STI Risks Calculator
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STI Risks Calculator

Compare published per-act transmission risk and prevalence for chlamydia, gonorrhea, syphilis, HIV, hepatitis B&C, HPV, HSV-2 and more across 21 countries — vaginal, anal and oral sex, with and without a condom. Based on WHO, ECDC and CDC data. An educational reference, not a personal diagnosis.

You
Hep B vaccinated (~75% in this country)
HPV vaccinated (~45% in this country)
Estimated STI carrier chance (per 1,000,000) — your profile
prevalence
Chlamydia11,500
Gonorrhea2,250
Syphilis300
HIV5,000
Hepatitis C4,500
National prevalence — add test result & partners below to adjust the estimate.

Partners since 30 days before your last negative test

Add partner
No encounters added yet.
with
Partner
Hep B vaccinated (~75% in this country)
HPV vaccinated (~45% in this country)
Estimated STI carrier chance (per 1,000,000) — partner's profile
prevalence
Chlamydia11,500
Gonorrhea2,250
Syphilis300
HIV5,000
Hepatitis C4,500
National prevalence — add test result & partners below to adjust the estimate.

Partners since 30 days before your last negative test

Add partner
No encounters added yet.
No condom
Unprotected: higher transmission rates shown.
Your risk per single sexual interaction

Risk per single act— compounds with repetition. Hover any value for the breakdown.

Figures are estimated cases per 1,000,000 (e.g. 50,000 ≈ 5%).

5 more STIs

Show ranges

Activity
ChlamydiaCurable
GonorrheaSerious
SyphilisSerious
HIVLife-threatening
Hepatitis CSerious
Vaginal sex: you are insertive
Penis-in-vagina intercourse.
455
600
150
4
11
Anal sex: you penetrate
Penis-in-anus intercourse.
455
45
170
6
68
Blowjob (fellatio): you receive
Oral sex on a penis.
95
1,417
70
1
negligible
Cunnilingus: you give
Oral sex on a vulva.
95
34
70
negligible
negligible
Grinding
Unclothed genital-to-genital contact without penetration.
negligible
negligible
25
negligible
negligible
Fingering
Manual stimulation of genitals or anus.
12
2
7
negligible
negligible
Numbers are illustrative: published population averages, not a personal prediction.A low number is not a guarantee of safety. If a specific partner is infected, the real risk is far higher. Weigh the severity badges alongside the odds.Severity badges show what's at stake if untreated: Curable, Serious (organ damage / cancer), or Life-threatening.Hep B and HPV show N/A when vaccinated — toggle per person in the form above.HIV is asymmetric: receptive partners face higher risk than insertive partners.Mpox (monkeypox) isn't shown: A sexually transmissible viral infection to watch out for during outbreaks. We don't show per-encounter statistics because its spread is outbreak-driven, not a steady background rate. Learn more.

Serious injury in a car accident

per car trip
10

Any cancer diagnosis (ages 20 to 40)

per year
800

Pregnancy from one act of protected vaginal sex (condom, typical use)

per encounter · Condom reduces per-act pregnancy risk by ~87% at typical use.
5,000

Pregnancy from one act of unprotected vaginal sex

per encounter · Varies hugely with cycle phase; higher mid-cycle, lower otherwise.
40,000

Catching the flu

per year
80,000

Physical/sexual violence by an intimate partner (women, the Netherlands)

lifetime · WHO 2021 estimates of intimate partner violence (lifetime, ages 15 to 49); country figures from national surveys where available.
250,000
Time bases differ (per encounter, per trip, per year, lifetime). Use these as rough magnitude anchors, not direct comparisons. Numbers are order-of-magnitude estimates.

Formula
R=T×P×MR = T \times P \times M

RR: risk per encounter of the susceptible person contracting the STI.

TT: per-act transmission rate (assuming the infectious partner has the STI), from peer-reviewed meta-analyses.

PP: probability the partner is infected. By default this is the country prevalence of the STI in the partner's country of residence; personal history can replace it entirely (below).

MM: male-male (MSM) multiplier. M=1M = 1 for mixed-sex pairings; when both partners are male it is the STI-specific factor in the table below. (When personal history is used, the MSM adjustment is folded into PP and M=1M = 1.)

