Event Risk Calculator
HealthEstimate the chance at least one attendee at an event is currently infectious, based on local case rate and event size. An educational probability model, not medical advice.
Chance At Least One Attendee Is Infectious
13.95%
Per-Person Probability
0.30%
What is a Event Risk?
The Event Risk Calculator estimates the probability that at least one attendee at a gathering is currently infectious with a contagious illness, based on the local case rate, an adjustment for under-reporting, and the number of people attending. It's a general educational probability tool, not a personalized medical risk assessment.
For a related estimate factoring in mask use, see the Mask vs No Mask Calculator.
How to use this Event Risk calculator
- Enter the active cases per 100,000 people for your local area, from public health data.
- Adjust the under-reporting multiplier to reflect how much true cases may exceed official reports.
- Enter the number of people expected at the event.
- Read the Chance At Least One Attendee Is Infectious result instantly.
- Try different event sizes to see how quickly the risk compounds.
Formula & Methodology
Per-person probability (p) = (Cases per 100k × Under-reporting multiplier) ÷ 100,000 Event risk = 1 − (1 − p)ⁿ, where n is the number of attendees. Worked example — 100 cases per 100k, a 3x under-reporting multiplier, and 50 attendees: p = (100 × 3) ÷ 100,000 = 0.3% Event risk = 1 − (1 − 0.003)⁵⁰ ≈ 13.9% This shows how a seemingly small 0.3% individual probability compounds to a meaningfully higher chance across 50 attendees.
Frequently Asked Questions
The Event Risk Calculator estimates the probability that at least one person at a gathering is currently infectious with a contagious illness, based on the local reported case rate, an under-reporting adjustment, and the number of people attending. It's an educational probability model, not a medical risk assessment.
The calculator first estimates a per-person probability of being infectious from the local case rate, then applies the standard 'at least one' probability formula across the number of attendees to estimate the chance at least one person is infectious.
Officially reported case counts typically undercount true infections because many cases go untested, asymptomatic, or unreported, so an under-reporting multiplier adjusts the reported rate to better estimate the true prevalence.
Because the 'at least one' probability compounds across every additional attendee — even a fairly low per-person probability can add up to a much higher chance that at least one of many attendees is infectious, which is a well-known feature of this type of probability calculation.
No — this calculator only estimates the chance that at least one attendee is currently infectious. It does not model actual transmission risk between attendees, which depends on many additional factors like ventilation, contact duration, and mitigation measures.
Local public health case rate data, often reported as cases per 100,000 people over a recent period, is a common input; the accuracy of the estimate depends heavily on how current and reliable that local data is.
This calculator doesn't include mask effectiveness directly — for a mask-specific estimate, see the companion [Mask vs No Mask Calculator](/mask-vs-no-mask-calculator/), which models how mask filtration affects transmission risk.
No — this is a general probability model that can be applied to any contagious illness given an appropriate local case rate estimate; it does not model illness-specific transmission dynamics.
This calculator is intended for general education about probability and risk scaling, not as a personal health decision tool — individual risk tolerance and health circumstances vary, and specific health decisions should involve your own judgment and, if relevant, medical advice.
Because the probability compounds multiplicatively across all attendees rather than adding linearly, even a 1% per-person chance can translate into a much higher chance that at least one person among 50 or 100 attendees is infectious.
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