Reference
API county codes by state
In an API well number, digits one and two are the state and digits three to five are the county the well was drilled in. So 42-317 is Texas, Martin County. The number does not carry the well's coordinates; those are in the state agency's well record.
Pick a state for its full county table, or paste a number into the API well number decoder to split it into every part.
States
| State code | State | County codes |
|---|---|---|
| 01 | Alabama | 67 counties |
| 02 | Arizona | 15 counties |
| 03 | Arkansas | 75 counties |
| 04 | California | 59 counties |
| 05 | Colorado | 64 counties |
| 06 | Connecticut | 8 counties |
| 07 | Delaware | 3 counties |
| 09 | Florida | 67 counties |
| 10 | Georgia | 159 counties |
| 11 | Idaho | 44 counties |
| 12 | Illinois | 102 counties |
| 13 | Indiana | 92 counties |
| 14 | Iowa | 99 counties |
| 15 | Kansas | 105 counties |
| 16 | Kentucky | 120 counties |
| 17 | Louisiana | 64 parishes |
| 18 | Maine | 16 counties |
| 19 | Maryland | 24 counties |
| 20 | Massachusetts | 14 counties |
| 21 | Michigan | 83 counties |
| 22 | Minnesota | 87 counties |
| 23 | Mississippi | 82 counties |
| 24 | Missouri | 115 counties |
| 25 | Montana | 56 counties |
| 26 | Nebraska | 93 counties |
| 27 | Nevada | 17 counties |
| 28 | New Hampshire | 10 counties |
| 29 | New Jersey | 21 counties |
| 30 | New Mexico | 33 counties |
| 31 | New York | 62 counties |
| 32 | North Carolina | 100 counties |
| 33 | North Dakota | 53 counties |
| 34 | Ohio | 88 counties |
| 35 | Oklahoma | 77 counties |
| 36 | Oregon | 36 counties |
| 37 | Pennsylvania | 67 counties |
| 38 | Rhode Island | 5 counties |
| 39 | South Carolina | 46 counties |
| 40 | South Dakota | 66 counties |
| 41 | Tennessee | 95 counties |
| 42 | Texas | 254 counties |
| 43 | Utah | 29 counties |
| 44 | Vermont | 14 counties |
| 45 | Virginia | 134 counties |
| 46 | Washington | 39 counties |
| 47 | West Virginia | 55 counties |
| 48 | Wisconsin | 72 counties |
| 49 | Wyoming | 23 counties |
| 51 | Hawaii | 5 counties |
Not listed: District of Columbia (08) and Alaska (50). Alaska numbers its wells by USGS quadrangle rather than borough, so its codes come from the Alaska Oil and Gas Conservation Commission. The District of Columbia has a single code.
What the county code tells you
The county is where the well starts at the surface. A horizontal well that runs under a county line keeps the code of the county it was drilled from. A few state regulators assign the producing county instead, so for a well near a boundary the state record decides.
The number stops there. It has no coordinates, lease, survey section or operator in it. Those are in the well record the state agency keeps, which you find by searching the full number.
Frequently asked questions
Which digits of an API number are the county?
Digits three to five. In 35-137-22789, 35 is Oklahoma and 137 is Stephens County. The five digits after that identify the well within the county.
Can you get a well's location from its API number?
Only the state and county. For the latitude and longitude, lease or survey section, search the full API number in the state agency database, which holds the well record.
Are API county codes the same as FIPS county codes?
Not by definition. The PPDM standard that replaced API Bulletin D12A says they are not necessarily the same. For onshore counties in the lower 48 the numbers line up in practice, which is why the tables here use Census names, checked against state well records. Alaska and offshore waters are the exceptions.
Why does an API county code not appear in any state table?
It is most often an offshore pseudo-county covering state or federal waters, such as the Texas and Louisiana codes in the 600s and 700s. A few states have also added codes, such as Kern County, California's second code, 030.
From API number to well record
TetraSoft Atlas maps about 2.7 million wells across eight states, each with its API-10 number beside the operator, status and production. Free 7-day trial, no credit card.
How to cite this page
Sources: American Petroleum Institute Bulletin D12A well numbering standard; US Census county FIPS codes.
TetraSoft. (2026). API county codes by state. Reference generated 2026-09-24. https://tetrasoftco.com/tools/api-county-codes/