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–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 — gender, country, activities, test history — stays in your browser and is never sent to any server.

STI Risks Calculator

Compare published STI prevalence and per-encounter transmission risk for 10 sexually transmitted infections across 21 countries and every sexual activity — with and without a condom. An educational reference, not a personal prediction.

Educational reference — not medical advice, and not a safety guarantee

These numbers are population averages with wide uncertainty. They don't predict your personal risk — that depends on factors not modelled here (viral load, mucosal health, other STIs, vaccination status, partner test history).

A low percentage is not a green light. These are population averages — if a specific partner is infected, your real risk is far higher than the number shown, and some outcomes (HIV, liver cancer from hepatitis) are lifelong or fatal. Weigh the consequences, not just the odds.
Many STIs are silent. Chlamydia, throat gonorrhea and HIV (for years) are often completely symptom-free, so a partner can be infectious while feeling perfectly healthy.
If you or a partner have any possible STI symptoms, stop and get tested before sex — and re-test, since early infections can be missed.
Make STI status part of the consent conversation. Before any sexual activity, share your recent test results and ask your partner about their full sexual history since their last test.
STI prevention is a shared responsibility. Protecting your partners — through testing, honest disclosure and protection — is part of protecting yourself.

For personal advice, get tested and speak to a healthcare professional.

You

Gender

Region of residence

Hep B vaccinated (~75% in this country)

HPV vaccinated (~45% in this country)

This estimate assumes a recent comprehensive negative STI test. Add every sexual partner from 30 days before that test onward — the 30-day window covers the incubation period during which a new infection may not yet show up on a test. Anything earlier is already reflected in the negative result.

Partners since 30 days before your last negative test

Add partner

No encounters added yet.

with

Partner

Gender

Region of residence

Hep B vaccinated (~75% in this country)

HPV vaccinated (~45% in this country)

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 encounter

Each value is the risk from a single act — risk compounds over repeated encounters. Hover any value for the breakdown.

5 more STIs

Show ranges

Activity	ChlamydiaCurable	GonorrheaSerious	SyphilisSerious	HIVLife-threatening	Hepatitis CSerious
Vaginal sex— you are insertive Penis-in-vagina intercourse.	0.05%	0.06%	0.01%	0.0004%	0.001%
Anal sex— you penetrate Penis-in-anus intercourse.	0.05%	0.005%	0.02%	0.0006%	0.007%
Blowjob (fellatio)— you receive Oral sex on a penis.	0.009%	0.14%	0.007%	<0.0001%	negligible
Cunnilingus— you give Oral sex on a vulva.	0.009%	0.003%	0.007%	negligible	negligible
Grinding Unclothed genital-to-genital contact without penetration.	negligible	negligible	0.003%	negligible	negligible
Fingering Manual stimulation of genitals or anus.	0.001%	0.0002%	0.0007%	negligible	negligible

By default the grid shows the most commonly discussed STIs. Switch on “5 more STIs” above to also show hepatitis B, HPV, herpes, M. genitalium and trichomoniasis.Numbers are illustrative — published averages × country prevalence, not a personal prediction.A low percentage is not a guarantee of safety. These are population averages: if a specific partner is infected, the real risk is far higher than the number shown. Weigh the consequences (severity badges) alongside the odds.Severity badges show what's at stake if untreated — Curable, Serious (possible organ damage / cancer), or Life-threatening.Hep B and HPV show N/A when the person is 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

0.001%

Any cancer diagnosis (ages 20–40)

per year

0.08%

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

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

0.50%

Pregnancy from one act of unprotected vaginal sex

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

4.0%

Catching the flu

per year

8.0%

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

lifetime · WHO 2021 estimates of intimate partner violence (lifetime, ages 15–49); country figures from national surveys where available.

25%

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 \times P \times M

$R$ — risk per encounter of the susceptible person contracting the STI.

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

$P$ — 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).

$M$ — male–male (MSM) multiplier. $M = 1$ 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 $P$ and $M = 1$ .)

Adjustments to $P$

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, $P$ is multiplied by an STI-specific factor drawn from UNAIDS / ECDC surveillance:

Chlamydia	Gonorrhea	Syphilis	HIV	Hepatitis B	Hepatitis C	HPV	Herpes (HSV-2)	M. genitalium	Trichomoniasis
×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 $P_{\text{hist}}$ , combining a Bayesian posterior after the negative test with the cumulative acquisition risk from each listed encounter:

P_{\text{base}} = \dfrac{(1 - \text{sens})\,P_0}{(1 - \text{sens})\,P_0 + \text{spec}\,(1 - P_0)}

P_{\text{acq}} = 1 - \prod_{i}\bigl(1 - p_i\,T_i\bigr)^{n_i}

P_{\text{hist}} = 1 - (1 - P_{\text{base}})\,(1 - P_{\text{acq}})

$P_0$ — prior (country prevalence); $P_{\text{base}} = P_0$ when no negative test is entered.

