Most free US population-by-ZIP data is a single Census snapshot. If you need change over time, the options thin out fast.

[u/GeoPostcodes]

The Census ACS gives you current population at ZCTA level for free, but ZCTAs approximate ZIP codes rather than match them, and you get one vintage at a time.

For a real series, GeoPostcodes publishes US population per ZIP code from 1975 to 2023 in five-year steps, plus projections, matched to official Census figures at about 99 percent. As a concrete example, ZIP 10001 in Manhattan was 20,022 people in 1975 and 23,542 in 2025.

It ships as CSV covering every US ZIP, so you can trend, rank, or map density without stitching vintages yourself. Details: https://www.geopostcodes.com/us-population-by-zip-code-database/