Economics
How to Read the ACS5 Margin of Error (and When to Ignore the Number)
Every ACS estimate ships with a margin of error. For small ZIP codes that margin can be larger than the estimate itself. Here's how to tell when a number is solid and when it's noise.
By City Zip Compare Editorial · February 8, 2026 · 6 min read
The American Community Survey is a sample, not a census of every household. Every published estimate is paired with a margin of error (MOE) at the 90% confidence level. For populous places — entire states, large counties, big cities — MOEs are tiny relative to the estimates and can be ignored. For small places they can swallow the number.
A practical rule of thumb
Compute the coefficient of variation: MOE divided by 1.645, then divided by the estimate. If the result is under 12%, the estimate is reliable. Between 12% and 40%, treat it as approximate. Above 40%, treat it as suggestive at best.
On City Zip Compare we surface the underlying ACS5 number on every page, but for ZIPs with very small populations (under ~500) you should expect noisier numbers, especially for income and housing-value medians.
Why ZCTAs are not exactly ZIP codes
The Census does not publish data by USPS ZIP code; it publishes by ZIP Code Tabulation Area (ZCTA), which is a Census-defined geographic approximation of a USPS ZIP. About 95% of residential ZIPs map cleanly to a ZCTA. P.O. Box-only ZIPs and unique-recipient ZIPs (large companies with their own code) usually do not.
When City Zip Compare cannot find a ZCTA for a ZIP query, that's why.
Frequently asked
›Why don't you publish the margin of error on every page?
We will. For now, treat ZIP-level numbers as approximate within ±5% for populous ZIPs and within ±15% for small ones.
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Source: U.S. Census Bureau, American Community Survey 5-year estimates. Data: census.gov/programs-surveys/acs.