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

$p_i$ — prevalence of the STI for encounter $i$ 's partner (with the MSM factor applied when both are male), $T_i$ its per-act transmission rate, and $n_i$ the number of acts.

Because the MSM factor is already inside $P_{\text{hist}}$ , this override takes precedence over the standalone $M$ 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 $(P)$ — Netherlands

STI	Prevalence	Source
Chlamydia	0.80% – 1.5%	`ecdc-sti-2023`
Gonorrhea	0.15% – 0.30%	`ecdc-sti-2023`
Syphilis	0.02% – 0.04%	`ecdc-sti-2023`
HIV	0.50%	`unaids-2024`
Hepatitis B	0.25%	`who-hbv-2024`
Hepatitis C	0.30% – 0.60%	`euro-hcv-sr-2019`
HPV	8.0% – 12%	`bruni-2010-hpv`
Herpes (HSV-2)	5.3% – 11%	`james-2020-hsv2`
M. genitalium	1.0% – 1.8%	`baumann-2018-mgen`
Trichomoniasis	0.30% – 2.0%	`rowley-2019-sti`

Per-act transmission rates $(T)$ — resolved for you

Direction follows the susceptible person's anatomy and the activity. Condom column matches the current selection (no condom).

Activity	Direction	Chlamydia	Gonorrhea	Syphilis	HIV	Hepatitis B	Hepatitis C	HPV	Herpes (HSV-2)	M. genitalium	Trichomoniasis
Vaginal	Female → Male	2.0% – 5.0%`althaus-2012-chlamydia-per-act`	20% – 30%`fairley-2019-gonorrhea-per-act`	30% – 60%`cdc-syphilis-transmission`	0.05% – 0.10%`patel-2014-hiv-per-act`	10% – 20%`cdc-hbv-transmission`	0.10% – 0.30%`terrault-2013-hcv-sexual`	8.0% – 20%`burchell-2006-hpv`	0.04% – 0.10%`corey-2004-hsv2`	3.0% – 10%`manhart-2011-mgen`	5.0% – 15%`cdc-trich-2021`
Anal	Receptive → Insertive	2.0% – 5.0%`althaus-2012-chlamydia-per-act`	2.0%`fairley-2019-gonorrhea-per-act`	50% – 60%`cdc-syphilis-transmission`	0.06% – 0.16%`patel-2014-hiv-per-act`	20% – 40%`cdc-hbv-transmission`	0.50% – 2.0%`terrault-2013-hcv-sexual`	8.0% – 18%`burchell-2006-hpv`	0.05% – 0.15%`corey-2004-hsv2`	2.0% – 5.0%`manhart-2011-mgen`	—
Blowjob	Receiver of blowjob at risk	0.50% – 1.0%`althaus-2012-chlamydia-per-act`	63%`fairley-2019-gonorrhea-per-act`	10% – 30%`cdc-syphilis-transmission`	0.01%`patel-2014-hiv-per-act`	1.0% – 3.0%`cdc-hbv-transmission`	negligible	1.0% – 4.0%`burchell-2006-hpv`	0.01% – 0.05%`corey-2004-hsv2`	0.30% – 1.0%`manhart-2011-mgen`	negligible
Cunnilingus	Giver of cunnilingus at risk	0.50% – 1.0%`althaus-2012-chlamydia-per-act`	0.50% – 2.0%`fairley-2019-gonorrhea-per-act`	10% – 30%`cdc-syphilis-transmission`	negligible	1.0% – 3.0%`cdc-hbv-transmission`	negligible	2.0% – 6.0%`burchell-2006-hpv`	0.01% – 0.05%`corey-2004-hsv2`	negligible	negligible
Grinding	Skin contact	negligible	negligible	5.0% – 10%`cdc-syphilis-transmission`	negligible	negligible	negligible	3.0% – 10%`burchell-2006-hpv`	0.05% – 0.15%`corey-2004-hsv2`	negligible	0.50% – 2.0%`cdc-trich-2021`
Fingering	Manual contact	0.10%`althaus-2012-chlamydia-per-act`	0.10%`fairley-2019-gonorrhea-per-act`	1.0% – 3.0%`cdc-syphilis-transmission`	negligible	negligible	negligible	0.50% – 2.0%`burchell-2006-hpv`	0.01% – 0.03%`corey-2004-hsv2`	0.10%`manhart-2011-mgen`	0.50% – 2.0%`cdc-trich-2021`

