Sperm wait; the egg does not
The tool assumes sperm can survive up to five days before ovulation and the egg about a day after. That is why intercourse two days before ovulation often beats ovulation day alone in the modeled odds.
Conception timing calendar
Conception calculator: fertile days from your cycle
Fertile window and ovulation estimate from last period, cycle length, and luteal phase.
By Jeff Beem
Updated
Calendar model, local only; not for avoiding pregnancy.
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Cycle
28 d
2128 typical45
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Period
Fertile
Peak
Ovulation
Cycle data stays in your browser.
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Timeline
Enter last period start.
July 2026
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12–24 hour rule
The egg is usually fertilizable for about half a day after ovulation. Sperm can last several days, so intercourse in the days before ovulation often lines up better with conception than relying only on the day after.
Signs of ovulation
Cervical mucus changes, LH kits, sustained BBT rise after ovulation, and one-sided cramping (mittelschmerz) can all add context beyond a calendar estimate.
Example: 28-day cycle, 14-day luteal
Enter the first day of your last period. With a 28-day average and 14-day luteal phase, ovulation lands around cycle day 14. The six-day fertile window is roughly days 9–15; peak modeled odds cluster on days 12–14 (about 27–33% per day in the chart).
Reading the calendar and chart
Peak vs full window
Peak fertile (two days before through ovulation day) shows the highest bars on the chart. The wider band adds days 9–11 and day 15 at lower percentages. Covering more than one day raises cumulative monthly odds.
Luteal phase slider
Default 14 days. If you tracked BBT for a few cycles and know you run 12 or 16, adjust advanced settings. Ovulation date shifts one day per luteal day you change.
Irregular cycle checkbox
Shortest and longest cycle widen the colored band when months vary. Calendar-only guesses get shakier; pair with LH tests or mucus if variation is more than a few days.
Conception calculator: fertile window and ovulation timing
28-day cycle, 14-day luteal: ovulation ~day 14, fertile window ~days 9–15, peak odds on days 12–14. Calendar, luteal slider, irregular-cycle band.
What this calculator does
Estimates ovulation day, a six-day fertile window (sperm survival −5 through egg viability +1), peak days, and a bar chart of relative conception odds by cycle day. Inputs: first day of last period, average cycle length, optional luteal phase (default 14), optional shortest/longest cycle for irregular mode. Data stays in the browser; not for contraception or medical diagnosis.
- Default math:28-day cycle, 14-day luteal → ovulation on cycle day 14; fertile window days 9–15; peak days 12–14.
- Chart (illustrative):D−5 ~10%, D−4 ~16%, D−3 ~14%, D−2 ~27%, D−1 ~31%, ovulation day ~33%, D+1 ~8%.
- Limits:Calendar-only; stress, illness, or hormonal conditions can shift ovulation. Add LH tests, mucus, or BBT for confirmation. Not a substitute for prenatal or fertility care.
How ovulation is estimated
Ovulation day (cycle day number) = cycle length − luteal phase length. Example: 28 − 14 = day 14. Fertile window starts five days before ovulation (sperm buffer) and ends one day after (late egg survival). Peak days are the two days before ovulation through ovulation day. Irregular mode uses shortest cycle for the early edge and longest for the late edge.
- Formula:Ovulation day = cycle length − luteal days
- Window:(Ovulation day − 5) through (ovulation day + 1)
- Late period:Usually late ovulation, not a longer luteal phase
Using the form
Pick the first day of your most recent period, leave cycle length at 28 unless you know your average, and open advanced only to change luteal length or widen the band for irregular cycles. The calendar colors period, fertile, peak, and ovulation days; the chart shows relative odds, not a personal medical probability.
Beyond the calendar
LH strips predict ovulation 24–36 hours ahead. Fertile cervical mucus (clear, stretchy) means the window is open now. BBT rises after ovulation and helps nail your luteal length for the next month. Mittelschmerz or libido shifts are hints, not enough alone.
Trying to conceive in practice
Intercourse every 1–2 days across the fertile window is enough for most couples. Start when mucus turns fertile or five days before expected ovulation if you are unsure. Prenatal folate (400–800 mcg) helps before pregnancy; both partners benefit from limiting smoking and heavy alcohol. Under-35 couples often wait 12 months before specialist referral; 35–40 often six months; over 40, talk earlier.
Already pregnant?
This page plans forward from your period. To estimate when conception likely happened from a due date or ultrasound, use the Pregnancy Conception Calculator.
FAQ
When am I most fertile during my cycle?
The three days ending on ovulation day are usually strongest. On a 28-day cycle with a 14-day luteal phase, that is often cycle days 12–14. Modeled odds run about 27–33% on those peak days versus 10–16% earlier in the window. Sperm need to be present before the egg releases, not only after.
How many days after my period can I conceive?
Earliest likely fertile day ≈ shortest recent cycle − luteal phase − 5. For 28 days and 14-day luteal, that is cycle day 9. A 24-day cycle can start as early as day 5 because sperm can wait several days for ovulation.
Can I get pregnant after ovulation?
The egg is usually viable about 12–24 hours after release. After that, conception waits until the next cycle. Intercourse in the days before ovulation lines up better than betting on the day after.
What is a luteal phase?
Time from ovulation to the next period, often 12–14 days and fairly stable for you (±1–2 days). Ovulation day ≈ cycle length − luteal length. Change the luteal slider here if BBT tracking gave you a personal number.
How accurate are calendar-based ovulation predictions?
With regular cycles (±2–3 days month to month), calendar math is often 80–85% accurate. Add LH test strips and accuracy climbs toward ~95%. Irregular cycles need mucus, tests, or specialist input, not calendar alone.
What is the most reliable way to confirm ovulation?
LH strips flag the surge 24–36 hours before ovulation (actionable now). BBT confirms ovulation after the fact and helps the next cycle. Stretchy egg-white mucus is a real-time clue that the window is open.
How often should we have intercourse when trying to conceive?
Every 1–2 days through the fertile window is enough for most couples; daily and every-other-day look similar in studies. Very long gaps (>5 days) or very high frequency (>3×/day) can hurt sperm quality.
When should I see a fertility specialist?
Under 35: after 12 months of timed trying. Ages 35–40: after 6 months. Over 40: talk to a clinician early. PCOS, endometriosis, very irregular cycles, or known partner factors warrant earlier visits.
Sources & citations
References used for the calculation method and definitions. Links open in a new tab when available.
[1]
ACOG Committee Opinion No. 700: Methods for Estimating the Due Date
ACOG guidance on ovulation timing, conception windows, and the relationship between LMP and gestational age.
[2]
Wilcox AJ, Dunson DB, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259-1262
Prospective BMJ study (2000) giving day-specific probabilities for the six fertile days; supports modeling conception likelihood relative to cycle day and ovulation.
Fitness Reference Note
Informational Use: These calculations (BMI, Calories, etc.) are based on standard statistical formulas and are intended for general reference and goal-setting purposes only.
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