During which two-year period did Isaac Newton develop calculus?

Isaac Newton developed calculus during 1665–1666, the remarkable period often called his annus mirabilis. Newton’s work began at Cambridge and continued during the university’s closure amid the Great Plague, when he returned to Woolsthorpe in Lincolnshire.

He called his method “fluxions.” By 1665, Newton’s manuscripts show that he could calculate tangents and curvature, while also connecting rates of change with areas under curves. He developed these ideas largely through private study, alongside major work in optics and gravitation.

The date can sometimes appear as 1664–1666 because Newton’s earliest mathematical investigations began in 1664. However, 1665–1666 is the standard two-year period used for the main development of his calculus. Newton kept the work private for years, and Gottfried Wilhelm Leibniz later developed calculus independently, publishing his version first. Their priority dispute became one of the most famous controversies in mathematical history.