COUNTY · DE
New Castle County, Delaware
New Castle County is a county in Delaware with FIPS code 10003 and an estimated population of 578,592 (Census 2023), 1st largest of 3 in the state.
County facts
- Name
- New Castle County
- State
- Delaware
- Type
- county
- FIPS code
- 10003
- State FIPS
- 10
- Population (2023)
- 578,592
- Change since 2020
- +1.3%
- State rank
- 1 of 3
Population is the Census Bureau’s Vintage 2023 estimate.
Questions
What is the FIPS code for New Castle County?
New Castle County, Delaware has the five-digit FIPS code 10003: 10 for the state and 003 for the county.
What state is New Castle County in?
New Castle County is in Delaware.
How do I know if an address is in this county?
Use the address lookup. ZIP codes and city names can cross county lines, so an address is the reliable check.
Sources: U.S. Census Bureau geographic reference files; Census Bureau Population Estimates, Vintage 2023. Last reviewed 2026-10-27.