Conception Calculator
Predict your most fertile days and best conception window from your cycle.
About Conception Calculator
Ovulation typically occurs around cycle length minus luteal phase days after the start of a period. Because sperm can survive up to 5 days in the reproductive tract, the fertile window extends from 5 days before ovulation through ovulation itself. The two days with the highest conception probability are the day before ovulation and ovulation day itself.
Formulas
Ovulation timing
Ovulation day = cycle length − luteal phase length
Fertile window
Fertile window = ovulation day − 5 through ovulation day
Worked example
With the default values below, Conception Calculator returns Estimated ovulation of July 15, 2025. Change any input above and every figure here is recalculated in your browser.
Inputs used
- First day of last period
- 2025-07-01
- Average cycle length (days)accepts 21–45
- 28
- Luteal phase length (days)accepts 10–16
- 14
Results
- Estimated ovulation
- July 15, 2025
- Fertile window opens
- July 10, 2025
- Fertile window closes
- July 15, 2025
- Peak fertility day 1
- July 14, 2025
- Peak fertility day 2
- July 15, 2025
- Expected next period
- July 29, 2025
- Ovulation on cycle day
- 14
Step by step
- 1Cycle length= 28 days
- 2Luteal phase= 14 days
- 3Ovulation day= Day 14 of cycle (July 15, 2025)
- 4Fertile window= July 10, 2025 – July 15, 2025
- 5Peak fertility= July 14, 2025 & July 15, 2025
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- This calculator is an estimate based on average cycle patterns. It is not a contraceptive method and does not account for individual cycle variations. Consult a healthcare provider for fertility guidance.
- This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
This calculator is an estimate based on average cycle patterns. It is not a contraceptive method and does not account for individual cycle variations. Consult a healthcare provider for fertility guidance.
Frequently Asked Questions
What is the luteal phase?
The luteal phase is the second half of the menstrual cycle, from ovulation until the next period begins. It is driven by progesterone from the corpus luteum and typically lasts 12–16 days. Unlike the follicular phase, its length is relatively stable from cycle to cycle for most people.
How can I tell when I am actually ovulating?
The most reliable home methods are ovulation predictor kits (OPKs), which detect the LH surge 24–48 hours before ovulation, and basal body temperature (BBT) charting. BBT rises slightly (0.2–0.5 °C) after ovulation and stays elevated through the luteal phase.
Does this apply if my cycles are irregular?
Calendar-based calculations assume a consistent cycle length, so they are less accurate for irregular cycles. Track several cycles to find your average, or use OPKs to detect ovulation directly.