Conception Calculator
Predict your most fertile days and best conception window from your cycle.
Inputs
The luteal phase is typically 12–16 days. Ovulation = cycle length − luteal phase.
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
Cycle length
= 28 days
Luteal phase
= 14 days
Ovulation day
Cycle length − luteal phase = 28 − 14
= Day 14 of cycle (July 15, 2025)
Fertile window
Day 9 to Day 14
= July 10, 2025 – July 15, 2025
Peak fertility
= July 14, 2025 & July 15, 2025
How it works
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
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.