). It detects the LH surge that triggers ovulation 12–36 hours later. The FIRST positive matters most — subsequent positives just mean LH is still elevated. Target intercourse on the day of the first positive and the next 1–2 days.
I have irregular cycles — what should I do?
Use the Irregular Cycles tab and enter your shortest and longest cycle lengths over the last 6–12 months. The Calendar (rhythm) method then gives a wider fertile window where ovulation could fall. Pair this with daily OPK testing from the earliest possible ovulation date; OPKs detect the actual LH surge wherever it lands in that window.
What's special about Clomid timing?
Clomid (clomiphene citrate) extends and intensifies the natural LH surge. Ovulation typically occurs 5–10 days after the LAST pill of the cycle. Start OPK testing on day 4 after the last pill so you catch the surge wherever it falls. About 75–80% of people taking Clomid will ovulate in a given cycle; not all responders conceive. Your reproductive endocrinologist may also use follicle-tracking ultrasound around cycle days 12–14.
What is luteal-phase length and why does it matter?
The luteal phase is the time from ovulation to your next period. A typical luteal phase is 12–16 days. Shorter than 10 days (luteal-phase defect) may make it harder for an embryo to implant. This is one reason charting BBT for a few cycles is useful — once you see your ovulation day, you can count to next-period to learn your luteal length.
Can I get pregnant outside the fertile window?
Pregnancy is rare outside the fertile window because sperm and egg have to meet. But ovulation timing varies — even in regular cycles it can shift earlier or later by a few days due to stress, illness, or sleep. If you're avoiding pregnancy, calendar methods alone are not reliable contraception.

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