hiv risk calculator

What this HIV risk calculator does

This calculator gives a rough estimate of HIV transmission risk based on common factors: type of exposure, number of exposures, source partner status, condom use, PrEP, PEP, and STI presence. It combines these inputs into an estimated per-act risk and cumulative risk.

The goal is practical education, not certainty. Real-world risk changes with viral load, timing of testing, biological factors, and local epidemiology. Use this as a guide for decision-making and follow-up testing, not as a final answer.

How the estimate is built

1) Baseline exposure risk

Different exposures carry different baseline probabilities. For example, receptive anal sex generally has higher per-act transmission risk than vaginal or oral sex. Blood exposures like needle sharing can also carry substantial risk.

2) Source probability

If the partner is known HIV-positive and not undetectable, source probability is treated as high. If status is unknown, the tool uses the prevalence value you provide. If known HIV-negative with a reliable recent test, source probability approaches zero.

3) Protection and modifiers

• Condoms reduce risk significantly when used correctly.
• PrEP can reduce sexual HIV risk by about 99% with high adherence.
• PEP can lower risk when started within 72 hours after exposure.
• Undetectable viral load means effectively no sexual transmission risk (U=U).
• STIs/inflammation can increase susceptibility in sexual exposure contexts.

How to interpret your result

The calculator returns:

• Per-act estimate (risk from one event).
• Cumulative estimate (risk over repeated events).
• Risk category to help communicate urgency.

Even when estimated risk is low, testing may still be appropriate if exposure is concerning. A low number is not the same as zero risk.

What to do after a possible exposure

Immediate next steps

• If within 72 hours, seek emergency or urgent care and ask about PEP.
• Do not wait for symptoms; acute HIV can be asymptomatic.
• Discuss full STI screening as co-infections are common and treatable.

Testing timeline (general guide)

• Baseline test: as soon as possible after exposure.
• Follow-up: around 4–6 weeks with a 4th-generation test.
• Final confirmation: often at 3 months, based on clinician guidance.

Prevention strategies that work

• Use condoms or barriers consistently.
• Consider daily or event-based PrEP if you have ongoing risk.
• Encourage regular HIV testing for you and partners.
• Support treatment adherence; an undetectable viral load prevents sexual transmission.
• Avoid sharing injection equipment; use syringe service programs where available.

Limitations of any HIV risk calculator

No simple tool can capture every clinical nuance. This model does not account for all biological variables, exact medication adherence, timing of antiretroviral therapy, or network-level epidemiology. Use this estimate as a starting point for informed conversation with a healthcare professional.