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

who-sti-2024
WHO — Sexually Transmitted Infections fact sheet (2020 global prevalence among adults 15–49: chlamydia 2.5%, gonorrhea 0.7%, syphilis 0.5%, trichomoniasis 0.5%; regional figures interpolated from WHO regional statements) (2024)
who-hbv-2024
WHO — Hepatitis B fact sheet (regional chronic-HBV counts: WPR 97M, AFR 65M, SEAR 61M, EMR 15M, EUR 11M, AMR 5M, global 254M) (2024)
who-hcv-2024
WHO — Hepatitis C fact sheet (regional chronic-HCV counts: EMR 12M, EUR 9M, SEAR 9M, AFR 8M, WPR 7M, AMR 5M, global 50M) (2024)
unaids-2024
UNAIDS — Global HIV statistics by region (2024 adult prevalence 15–49: global 0.7%, AFR 3.1%, AMR 0.4%, EUR 0.3%, SEAR 0.2%, EMR 0.1%, WPR 0.1%) (2024)
james-2020-hsv2
James C, Harfouche M, Welton NJ, et al. — Herpes simplex virus: global infection prevalence and incidence estimates, 2016 (Bull World Health Organ 98:315–329). HSV-2 prevalence by WHO region and sex; global 13.2% of adults 15–49. (2020)
bruni-2010-hpv
Bruni L, Diaz M, Castellsagué X, et al. — Cervical HPV prevalence in 5 continents: meta-analysis of 1 million women with normal cytology (J Infect Dis 202:1789–1799). Global 11.7%; Africa 22.1%, C. America/Mexico 20.4%, N. America 11.3%, Europe 8.1%, Asia 8.0%. (2010)
baumann-2018-mgen
Baumann L, Cina M, Egli-Gany D, et al. — Prevalence of Mycoplasma genitalium in different population groups: systematic review and meta-analysis (Sex Transm Infect 94:255–262). General population ~1.3% (developed) / ~3.9% (developing). (2018)
rowley-2019-sti
Rowley J, Vander Hoorn S, Korenromp E, et al. — Chlamydia, gonorrhoea, trichomoniasis and syphilis: global prevalence and incidence estimates, 2016 (Bull World Health Organ 97:548–562). Trichomoniasis ~156M cases; female prevalence far exceeds male. (2019)
patel-2014-hiv-per-act
Patel et al. — Estimating per-act HIV transmission risk: a systematic review (AIDS 28:1509–1519) (2014)
fairley-2019-gonorrhea-per-act
Fairley et al. — Per-act probability of Neisseria gonorrhoeae transmission by sexual practice (Sex Transm Infect) (2019)
althaus-2012-chlamydia-per-act
Althaus et al. — Transmission of Chlamydia trachomatis through sexual partnerships: a comparison between three individual-based models (2012)
cdc-syphilis-transmission
CDC — Syphilis transmission and STI treatment guidelines (sore/chancre contact is the primary route; condoms protect only where they cover the sore) (2021)
cdc-hbv-transmission
CDC — Hepatitis B sexual-exposure guidance (HBV is 50–100× more infectious than HIV; high efficiency through semen/vaginal fluid) (2023)
terrault-2013-hcv-sexual
Terrault et al. — Sexual transmission of hepatitis C virus among monogamous heterosexual couples (HCV is primarily blood-borne; sexual transmission inefficient) (2013)
burchell-2006-hpv
Burchell AN, Winer RL, de Sanjosé S, Franco EL — Epidemiology and transmission dynamics of genital HPV infection (Vaccine 24 Suppl 3:S52–61). HPV is highly transmissible skin-to-skin; per-act rates are poorly quantified and condoms only partially protect. (2006)
corey-2004-hsv2
Corey L, Wald A, Patel R, et al. — Once-daily valacyclovir to reduce the risk of transmission of genital herpes (N Engl J Med 350:11–20). Source for low per-act HSV-2 transmission in discordant couples and effect of condoms/suppression. (2004)
manhart-2011-mgen
Manhart LE — Mycoplasma genitalium: an emergent sexually transmitted disease? (Infect Dis Clin North Am 25:289–303). Transmission is genital/fluid-borne, epidemiologically similar to chlamydia; per-act rates are not precisely established. (2011)
cdc-trich-2021
CDC — Trichomoniasis (2021 STI Treatment Guidelines). T. vaginalis infects the lower genital tract (vagina/urethra) but not the pharynx or rectum; condoms reduce transmission and partners must be treated together. (2021)
ecdc-sti-2023
ECDC — Annual Epidemiological Reports 2023 for chlamydia, gonorrhoea, and syphilis in EU/EEA countries (2022 data year; notification rates per 100,000 — true prevalence is higher due to underdiagnosis) (2023)
gbd-2019-hbv
GBD 2019 Hepatitis B Collaborators — Global, regional, and national burden of hepatitis B, 1990–2019: a systematic analysis (Lancet Gastroenterology & Hepatology 2022). Country-level HBsAg all-age prevalence estimates. (2022)
euro-hcv-sr-2019
van der Meer et al. — Prevalence of hepatitis C infection among the general population and high-risk groups in the EU/EEA: a systematic review update (BMC Infectious Diseases 2019). General-population anti-HCV estimates by European country. (2019)
cdc-naat-2021
CDC — 2021 STI Treatment Guidelines: NAAT sensitivity 97–99% for chlamydia, 98–100% for gonorrhea; specificity ≥99% for both (2021)
larsson-2014-syphilis-serology
Larsson et al. — Pooled sensitivity 87% / specificity 98.5% for combined treponemal serological syphilis testing (STI 2014) (2014)
deeks-2017-hiv-test
Deeks et al. — Systematic review of 4th-generation HIV combo tests: sensitivity ≥99.9%, specificity 99.94% after window period (BMJ 2017) (2017)
who-hbv-hbsag-2020
WHO — HBsAg test accuracy guidance: sensitivity 98.5%, specificity 99% (WHO Technical Guidance on Hepatitis Testing 2020) (2020)
who-hcv-test-2021
WHO — Anti-HCV antibody tests: pooled sensitivity 97.4%, specificity 99.7% (WHO Technical Guidance on Hepatitis Testing 2021) (2021)
hpv-dna-test-2020
HPV DNA assays (e.g. Hybrid Capture 2 / PCR) — clinical sensitivity ~90% and specificity ~90% for high-risk HPV. HPV is not part of a standard STI panel; cervical screening is the usual detection route, and there is no routine test for men. (2020)
hsv2-serology-2017
Type-specific HSV-2 glycoprotein-G (gG2) serology — sensitivity ~98% but specificity ~95% (false positives are common at low index values). Herpes is generally not included in routine asymptomatic STI screening. (2017)
unaids-ecdc-msm-2022
UNAIDS / ECDC — MSM prevalence multipliers: HIV ~26× (UNAIDS 2022 Key Populations report); syphilis ~25× (ECDC AER 2022: MSM ~70% of cases, ~3–5% of population); gonorrhea ~12× (ECDC AER 2022 surveillance); chlamydia ~4× (rectal/pharyngeal burden); Hep B ~4× (CDC/ECDC); Hep C ~3× (van de Laar 2021, ECDC). Applied to general-population country prevalence when both participants are male. (2022)
who-vaw-2021
WHO — Violence Against Women fact sheet (lifetime physical/sexual intimate partner violence by WHO region: AFR 33%, EMR 31%, SEAR 33%, AMR 25%, EUR 22%, WPR 20%, global 30%) (2025)

