ZIP codes and county lines in Oklahoma

277 of the 666 ZIP code areas in Oklahoma, or 41.6%, overlap more than one county. If your territories, tax rules or service areas are set by county and your list is by ZIP, these are the rows where the two disagree.

ZIP code areas: 666
In more than one county: 277
At least 5% in a second county: 219
Cross a state line: 3

How Oklahoma compares

Oklahoma ranks 12 of 52 (the 50 states, DC and Puerto Rico) by the share of its ZIP code areas that overlap more than one county. 77 of its counties have at least one split ZIP code area along an edge. 3 of its ZIP code areas also reach into Arkansas and Texas, so a ZIP list that is meant to stop at the state line does not.

Counties with the most split ZIP code areas

Oklahoma County: 17 ZIP codes shared with another county
Tulsa County: 17 ZIP codes shared with another county
Creek County: 14 ZIP codes shared with another county
Mayes County: 14 ZIP codes shared with another county
Cleveland County: 13 ZIP codes shared with another county

Every split ZIP code area in Oklahoma

Each row is one ZCTA and the counties its land falls in, largest share first. Counties in another state are marked with that state. Sort by the share outside the main county to put the most evenly divided ones first.

	Share of land by county
71937	Polk County, AR 93.8% McCurtain County 6.3%	6.3%	2
71953	Polk County, AR 75.3% Scott County, AR 19.5% Le Flore County 5.2%	24.7%	3
73002	Grady County 99.4% McClain County 0.6%	0.6%	2
73006	Caddo County 82.6% Comanche County 17.4%	17.4%	2
73007	Oklahoma County 84.0% Logan County 16.0%	16.0%	2
73010	Grady County 51.8% McClain County 48.2%	48.2%	2
73015	Caddo County 74.0% Kiowa County 14.6% Washita County 11.3%	25.9%	3
73016	Kingfisher County 62.8% Logan County 37.2%	37.2%	2
73017	Caddo County 48.5% Grady County 47.5% Comanche County 4.1%	51.5%	3
73020	Oklahoma County 94.5% Cleveland County 5.5%	5.5%	2
73021	Washita County 78.3% Caddo County 21.7%	21.7%	2
73025	Logan County 57.7% Oklahoma County 42.3%	42.3%	2
73027	Logan County 50.6% Payne County 49.4%	49.4%	2
73028	Logan County 79.2% Kingfisher County 20.8%	20.8%	2
73030	Murray County 89.1% Garvin County 10.9%	10.9%	2
73034	Oklahoma County 68.2% Logan County 31.8%	31.8%	2
73040	Blaine County 68.9% Canadian County 31.1%	31.1%	2
73041	Kiowa County 79.8% Washita County 20.2%	20.2%	2
73045	Oklahoma County 58.5% Lincoln County 39.5% Pottawatomie County 1.9%	41.5%	3
73047	Caddo County 56.0% Canadian County 44.0%	44.0%	2
73048	Caddo County 71.5% Blaine County 21.5% Custer County 7.0%	28.4%	3
73051	Cleveland County 99.5% Pottawatomie County 0.5%	0.5%	2
73052	Garvin County 51.7% McClain County 38.0% Grady County 10.3%	48.3%	3
73054	Oklahoma County 71.4% Logan County 19.4% Lincoln County 9.2%	28.6%	3
73055	Stephens County 74.6% Grady County 23.7% Comanche County 1.7%	25.4%	3
73056	Garfield County 52.5% Logan County 36.0% Kingfisher County 11.5%	47.5%	3
73057	Garvin County 66.5% McClain County 33.5%	33.5%	2
73059	Grady County 59.0% Caddo County 28.1% Canadian County 12.8%	41.0%	3
73061	Noble County 60.0% Pawnee County 40.0%	40.0%	2
73062	Kiowa County 75.3% Washita County 24.7%	24.7%	2
73063	Logan County 77.4% Payne County 22.6%	22.6%	2
73072	Cleveland County 66.3% McClain County 33.7%	33.7%	2
73073	Noble County 31.0% Logan County 26.1% Payne County 25.0% Garfield County 17.9%	69.0%	4
73074	Garvin County 78.3% McClain County 21.7%	21.7%	2
73078	Canadian County 99.8% Oklahoma County 0.2%	0.2%	2
73079	Grady County 79.2% Caddo County 20.8%	20.8%	2
73082	Grady County 97.5% Comanche County 2.5%	2.5%	2