Adjustments to PP

1. Male-male contact (MSM). Country prevalence figures are general-population averages and substantially underestimate risk for men who have sex with men. When both participants are male, PP is multiplied by an STI-specific factor drawn from UNAIDS / ECDC surveillance:

ChlamydiaGonorrheaSyphilisHIVHepatitis BHepatitis CHPVHerpes (HSV-2)M. genitaliumTrichomoniasis
×4×12×25×25×4×3×4×3×3×1
Source: unaids-ecdc-msm-2022

2. Personal history. When you enter a partner's recent negative test and their encounter history, the country prevalence is replaced entirely by a personalised estimate PhistP_{\text{hist}}, combining a Bayesian posterior after the negative test with the cumulative acquisition risk from each listed encounter:

Pbase=(1−sens) P0(1−sens) P0+spec (1−P0)P_{\text{base}} = \dfrac{(1 - \text{sens})\,P_0}{(1 - \text{sens})\,P_0 + \text{spec}\,(1 - P_0)}
Pacq=1−∏i(1−pi Ti)niP_{\text{acq}} = 1 - \prod_{i}\bigl(1 - p_i\,T_i\bigr)^{n_i}
Phist=1−(1−Pbase) (1−Pacq)P_{\text{hist}} = 1 - (1 - P_{\text{base}})\,(1 - P_{\text{acq}})

P0P_0: prior (country prevalence); Pbase=P0P_{\text{base}} = P_0 when no negative test is entered.

sens,spec\text{sens}, \text{spec}: test sensitivity and specificity for that STI.

pip_i: prevalence of the STI for encounter ii's partner (with the MSM factor applied when both are male), TiT_i its per-act transmission rate, and nin_i the number of acts.

Because the MSM factor is already inside PhistP_{\text{hist}}, this override takes precedence over the standalone MM term above.

Vaccination. There is no vaccination term in the formula. For the vaccine-preventable STIs (Hep B and HPV), if the susceptible person is marked as vaccinated their cell is simply shown as N/A. This is a simplification: real-world vaccines are highly but not 100% protective (and the HPV vaccine covers only the high-risk types), so treat a vaccinated N/A as "very low", not "zero".

Country prevalence (P)(P): Netherlands
STIPrevalence (per 1,000,000)Source
Chlamydia8,000 to 15,000ecdc-sti-2023
Gonorrhea1,500 to 3,000ecdc-sti-2023
Syphilis200 to 400ecdc-sti-2023
HIV5,000unaids-2024
Hepatitis B2,500who-hbv-2024
Hepatitis C3,000 to 6,000euro-hcv-sr-2019
HPV80,000 to 120,000bruni-2010-hpv
Herpes (HSV-2)53,000 to 107,000james-2020-hsv2
M. genitalium10,000 to 18,000baumann-2018-mgen
Trichomoniasis3,000 to 20,000rowley-2019-sti
Per-act transmission rates (T)(T): resolved for you
Direction follows the susceptible person's anatomy and the activity. Condom column matches the current selection (no condom). Rates shown as cases per 1,000,000.
ActivityDirectionChlamydiaGonorrheaSyphilisHIVHepatitis BHepatitis CHPVHerpes (HSV-2)M. genitaliumTrichomoniasis
VaginalFemale → Male
20,000 to 50,000althaus-2012-chlamydia-per-act
200,000 to 300,000fairley-2019-gonorrhea-per-act
300,000 to 600,000cdc-syphilis-transmission
500 to 1,000patel-2014-hiv-per-act
100,000 to 200,000cdc-hbv-transmission
1,000 to 3,000terrault-2013-hcv-sexual
80,000 to 200,000burchell-2006-hpv
400 to 1,000corey-2004-hsv2
30,000 to 100,000manhart-2011-mgen
50,000 to 150,000cdc-trich-2021
AnalReceptive → Insertive
20,000 to 50,000althaus-2012-chlamydia-per-act
20,000fairley-2019-gonorrhea-per-act
500,000 to 600,000cdc-syphilis-transmission
600 to 1,600patel-2014-hiv-per-act
200,000 to 400,000cdc-hbv-transmission
5,000 to 20,000terrault-2013-hcv-sexual
80,000 to 180,000burchell-2006-hpv
500 to 1,500corey-2004-hsv2
20,000 to 50,000manhart-2011-mgen
—
BlowjobReceiver of blowjob at risk
5,000 to 10,000althaus-2012-chlamydia-per-act
630,000fairley-2019-gonorrhea-per-act
100,000 to 300,000cdc-syphilis-transmission
100patel-2014-hiv-per-act
10,000 to 30,000cdc-hbv-transmission
negligible
10,000 to 40,000burchell-2006-hpv
100 to 500corey-2004-hsv2
3,000 to 10,000manhart-2011-mgen
negligible
CunnilingusGiver of cunnilingus at risk
5,000 to 10,000althaus-2012-chlamydia-per-act
5,000 to 20,000fairley-2019-gonorrhea-per-act
100,000 to 300,000cdc-syphilis-transmission
negligible
10,000 to 30,000cdc-hbv-transmission
negligible
20,000 to 60,000burchell-2006-hpv
100 to 500corey-2004-hsv2
negligiblenegligible
GrindingSkin contactnegligiblenegligible
50,000 to 100,000cdc-syphilis-transmission
negligiblenegligiblenegligible
30,000 to 100,000burchell-2006-hpv
500 to 1,500corey-2004-hsv2
negligible
5,000 to 20,000cdc-trich-2021
FingeringManual contact
1,000althaus-2012-chlamydia-per-act
1,000fairley-2019-gonorrhea-per-act
10,000 to 30,000cdc-syphilis-transmission
negligiblenegligiblenegligible
5,000 to 20,000burchell-2006-hpv
100 to 300corey-2004-hsv2
1,000manhart-2011-mgen
5,000 to 20,000cdc-trich-2021

