About

Conception is only biologically possible during a brief window each cycle, dictated by the lifespan of sperm (3-5 days) and the ovum (12-24 hours). This tool cuts through the noise of general tracking to answer a single question: "What is the probability of conception today?"

Using a weighted probability model based on clinical studies of sperm viability, the tool categorizes days into a traffic light system: Green (Peak Fertility), Yellow (Viable), and Red (Low Probability). This focus allows for targeted timing without the need for complex basal body temperature charts or ambiguous cervical mucus interpretation.

Formulas

The calculator estimates the Ovulation Day O using the standard luteal phase subtraction method, where LMP is the start of the cycle and C is the cycle length:

O = LMP + (C − 14)

Daily Probability P is then assigned based on the offset d from ovulation (Date − O):

P(d) =

{

33% if d = 031% if d = -127% if d = -2<1% otherwise

Reference Data

Day Relative to Ovulation	Conception Probability	Status	Biological Context
Ovulation − 5	10%	Yellow	Sperm survival limit
Ovulation − 4	14%	Yellow	Viable window opens
Ovulation − 3	16%	Yellow	Estrogen rising
Ovulation − 2	27%	Green	Peak mucus quality
Ovulation − 1	31%	Green	Optimal timing
Ovulation Day	33%	Green	Egg release
Ovulation + 1	< 1%	Red	Window closing
Luteal Phase	0%	Red	Progesterone dominance

Frequently Asked Questions

Sperm require time to undergo "capacitation" within the reproductive tract before they can fertilize an egg. Intercourse 1-2 days prior ensures a ready supply of capacitated sperm waiting in the fallopian tubes the moment the egg is released.

It is extremely unlikely. Once the egg dies (12-24 hours after release), conception is impossible until the next cycle. Sperm surviving longer than 5 days is also biologically rare. The "Red" days represent this physiological impossibility.

Yes. You must input your specific average cycle length. The tool dynamically adjusts the ovulation date (typically Cycle Length minus 14) and shifts the probability window accordingly.

If your cycle varies by more than 2-3 days, prediction accuracy drops. In such cases, widen the "Green" window by 2 days on either side or consider using the "Irregular Period Calculator" specifically designed for variable cycles.