Predict your ovulation date and fertile window from your last period and average cycle length with daily conception probability
| Cycle day | Date | Fertility | Conception chance | Notes |
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Founder & CEO, Toolraxy
Faiq Ur Rahman is a web designer, digital product developer, and founder of Toolraxy, a growing platform of web-based calculators and utility tools. He specializes in building structured, user-friendly tools focused on health, finance, productivity, and everyday problem-solving.
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Ovulation doesn’t land on the same day for everyone. A 28-day cycle with a 14-day luteal phase puts it on day 14. A 32-day cycle pushes it to day 18. A 25-day cycle pulls it to day 11. That’s why counting “day 14” from your period works for some women and misses entirely for others.
The luteal phase is the fixed part. Once ovulation happens, the corpus luteum runs its course in roughly 12 to 14 days, then your period starts. So the calculation flips: subtract the luteal phase from your cycle length, and you get the cycle day ovulation lands on. A 30-day cycle with a 14-day luteal phase puts ovulation on day 16. Change the luteal phase to 12 days and it moves to day 18.
This calculator handles that math and produces a dated fertile window from five days before ovulation through one day after. That’s the biologically relevant stretch, since sperm survive up to five days and the egg lives only 12 to 24 hours.
Enter the first day of your last period in the date field.
Set your average cycle length. The default is 28 days. The range is 21 to 35.
Adjust the luteal phase if yours differs from the standard 14 days. Options run 12 to 16.
Read the hero panel. Your estimated ovulation date sits at the top in large type.
Check the four tiles for ovulation date, fertile window range, peak fertility days, and next period.
Scan the day-by-day fertile window grid to see each day’s conception probability.
Follow the cycle progress bar to see where you sit today.
Use the table below the grid for a fuller breakdown with cycle day numbers and notes.
Formula: Ovulation day = cycle length − luteal phase
That’s the core calculation. It works because the luteal phase is the more stable part of the cycle. Once ovulation happens, the time from ovulation to the next period stays within a narrow range for most women, typically 12 to 14 days. The follicular phase, which runs from the first day of your period to ovulation, is the variable stretch.
Once ovulation day is known:
Formula: Ovulation date = LMP + (ovulation day − 1)
The fertile window opens 5 days before ovulation and closes 1 day after. So the window spans 7 days total. The peak fertility days are the day before ovulation and ovulation day itself, each carrying roughly a 33% conception chance per act of intercourse in a healthy couple. Earlier days in the window carry progressively lower probabilities, dropping to about 5% at 5 days before ovulation.
Cycle length is clamped between 21 and 35 days. Luteal phase options run 12 to 16 days. The tool defaults to 28 days and 14 days, which matches the textbook cycle. Next period is calculated as LMP plus cycle length.
Suppose your last period started on March 5, 2026, your average cycle runs 32 days, and your luteal phase sits at 14 days.
LMP: March 5, 2026
Cycle length: 32 days
Luteal phase: 14 days
Ovulation day = 32 − 14 = day 18
Ovulation date = March 5 + 17 days = March 22, 2026
Fertile window: March 17 to March 23
Peak fertility: March 21 to March 22
Next period: April 6, 2026
Conception chance on ovulation day: about 33%
The calculator would show March 22, 2026 in the hero panel, “Mar 17 – Mar 23” in the fertile window tile, and “Mar 21 – Mar 22” as peak fertility. The cycle progress bar would place March 5 at the start and April 6 at the end, with ovulation sitting at roughly 53% of the way through.
If your cycle runs 30 days instead, the ovulation date shifts two days earlier to March 20. A 35-day cycle moves it to March 25. Small changes in cycle length can shift the fertile window by several days, which is why measuring matters more than guessing.
Ovulation in a 28-day cycle typically happens on day 14, assuming a 14-day luteal phase. The range across women runs from about day 12 to day 16, since luteal phase length varies between 12 and 16 days.
Ovulation date = LMP + (cycle length − luteal phase). For a 30-day cycle with a 14-day luteal phase, ovulation lands on day 16, which is 15 days after the LMP.
The fertile window is about six to seven days. It opens five days before ovulation and closes one day after. Sperm survive up to five days, and the egg lives 12 to 24 hours.
Five days before ovulation, about 5%. Four days before, about 10%. Three days before, about 15%. Two days before, about 25%. The day before and ovulation day each carry about 33%. One day after drops to about 10%.
The luteal phase is the stretch from ovulation to the first day of your next period. It runs about 12 to 14 days for most women and is more consistent than the follicular phase.
An OPK detects the LH surge before ovulation, a BBT shift confirms it after, and a mid-luteal progesterone blood test is the most direct confirmation. Combining two or three methods gives a clearer picture.
Not in the usual sense. A single ovulation per cycle is standard. Two eggs released within 24 hours is the pathway for fraternal twins. A second, separate ovulation later in the cycle is extremely rare.
No. Ovulation can shift by several days between cycles due to stress, illness, travel, or sleep disruption. Tracking across several cycles gives a better sense of your typical range.
The most common reasons are a cycle that ran shorter or longer than average this month, a luteal phase that isn’t actually 14 days, or an ovulatory disorder like PCOS. Cross-check with an OPK or BBT chart.
The egg survives only 12 to 24 hours after release. Conception after ovulation is possible but limited to that short window. Intercourse more than a day after ovulation is unlikely to lead to pregnancy.
Calendar predictions are less reliable for irregular cycles. An OPK or BBT chart will be more useful. Talk to a provider if cycles routinely vary by more than 8 days or if irregularity is new.
No. Calendar-based methods have a high failure rate because ovulation timing varies. Use a dedicated contraception method and talk to your provider about what fits your situation.
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