73086	Murray County 98.4% Johnston County 1.6%	1.6%	2
73092	Grady County 54.8% Caddo County 45.2%	45.2%	2
73096	Custer County 69.5% Washita County 29.5% Blaine County 1.0%	30.5%	3
73098	Garvin County 81.1% Murray County 18.9%	18.9%	2
73127	Oklahoma County 74.8% Canadian County 25.2%	25.2%	2
73128	Oklahoma County 76.5% Canadian County 23.5%	23.5%	2
73135	Oklahoma County 97.1% Cleveland County 2.9%	2.9%	2
73139	Oklahoma County 67.5% Cleveland County 32.5%	32.5%	2
73149	Oklahoma County 93.1% Cleveland County 6.9%	6.9%	2
73150	Oklahoma County 98.1% Cleveland County 1.9%	1.9%	2
73159	Oklahoma County 68.6% Cleveland County 31.4%	31.4%	2
73169	Oklahoma County 60.7% Cleveland County 39.3%	39.3%	2
73179	Oklahoma County 83.0% Canadian County 17.0%	17.0%	2
73401	Carter County 98.0% Love County 1.7% Johnston County 0.3%	2.0%	3
73425	Stephens County 52.9% Carter County 47.1%	47.1%	2
73432	Johnston County 67.5% Atoka County 32.5%	32.5%	2
73434	Stephens County 55.2% Garvin County 44.8%	44.8%	2
73442	Stephens County 83.1% Jefferson County 16.9%	16.9%	2
73444	Carter County 86.2% Murray County 12.8% Garvin County 1.0%	13.8%	3
73446	Marshall County 99.7% Carter County 0.3%	0.3%	2
73453	Love County 98.8% Carter County 1.2%	1.2%	2
73456	Jefferson County 79.3% Love County 10.8% Carter County 9.9%	20.7%	3
73461	Johnston County 66.2% Atoka County 33.8%	33.8%	2
73463	Carter County 68.0% Love County 32.0%	32.0%	2
73481	Carter County 74.2% Stephens County 17.0% Garvin County 8.8%	25.8%	3
73526	Jackson County 85.2% Greer County 14.8%	14.8%	2
73528	Cotton County 47.2% Tillman County 39.8% Comanche County 13.0%	52.8%	3
73529	Stephens County 74.5% Jefferson County 23.8% Cotton County 1.7%	25.5%	3
73532	Jackson County 87.8% Harmon County 12.2%	12.2%	2
73533	Stephens County 99.7% Cotton County 0.3%	0.3%	2
73537	Jackson County 80.3% Harmon County 19.7%	19.7%	2
73540	Comanche County 96.7% Cotton County 3.3%	3.3%	2
73543	Comanche County 71.8% Cotton County 28.2%	28.2%	2
73546	Tillman County 92.0% Cotton County 8.0%	8.0%	2
73548	Cotton County 53.1% Jefferson County 41.4% Stephens County 5.5%	46.9%	3
73552	Comanche County 98.4% Kiowa County 1.6%	1.6%	2
73554	Greer County 85.5% Harmon County 13.4% Jackson County 1.0%	14.5%	3
73559	Kiowa County 84.6% Comanche County 15.4%	15.4%	2
73566	Kiowa County 57.5% Tillman County 42.5%	42.5%	2
73572	Cotton County 96.2% Comanche County 3.8%	3.8%	2
73573	Jefferson County 99.7% Stephens County 0.3%	0.3%	2
73601	Custer County 73.6% Washita County 26.4%	26.4%	2
73626	Washita County 60.0% Custer County 40.0%	40.0%	2
73627	Beckham County 98.1% Washita County 1.9%	1.9%	2
73644	Beckham County 80.3% Washita County 14.9% Roger Mills County 2.8% Custer County 2.1%	19.8%	4
73646	Blaine County 61.3% Dewey County 29.3% Custer County 9.4%	38.7%	3
73647	Washita County 59.7% Custer County 40.3%	40.3%	2
73650	Roger Mills County 69.3% Custer County 30.7%	30.7%	2
73654	Roger Mills County 49.7% Dewey County 35.9% Custer County 14.4%	50.3%	3
73655	Kiowa County 97.2% Washita County 2.8%	2.8%	2
73658	Dewey County 95.2% Blaine County 4.8%	4.8%	2
73662	Beckham County 90.2% Roger Mills County 9.8%	9.8%	2
73663	Dewey County 94.5% Major County 3.3% Woodward County 2.2%	5.5%	3
73666	Roger Mills County 53.2% Beckham County 46.8%	46.8%	2
73669	Custer County 75.9% Dewey County 18.4% Blaine County 5.6%	24.1%	3
73673	Greer County 91.1% Beckham County 8.9%	8.9%	2
73703	Garfield County 96.5% Grant County 3.5%	3.5%	2
73716	Woods County 57.4% Alfalfa County 34.4% Major County 8.2%	42.6%	3
73717	Woods County 94.5% Alfalfa County 5.5%	5.5%	2
73718	Major County 76.3% Kingfisher County 16.7% Garfield County 7.0%	23.7%	3
