State Route 21 is a 278 miles (447.4 km) state highway

State highway

State highway, state road or state route can refer to one of three related concepts, two of them related to a state or provincial government in a country that is divided into states or provinces :#A...

 that extends from Piedmont

Piedmont, Alabama

Piedmont is a city in Calhoun and Cherokee counties in the U.S. state of Alabama. The population was 5,120 at the 2000 census. It is included in the Anniston-Oxford, Alabama Metropolitan Statistical Area. Many surrounding communities utilize the 36272 ZIP code, including Spring Garden, Rock Run,...

 in Calhoun County

Calhoun County, Alabama

Calhoun County is a county in the U.S. state of Alabama. Its name is in honor of John C. Calhoun, famous member of the United States Senate from South Carolina. As of 2010 the population was 118,572. It is included in the Anniston-Oxford Metropolitan Statistical Area...

 to the Florida

Florida

Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...

 state line near Atmore

Atmore, Alabama

Atmore is a city in Escambia County, Alabama, United States. At the 2000 census the population was 7,676. According to the 2005 U.S. Census estimates, the city had a population of 7,530. The city was named for Mr. C.P. Atmore....

 in Escambia County

Escambia County, Alabama

-2010:Whereas according to the 2010 U.S. Census Bureau:*62.1% White*31.9% Black*4.4% Native American*0.2% Asian*0.0% Native Hawaiian or Pacific Islander*1.5% Two or more races*1.9% Hispanic or Latino -2000:...

. The route runs almost the entire length of the state from the northeast to the southwest.
Starting at a junction with SR-9 in Piedmont, the route runs southwest through Anniston

Anniston, Alabama

Anniston is a city in Calhoun County in the state of Alabama, United States.As of the 2000 census, the population of the city is 24,276. According to the 2005 U.S. Census estimates, the city had a population of 23,741...

, Talledega

Talladega, Alabama

Talladega is a city in Talladega County, Alabama, United States. At the 2000 census the population was 15,143. The city is the county seat of Talladega County. Talladega is approximately 50 miles east of Birmingham, Alabama....

 and Sylacauga

Sylacauga, Alabama

Sylacauga is a city in Talladega County, Alabama, United States. At the 2000 census the population was 12,616.Nicknames for Sylacauga include: "The Marble City," "Buzzard's Roost" and "Sly Town"....

. The route passes close to the Talladega Superspeedway

Talladega Superspeedway

Talladega Superspeedway is a motorsports complex located north of Talladega, Alabama, United States. It is located on the former Anniston Air Force Base just outside the small city of Lincoln. It was constructed by International Speedway Corporation, a business controlled by the France Family, in...

 and the Talladega National Forest

Talladega National Forest

The Talladega National Forest is located in the U.S. state of Alabama and covers 392,567 acres at the southern edge of the Appalachian Mountains....

. After Sylacauga the road is co-routed with U.S.