Citation IDs appear next to each per-act transmission rate in the "How is this calculated?" section.

All inputs stay in your browser. Nothing is sent to any server.

Understand STI risk per encounter

Headlines about STIs rarely tell you what a single encounter actually means. The STI Risks Calculator combines published per-act transmission rates with real prevalence data so you can compare the relative risk of chlamydia, gonorrhea, syphilis, HIV, hepatitis B and C, HPV, herpes, Mycoplasma genitalium and trichomoniasis side by side, for vaginal, anal, and oral sex, with or without a condom.

STI prevalence varies a lot by country

How likely a partner is to carry an STI depends heavily on where they live. The calculator covers 21 countries, including the Netherlands, France, Germany, the UK, Spain, Italy, the United States, Brazil, Australia, Japan and India, plus WHO regional averages, drawing on ECDC, UNAIDS and WHO surveillance data, so you can see how the same activity carries different risk in different places.

Frequently asked questions

What is the STI Risks Calculator?

It's a free, private educational tool that compares published STI prevalence and per-encounter transmission rates so you can see how risk changes by country, partner, sexual activity, and condom use. It shows illustrative population averages, not a personal diagnosis or prediction.

Which STIs does it cover?

The five most common (the 'Big 5': chlamydia, gonorrhea, syphilis, HIV, and hepatitis B/C), plus four supplementary infections: HPV, genital herpes (HSV-2), Mycoplasma genitalium, and trichomoniasis. Mpox is included as an awareness 'watch list' entry without per-encounter statistics, because it spreads in outbreaks rather than at a steady rate.

How is per-encounter STI transmission risk calculated?

For each activity the illustrative risk is the published per-act transmission rate multiplied by the prevalence of that STI in the partner's country of residence, shown as a low-to-high range. Male-to-male contact applies an elevated-prevalence (MSM) adjustment, and you can replace country prevalence with a personalised estimate based on a recent negative test and partner history. Every number is traceable to a cited source (WHO, ECDC, UNAIDS, CDC, and peer-reviewed meta-analyses).

Do condoms reduce STI risk?

Yes. For fluid-borne infections such as HIV, gonorrhea, chlamydia and hepatitis B, condoms reduce per-encounter risk substantially. For skin-to-skin infections such as HPV and herpes they help less, because the virus can live on skin a condom doesn't cover. Toggle 'condom' in the calculator to compare both.

Is this medical advice?

No. The STI Risks Calculator is an educational reference only. A low percentage is never a guarantee of safety. For testing, diagnosis, vaccination (for example hepatitis B or HPV), or treatment, speak to a healthcare professional.

Does it store or share my data?

No. There is no account, no database, and no analytics on your selections. Everything you enter, such as gender, country, activities, and test history, stays in your browser and is never sent to any server.