73724	Dewey County 57.7% Blaine County 42.3%	42.3%	2
73726	Alfalfa County 60.1% Woods County 39.9%	39.9%	2
73729	Major County 84.1% Woods County 15.9%	15.9%	2
73735	Garfield County 96.3% Major County 3.7%	3.7%	2
73739	Alfalfa County 76.9% Garfield County 23.1%	23.1%	2
73744	Blaine County 95.3% Kingfisher County 4.7%	4.7%	2
73754	Garfield County 62.0% Major County 36.0% Alfalfa County 2.0%	38.0%	3
73755	Blaine County 56.7% Major County 29.3% Dewey County 14.0%	43.3%	3
73757	Noble County 73.3% Garfield County 26.8%	26.8%	2
73758	Grant County 59.5% Alfalfa County 40.5%	40.5%	2
73760	Major County 95.2% Alfalfa County 4.8%	4.8%	2
73761	Grant County 90.0% Garfield County 10.0%	10.0%	2
73762	Canadian County 55.7% Kingfisher County 44.3%	44.3%	2
73763	Blaine County 74.8% Kingfisher County 13.4% Major County 11.8%	25.2%	3
73764	Kingfisher County 58.2% Blaine County 41.8%	41.8%	2
73768	Major County 98.2% Alfalfa County 1.8%	1.8%	2
73801	Woodward County 78.3% Harper County 21.7%	21.7%	2
73832	Ellis County 92.5% Woodward County 7.5%	7.5%	2
73838	Major County 82.7% Woodward County 17.3%	17.3%	2
73840	Ellis County 74.4% Woodward County 25.6%	25.6%	2
73841	Ellis County 59.4% Woodward County 40.6%	40.6%	2
73842	Woods County 66.8% Woodward County 24.6% Harper County 8.6%	33.1%	3
73843	Ellis County 93.8% Beaver County 6.2%	6.2%	2
73844	Beaver County 89.3% Harper County 10.7%	10.7%	2
73848	Beaver County 56.7% Harper County 43.3%	43.3%	2
73851	Ellis County 69.2% Harper County 30.8%	30.8%	2
73852	Woodward County 98.4% Major County 1.6%	1.6%	2
73853	Woodward County 98.7% Dewey County 1.3%	1.3%	2
73859	Dewey County 64.2% Woodward County 24.4% Ellis County 11.4%	35.8%	3
73860	Woods County 61.4% Major County 38.6%	38.6%	2
73947	Cimarron County 92.9% Texas County 7.1%	7.1%	2
73949	Texas County 59.0% Cimarron County 27.3% Sherman County, TX 13.7%	41.0%	3
73950	Beaver County 80.1% Texas County 19.9%	19.9%	2
74003	Osage County 84.7% Washington County 15.3%	15.3%	2
74008	Tulsa County 95.8% Wagoner County 4.2%	4.2%	2
74010	Creek County 99.2% Okfuskee County 0.8%	0.8%	2
74014	Wagoner County 99.9% Tulsa County under 0.1%	under 0.1%	2
74015	Rogers County 69.1% Wagoner County 30.7% Tulsa County 0.2%	30.9%	3
74016	Rogers County 61.5% Mayes County 15.9% Nowata County 11.9% Craig County 10.7%	38.5%	4
74021	Tulsa County 66.2% Rogers County 18.9% Washington County 14.9%	33.8%	3
74022	Washington County 55.2% Osage County 44.8%	44.8%	2
74023	Payne County 58.9% Lincoln County 41.1%	41.1%	2
74028	Creek County 99.4% Lincoln County 0.6%	0.6%	2
74030	Creek County 91.7% Payne County 8.3%	8.3%	2
74032	Payne County 71.3% Pawnee County 19.5% Noble County 9.2%	28.7%	3
74036	Rogers County 80.8% Mayes County 10.6% Wagoner County 8.5%	19.1%	3
74038	Creek County 51.5% Pawnee County 48.5%	48.5%	2
74044	Creek County 92.8% Pawnee County 7.2%	7.2%	2
74045	Pawnee County 99.7% Payne County 0.3%	0.3%	2
74047	Okmulgee County 55.9% Tulsa County 24.5% Creek County 19.6%	44.1%	3
74048	Nowata County 99.6% Washington County 0.4%	0.4%	2
74050	Creek County 90.4% Tulsa County 9.6%	9.6%	2
74051	Washington County 90.1% Osage County 9.9%	9.9%	2
74055	Tulsa County 54.5% Rogers County 45.5%	45.5%	2
74058	Pawnee County 100.0% Osage County under 0.1%	under 0.1%	2
74059	Payne County 63.3% Lincoln County 28.8% Logan County 7.9%	36.7%	3
74063	Tulsa County 49.1% Osage County 41.4% Creek County 9.5%	50.9%	3
74070	Osage County 74.2% Tulsa County 17.2% Washington County 8.6%	25.8%	3
74072	Nowata County 88.0% Craig County 12.0%	12.0%	2
74073	Osage County 57.7% Tulsa County 42.3%	42.3%	2
