Arithmetic Sequence Calculator — nth Term, Sum & Common Difference

A focused arithmetic sequence and arithmetic series calculator. Enter the first term a, the common difference d, and a count n, and read two answers at once: the nth term aₙ = a + (n-1)·d and the sum of the first n terms Sₙ = n(2a + (n-1)d)/2, which is the same as n(a + aₙ)/2. The tool lists the actual terms a, a+d, a+2d, a+3d, … so you can sanity-check the pattern by eye, and a one-click copy puts the whole sequence on your clipboard. A built-in reverse solver recovers the common difference from any two known terms and how many positions apart they sit, so if you only know the 1st term is 2 and the 4th is 11 it returns d = 3. Negative differences (a decreasing run like 20, 17, 14, 11) and fractional differences are handled, and a constant sequence with d = 0 still sums correctly to n·a. Everything runs locally in the browser with a shareable link that reopens the exact a, d and n you typed.