Estimate who is more likely to be the father by comparing intercourse dates against conception timing.
Relative Timing Breakdown*
* These percentages compare each candidate’s share of the modeled timing support. They are not the probability that someone is the biological father. Pregnancy dating and ovulation can vary by several days. Only DNA testing can determine paternity.
How to use this calculator
Select your dating method
The accuracy of this tool depends on knowing when you ovulated. The most accurate method is an Ultrasound Date from the first trimester (measuring the Crown Rump Length). If you don't have that, use your Last Period (LMP), but ensure your "Average Cycle Length" is accurate.
Enter dates for both partners
Input all intercourse dates that occurred roughly 2 weeks before and 1 week after your estimated conception timing. You can add as many dates as needed for "Partner A" and "Partner B."
Interpret the Probability Score
The calculator compares the dates against your fertile window (the 5 days leading up to ovulation + ovulation day).
How scoring works:
ā¢ Intercourse 1-2 days before ovulation scores highest (Peak Fertility).
ā¢ Intercourse 4-5 days before scores lower (sperm survival is less likely).
ā¢ Intercourse 24+ hours after ovulation scores near zero (the egg dissolves quickly).
Clinical Methodology
This calculator compares intercourse dates and times against an estimated ovulation date. It shows a relative modeled share based mainly on timing, with a small optional partner-age adjustment when complete ages are entered. It is not a DNA-based probability of paternity.
1. The fertile window calculation
Conception is possible only during a short part of the menstrual cycle. Sperm can survive in the reproductive tract for up to five days in favourable cervical mucus, while an egg is usually viable for about 12 to 24 hours after ovulation. The calculator's visible fertile window is the six calendar days ending on ovulation: the five days before ovulation and ovulation day.
2. How the timing model works
The model uses published estimates of clinical pregnancy after intercourse on one day in that six-day interval. These estimates describe conception timing in a study population; they do not identify a biological father.
| Day relative to ovulation | ā5 | ā4 | ā3 | ā2 | ā1 | 0 |
|---|---|---|---|---|---|---|
| Published clinical-pregnancy estimate | 4% | 13% | 8% | 29% | 27% | 8% |
3. How time, multiple dates and optional partner age are used
The research provides day-level estimates, not hourly measurements. When you enter an intercourse time, the calculator converts it to a fraction of that day and interpolates between neighbouring daily estimates. Pregnancy dating estimates an ovulation date rather than an ovulation hour, so the calculation averages across possible ovulation times. If a time is blank, it averages across the entered date instead.
For each possible father, timing support from distinct dates is combined. Repeated entries on the same calendar date count once, using the strongest timing support for that date. Each candidate's combined timing support is then divided by the total across candidates to show the relative timing share.
Partner age is optional and should be the age around the possible conception date. Research on male age and natural conception is mixed, and any effect appears much smaller than the effect of intercourse timing. When an age is entered for every possible father included in the comparison, each partner's complete timing score is multiplied once by a small, capped modifier: 1.000 below age 40, then 0.0025 less for each year from age 40, with a minimum of 0.950 from age 59 onward. If any included partner age is blank, no age modifier is applied and the percentages remain timing-only. This conservative population-level assumption is not a validated paternity formula and does not measure individual sperm quality.
4. Ovulation estimation
The accuracy of the comparison depends on the estimated ovulation date. Early ultrasound dating generally provides the strongest dating estimate. Last-period dating uses the first day of the last period and the cycle length entered. Due-date and ultrasound modes work backward from the details entered. Any estimated ovulation date can still be off by days.
Important assumptions and limits
The comparison assumes equal fertility except for the small optional age modifier, no contraception, complete intercourse dates, and the same pregnancy for all entries. It does not model individual semen differences, contraception, infertility, sperm competition, or a personal distribution for the true ovulation date. Age alone cannot confirm or exclude fertility or paternity. Only prenatal or postnatal DNA testing can determine paternity.
Medical disclaimer
The information and calculators on this site are educational tools only. They provide statistical estimates based on published research and the details you enter.
They cannot diagnose, predict what will happen for you, or replace personalized advice from a licensed health care professional who knows your full history. Always talk with your doctor, midwife, or other qualified clinician before making decisions about your health, fertility, or pregnancy.
Never ignore, avoid, or delay seeking professional medical advice because of something you read here or a result you see in a calculator. If you think you may be having a medical emergency, call 911 in the United States or your local emergency number.