Sum of Series Calculator Sum 1..n. n Sum 5,050 Step-by-step with your numbers: 1. Values used: 2. n = 100 3. Sum = 5,050 Did we solve your problem today? 👍 Yes 👎 No Thanks for the feedback! n(n+1)/2. FAQ Squares? n(n+1)(2n+1)/6. Related calculators Triangular Number Calculator Find the nth triangular number. Arithmetic Sequence Calculator Find the nth term and sum of an arithmetic sequence.