Social Distancing Calculator - Coronavirus

Estimate the chance that at least one person in a group or gathering is currently infectious, using population, active case counts, and an underreporting multiplier.

Quick Facts

Core formula
R = 1 - (1 - p)^n
p is the chance any one person is currently infectious; n is your group size.
Undercounting
Reported cases understate true active infections
The ascertainment-bias multiplier scales reported cases up toward an actual-infections estimate.

Your Results

Calculated
Individual risk (p)
-
Chance any one person is currently infectious
Estimated active infections
-
Reported cases x ascertainment bias
Event risk (R)
-
Chance at least 1 person in your group is infectious
Risk level
-
Interpretation band for event risk

Ready

Enter population, active cases, and group size, then calculate the event risk.

About the Social Distancing Calculator - Coronavirus

This calculator estimates the probability that at least one person in a group or gathering is currently infectious with a communicable disease such as COVID-19. It uses a standard two-step event-risk model: first the chance that any one person you might encounter is infectious (individual risk), then the chance that at least one person among several people is infectious (event risk).

How the calculation works

  • Individual risk (p): reported active cases are multiplied by an ascertainment-bias multiplier, then divided by the population: p = (active cases x bias) / population. The multiplier exists because confirmed case counts undercount true infections due to asymptomatic cases, limited testing, and reporting lag.
  • Event risk (R): R = 1 - (1 - p)^n, where n is the number of people in your group. This is the complement of the probability that none of n independently-encountered people are infectious.

Putting the result in context

Because probabilities compound, event risk climbs faster than individual risk as group size grows. A low individual risk can still produce a meaningfully higher event risk once dozens of people are involved. Use the risk band shown with your result as a rough interpretation guide, and re-run the calculation as case counts change - this is a snapshot based on current inputs, not a trend.

When to consult a professional

This tool performs a mathematical estimate; it does not model transmission dynamics, masking, ventilation, or vaccination status, and it is not a substitute for local public health guidance. For decisions about gatherings, workplaces, or event policy, consult official public health sources and, where relevant, a qualified professional.

Frequently Asked Questions

What formula does the Social Distancing Calculator use?
It uses a two-step event-risk model. First, individual risk p equals active cases multiplied by an ascertainment-bias (undercount) multiplier, divided by the population. Second, event risk R equals 1 minus (1 minus p) raised to the power of the group size n, which is the probability that at least one person among n people is currently infectious.
What is ascertainment bias and why does the calculator use it?
Reported case counts undercount true active infections because of asymptomatic cases, limited testing, and reporting delays. The ascertainment-bias multiplier scales reported cases up to approximate actual active infections; a value of 5 means about 5 true infections exist for every 1 officially reported.
Why does risk rise so quickly with group size?
Because probabilities compound across independent people. Even when the individual risk p is small, the chance that none of n people are infectious, (1 minus p)^n, shrinks quickly as n grows, so the complementary event risk 1 minus (1 minus p)^n climbs faster than p alone would suggest.

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