The summation calculator (Σ, sigma) computes the total of a sequence of numbers defined by a starting and ending index or a mathematical formula. Summation notation is a fundamental concept in mathematics, statistics, and finance — used to calculate everything from series of payments to polynomial approximations. This calculator supports four modes: a simple integer range sum, an arithmetic series (where each term increases by a fixed amount), a geometric series (where each term is multiplied by a constant ratio), and a fully custom formula in n so you can compute Σ n², Σ 1/n, or any expression you define. The term sequence is displayed so you can verify each individual value before the total.