74075	Payne County 88.2% Noble County 11.8%	11.8%	2
74079	Lincoln County 69.5% Creek County 29.3% Okfuskee County 1.1%	30.5%	3
74080	Rogers County 87.1% Washington County 12.9%	12.9%	2
74081	Pawnee County 68.5% Creek County 31.5%	31.5%	2
74083	Nowata County 69.7% Washington County 30.3%	30.3%	2
74085	Payne County 86.9% Pawnee County 8.3% Creek County 4.8%	13.1%	3
74106	Tulsa County 99.2% Osage County 0.8%	0.8%	2
74108	Tulsa County 86.1% Wagoner County 13.9%	13.9%	2
74116	Tulsa County 80.7% Rogers County 19.3%	19.3%	2
74126	Osage County 74.2% Tulsa County 25.8%	25.8%	2
74127	Osage County 73.0% Tulsa County 27.0%	27.0%	2
74131	Creek County 99.9% Tulsa County 0.1%	0.1%	2
74132	Tulsa County 74.1% Creek County 25.9%	25.9%	2
74301	Craig County 93.9% Mayes County 3.6% Nowata County 2.5%	6.1%	3
74330	Mayes County 94.9% Craig County 5.1%	5.1%	2
74331	Delaware County 50.6% Ottawa County 32.8% Craig County 16.6%	49.4%	3
74332	Craig County 44.1% Mayes County 35.1% Rogers County 20.8%	55.9%	3
74333	Craig County 88.0% Ottawa County 12.0%	12.0%	2
74337	Mayes County 89.9% Wagoner County 10.1%	10.1%	2
74338	Delaware County 99.7% Adair County 0.3%	0.3%	2
74342	Delaware County 96.8% Mayes County 3.2%	3.2%	2
74344	Delaware County 97.3% Ottawa County 2.6%	2.6%	2
74347	Delaware County 79.6% Adair County 12.5% Cherokee County 7.9%	20.4%	3
74349	Craig County 81.8% Mayes County 18.1%	18.1%	2
74352	Mayes County 80.7% Wagoner County 13.1% Cherokee County 6.2%	19.3%	3
74354	Ottawa County 96.8% Craig County 3.2%	3.2%	2
74359	Delaware County 94.7% Cherokee County 5.3%	5.3%	2
74361	Mayes County 100.0% Rogers County under 0.1%	under 0.1%	2
74364	Delaware County 66.0% Cherokee County 21.1% Mayes County 12.9%	34.0%	3
74365	Mayes County 72.9% Delaware County 27.1%	27.1%	2
74366	Mayes County 89.6% Delaware County 10.4%	10.4%	2
74370	Ottawa County 93.1% Delaware County 6.9%	6.9%	2
74403	Muskogee County 95.0% Wagoner County 5.0%	5.0%	2
74421	Okmulgee County 98.7% Creek County 1.3%	1.3%	2
74422	Muskogee County 57.0% Okmulgee County 43.0%	43.0%	2
74426	McIntosh County 99.8% Muskogee County 0.2%	0.2%	2
74428	McIntosh County 52.5% Muskogee County 47.5%	47.5%	2
74432	McIntosh County 80.0% Pittsburg County 19.9%	19.9%	2
74434	Muskogee County 62.0% Cherokee County 26.5% Wagoner County 11.5%	38.0%	3
74435	Sequoyah County 74.2% Muskogee County 25.8%	25.8%	2
74436	Muskogee County 64.1% Wagoner County 20.8% Okmulgee County 15.1%	35.9%	3
74437	Okmulgee County 68.0% McIntosh County 25.5% Okfuskee County 6.5%	32.0%	3
74445	Okmulgee County 97.7% McIntosh County 2.3%	2.3%	2
74452	Cherokee County 91.8% Mayes County 8.2%	8.2%	2
74455	Muskogee County 89.9% McIntosh County 10.1%	10.1%	2
74457	Cherokee County 61.8% Adair County 38.2%	38.2%	2
74462	Haskell County 96.6% Pittsburg County 3.4%	3.4%	2
74469	Muskogee County 88.4% McIntosh County 11.6%	11.6%	2
74523	Pushmataha County 92.7% Choctaw County 5.6% Atoka County 1.7%	7.3%	3
74525	Atoka County 99.2% Coal County 0.8%	0.8%	2
74530	Johnston County 65.0% Coal County 35.0%	35.0%	2
74531	Hughes County 94.7% Coal County 5.3%	5.3%	2
74536	Pushmataha County 90.0% Latimer County 6.6% Pittsburg County 3.4%	10.0%	3
74538	Coal County 99.1% Atoka County 0.9%	0.9%	2
74540	Atoka County 79.6% Pushmataha County 20.4%	20.4%	2
74547	Pittsburg County 96.2% Latimer County 3.8%	3.8%	2
74549	Le Flore County 65.6% Pushmataha County 34.4%	34.4%	2
74552	Haskell County 99.2% Pittsburg County 0.8%	0.8%	2
74553	Pittsburg County 70.1% Atoka County 29.9%	29.9%	2
74560	Pittsburg County 79.6% Atoka County 20.4%	20.4%	2
74561	Pittsburg County 75.8% Haskell County 24.3%	24.3%	2