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STI Risks Calculator

STI Risks Calculator

No condom

Your risk per single sexual encounter

For comparison

How is this calculated?

Formula

Adjustments to $P$

Country prevalence $(P)$ — Netherlands

Per-act transmission rates $(T)$ — resolved for you

Sources

Understand STI risk per encounter

STI prevalence varies a lot by country

Frequently asked questions

What is the STI Risks Calculator?

Which STIs does it cover?

How is per-encounter STI transmission risk calculated?

Do condoms reduce STI risk?

Is this medical advice?

Does it store or share my data?

STI Risks Calculator

Personalise partner's estimateUse a recent test result & partner history instead of country averages

No condom

For comparison

How is this calculated?Formula, inputs, and per-cell breakdown.

How is this calculated?

Formula

Adjustments to PPP

Country prevalence (P)(P)(P) — Netherlands

Per-act transmission rates (T)(T)(T) — resolved for you

Sources30 references — every number is traceable to one.

Sources

Understand STI risk per encounter

STI prevalence varies a lot by country

Frequently asked questions

What is the STI Risks Calculator?

Which STIs does it cover?

How is per-encounter STI transmission risk calculated?

Do condoms reduce STI risk?

Is this medical advice?

Does it store or share my data?

Adjustments to $P$

Country prevalence $(P)$ — Netherlands

Per-act transmission rates $(T)$ — resolved for you