How This Calculator Works

The model, its inputs, and its limits, in plain language.

This calculator estimates the statistical probability that a described exposure resulted in HIV transmission. It is a transparent, source-based model, not a diagnostic test. This page explains exactly how the number is produced so you can judge it for yourself.

Step 1: Per-act transmission probability

Every exposure type has an estimated probability of transmitting HIV in a single act, assuming the other person has HIV and is not virally suppressed. We use figures consistent with CDC estimates:

Receptive anal sex: 1.4% per act
Insertive anal sex: 0.11%
Receptive vaginal sex: 0.08%
Insertive vaginal sex: 0.04%
Oral sex: about 0.01% or lower
Sharing injection equipment: 0.63%
Accidental needlestick: 0.23%
Blood on mucous membranes or broken skin: 0.09%

Step 2: Probability the partner actually has HIV

If the partner is known HIV-positive, this factor is 100%. If their status is unknown, we weight the risk by HIV prevalence for the selected country. Sex-worker and men-who-have-sex-with-men options raise that assumed prevalence, because those groups carry higher prevalence than the general population in most regions. These are approximations, not precise local figures.

Step 3: Biological modifiers (sexual exposure only)

Condom used: about an 80% reduction
No ejaculation (with a male partner): about a 50% reduction
Circumcision (for insertive partners): about a 50% reduction
A co-existing STI in the partner: roughly doubles risk
Acute (very recent) infection in the partner: high viral load, modelled as a large increase

These modifiers apply only to sexual exposures. They are not applied to needle or blood exposures.

Step 4: Repeated exposures

For more than one exposure we use the standard cumulative-risk formula, 1 − (1 − p)ⁿ, where p is the per-act risk and n is the number of acts. Risk accumulates but never reaches certainty from repetition alone.

Step 5: Prevention

PrEP reduces risk by about 99% when taken consistently; we scale this down for imperfect adherence.
PEP started within 72 hours reduces risk, with earlier initiation being more effective.
U=U: a partner with a durably undetectable viral load does not transmit HIV sexually, so risk is reduced to effectively zero.

Step 6: Negative test adjustment

If you enter a negative HIV test taken after its window period, the estimate is reduced according to the test type and how long after exposure it was taken. A conclusive negative test after the full window effectively rules out HIV from that exposure.

Important limitations. This is a simplified statistical model. Real transmission depends on factors it cannot fully capture, including exact viral load, individual biology, and reporting accuracy. It can never confirm or exclude infection in your specific case. Only testing can. Always confirm with a healthcare provider.

Sources

Estimates are drawn from the U.S. Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), and peer-reviewed epidemiological literature. See the FAQ and Understanding Risk pages for more.