74563	Latimer County 93.7% Le Flore County 6.3%	6.3%	2
74570	Hughes County 59.5% Pittsburg County 40.5%	40.5%	2
74571	Latimer County 42.9% Le Flore County 41.4% Pushmataha County 15.7%	57.1%	3
74572	Coal County 83.2% Pontotoc County 16.8%	16.8%	2
74574	Pushmataha County 55.9% Latimer County 44.1%	44.1%	2
74576	Atoka County 54.9% Pittsburg County 45.1%	45.1%	2
74601	Kay County 95.7% Noble County 4.3%	4.3%	2
74604	Osage County 50.0% Kay County 50.0%	50.0%	2
74630	Noble County 73.3% Garfield County 26.7%	26.7%	2
74640	Garfield County 74.2% Grant County 25.8%	25.8%	2
74643	Grant County 93.1% Kay County 6.9%	6.9%	2
74646	Kay County 76.9% Grant County 23.1%	23.1%	2
74650	Osage County 76.0% Pawnee County 24.0%	24.0%	2
74651	Noble County 96.0% Pawnee County 4.0%	4.0%	2
74652	Osage County 90.7% Kay County 9.3%	9.3%	2
74653	Kay County 90.4% Noble County 9.6%	9.6%	2
74727	Choctaw County 76.5% Bryan County 13.4% Atoka County 10.1%	23.6%	3
74728	McCurtain County 89.3% Pushmataha County 10.7%	10.7%	2
74729	Bryan County 65.0% Atoka County 35.0%	35.0%	2
74735	Choctaw County 65.9% Pushmataha County 34.1%	34.1%	2
74748	Johnston County 46.2% Atoka County 35.3% Bryan County 18.5%	53.8%	3
74754	McCurtain County 62.0% Pushmataha County 35.1% Choctaw County 2.9%	38.0%	3
74755	McCurtain County 65.5% Choctaw County 34.5%	34.5%	2
74760	Pushmataha County 54.9% Choctaw County 45.1%	45.1%	2
74764	McCurtain County 93.0% Choctaw County 7.0%	7.0%	2
74820	Pontotoc County 100.0% Pottawatomie County under 0.1%	under 0.1%	2
74824	Lincoln County 91.6% Payne County 8.4%	8.4%	2
74825	Pontotoc County 47.8% Hughes County 33.2% Coal County 19.0%	52.2%	3
74831	McClain County 86.9% Pontotoc County 7.4% Garvin County 5.8%	13.1%	3
74839	Hughes County 66.8% Okfuskee County 18.2% McIntosh County 14.9%	33.2%	3
74840	Seminole County 58.7% Pottawatomie County 41.3%	41.3%	2
74848	Hughes County 97.4% Seminole County 2.6%	2.6%	2
74849	Seminole County 71.1% Pottawatomie County 28.9%	28.9%	2
74851	Pottawatomie County 58.3% Lincoln County 34.2% Cleveland County 7.5%	41.7%	3
74852	Pottawatomie County 98.2% Cleveland County 1.8%	1.8%	2
74854	Pottawatomie County 50.0% Seminole County 50.0%	50.0%	2
74855	Lincoln County 90.3% Pottawatomie County 9.8%	9.8%	2
74856	Johnston County 83.6% Murray County 16.4%	16.4%	2
74857	Cleveland County 70.0% Oklahoma County 26.4% Pottawatomie County 3.6%	30.0%	3
74859	Okfuskee County 88.6% Seminole County 11.4%	11.4%	2
74864	Lincoln County 61.9% Pottawatomie County 36.4% Okfuskee County 1.6%	38.1%	3
74865	Pontotoc County 88.4% Murray County 9.0% Garvin County 2.5%	11.6%	3
74867	Seminole County 87.7% Hughes County 12.3%	12.3%	2
74871	Pontotoc County 67.9% Coal County 17.6% Johnston County 14.5%	32.1%	3
74872	Garvin County 73.2% Pontotoc County 24.1% McClain County 2.7%	26.8%	3
74878	Pottawatomie County 93.8% Cleveland County 6.2%	6.2%	2
74880	Okfuskee County 91.4% Okmulgee County 8.6%	8.6%	2
74881	Lincoln County 87.6% Logan County 12.4%	12.4%	2
74883	Hughes County 83.5% Okfuskee County 16.5%	16.5%	2
74884	Seminole County 98.2% Hughes County 1.8%	1.8%	2
74931	Adair County 67.7% Cherokee County 25.3% Sequoyah County 7.0%	32.3%	3
74941	Haskell County 74.3% Le Flore County 25.7%	25.7%	2
74944	Haskell County 82.1% Le Flore County 17.9%	17.9%	2
74948	Sequoyah County 97.6% Adair County 2.4%	2.4%	2
74957	Le Flore County 68.1% McCurtain County 31.9%	31.9%	2
74960	Adair County 95.7% Cherokee County 4.3%	4.3%	2
74962	Sequoyah County 97.2% Cherokee County 2.8%	2.8%	2
74964	Adair County 93.3% Delaware County 5.8% Cherokee County 0.9%	6.7%	3
74966	Le Flore County 92.8% Latimer County 7.2%	7.2%	2

