Geometric Sequence Calculator

Find the nth term, the partial sum, and (when it converges) the infinite sum of a geometric sequence.

nth term (aₙ)
Sum of n terms (Sₙ)
Infinite sum (|r| < 1)

How to Work With a Geometric Sequence

Enter the first term, common ratio and term count.

1

Enter the sequence

a₁ is the first term, r is the constant factor each term is multiplied by, and n is how many terms.

2

Read the nth term

aₙ = a₁·r⁽ⁿ⁻¹⁾ gives the nth term.

3

Read the sums

The partial sum uses Sₙ = a₁(rⁿ − 1) ⁄ (r − 1); the infinite sum a₁ ⁄ (1 − r) exists only when |r| < 1.

Frequently Asked Questions

What is a geometric sequence?
A geometric sequence is a list of numbers in which each term is the previous one multiplied by a fixed number, the common ratio r. In 3, 6, 12, 24 the common ratio is r = 2, because every step doubles. A ratio between 0 and 1 shrinks the terms, as in 80, 40, 20, 10 with r = 0.5, and a negative ratio makes the signs alternate. Only the first term and the ratio are needed to generate the whole sequence.
What is the formula for the nth term?
The nth term of a geometric sequence is aₙ = a₁·r⁽ⁿ⁻¹⁾, where a₁ is the first term and r is the common ratio. For 3, 6, 12, … with a₁ = 3 and r = 2, the 8th term is 3 × 2⁷ = 384. The exponent is n − 1 rather than n because the first term has been multiplied by r zero times. The sum of the first n terms is Sₙ = a₁(1 − rⁿ) ⁄ (1 − r) whenever r is not 1.
When does the infinite sum exist?
The infinite geometric series converges to a₁ ⁄ (1 − r) only when the absolute value of r is below 1. With a₁ = 1 and r = 0.5 the sum is 1 ⁄ 0.5 = 2, which matches 1 + 0.5 + 0.25 + 0.125 and so on. If the absolute value of r is 1 or more the terms never shrink toward zero, so the partial sums grow without bound and the series diverges. The case r = 1 fails outright, because the formula would divide by zero.
Is my data sent anywhere?
Nothing you type is sent anywhere. The geometric sequence calculator runs entirely in your browser with JavaScript, so your first term, ratio and term count never leave your device. There is no account, no upload step and nothing is logged. The page keeps working offline once it has loaded.