Alexander "Sandy" Calder was born in 1898 in Lawnton, Pennsylvania. His actual birthday, however, remains a source of much confusion. According to Calder's mother, Nanette (née Lederer), Calder was born on August 22, yet his birth certificate at Philadelphia City Hall, based on a hand-written ledger, stated July 22. When Calder's family learned about the birth certificate, they reasserted with certainty that city officials had made a mistake.
Calder's grandfather, sculptor Alexander Milne Calder, was born in Scotland, had immigrated to Philadelphia in 1868, and is best known for the colossal statue of William Penn on top of Philadelphia City Hall's tower. His father, Alexander Stirling Calder, was a well-known sculptor who created many public installations, a majority of them in nearby Philadelphia.
Calder's mother was a professional portrait artist, who had studied at the Académie Julian and the Sorbonne in Paris from around 1888 until 1893. She moved to Philadelphia, where she met Stirling Calder while studying at the Pennsylvania Academy of the Fine Arts. Calder's parents married on February 22, 1895; his sister, Mrs. Margaret Calder Hayes, was instrumental in the development of the UC Berkeley Art Museum.
Constructing objects from a very young age, his first known art tool was a pair of pliers. At eight, Calder was creating jewelry for his sister's dolls from beads and copper wire. Over the next few years, as his family moved to Pasadena, Philadelphia, New York, and San Francisco, he crafted small animal figures and game boards from scavenged wood and brass.