What to do with this

If a ZIP code in your list is split, the ZIP alone cannot tell you which county an address is in. For most work that is fine. It matters when the county decides something: a sales tax rate, a licence, a franchise agreement written by county, or a service area somebody promised by county name.

The ZIP list audit checks your own list against the same Census data in your browser, and the ZIP spreadsheet comparison covers when a ZIP list is enough and when it is not.

What these numbers are, and are not

A ZCTA is not quite a ZIP code. ZIP Code Tabulation Areas are the Census Bureau's approximations of ZIP code areas, built by grouping census blocks by the ZIP codes of the addresses in them. USPS ZIP codes are delivery routes, not areas, and the two do not always match.
Some ZIP codes have no ZCTA at all. PO box ZIPs, ZIPs for a single building or a single large organisation, and military ZIPs usually have no area of their own, so they cannot be split across counties here and will not match any area in a map.
Land share is not population share. A ZCTA with 20% of its land in a second county might have almost none of its people there, or most of them. The shares here measure area, which is what the Census file publishes.
Tiny overlaps are included. A ZCTA counts as overlapping a county if the Census file lists any part of it there, including slivers along a road or river. That is why the share with at least 5% of land in a second county is shown alongside.

Source: U.S. Census Bureau, 2020 ZCTA to County relationship file, public domain. Counts use 2020 ZCTAs and 2020 county boundaries. ZIP codes added by USPS since then are not in it, and Connecticut's counties have since been replaced by planning regions in Census data.

Route by the boundary, not the ZIP

MapLogics decides who owns an address from a boundary you draw or import, so a ZIP that straddles two counties is answered by where the address actually is. There is a free trial if you want to see it on your own addresses.

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