Geometric Sequence Calculator
Enter the first term and the common ratio to get the n-th term, the sum of the first n terms and, when |r| < 1, the sum of the infinite series. You can also find r and a₁ from two known terms or check whether a value is in the sequence.
Fractions like 0.5 or negative ratios like −2 work too
How to use the Geometric Sequence Calculator
- Choose what you know: a₁ and r, two terms, or a value to find.
- Enter the numbers, for example a₁ = 2 and r = 3.
- Enter n, the number of terms.
- Press Calculate to see the n-th term, the sum and the infinite sum.
- Check the steps, table and chart, or copy the results.
Geometric sequence formulas
| What | Formula |
|---|---|
| n-th term (explicit) | aₙ = a₁ · r^(n − 1) |
| Recursive form | aₙ = r · aₙ₋₁ |
| Common ratio | r = aₙ / aₙ₋₁ = (a_q / aₚ)^(1/(q − p)) |
| Sum of n terms (r ≠ 1) | Sₙ = a₁(1 − rⁿ)/(1 − r) |
| Sum of n terms (r = 1) | Sₙ = n · a₁ |
| Infinite sum (|r| < 1) | S∞ = a₁/(1 − r) |
| Position of a value x | n = log(x/a₁)/log(r) + 1 |
Examples
| Sequence | a₁ | r | a₁₀ | S₁₀ | S∞ |
|---|---|---|---|---|---|
| 1, 2, 4, 8, … | 1 | 2 | 512 | 1,023 | diverges |
| 2, 6, 18, 54, … | 2 | 3 | 39,366 | 59,048 | diverges |
| 1/2, 1/4, 1/8, … | 0.5 | 0.5 | 0.000977 | 0.999023 | 1 |
| 9, 3, 1, 1/3, … | 9 | 1/3 | 0.000457 | 13.499771 | 13.5 |
| 8, −4, 2, −1, … | 8 | −0.5 | −0.015625 | 5.328125 | 5.333333 |
| 100, 105, 110.25, … (5% growth) | 100 | 1.05 | 155.132822 | 1,257.789254 | diverges |
Why the infinite sum works
Since Sₙ = a₁(1 − rⁿ)/(1 − r), the only part that changes with n is rⁿ. When |r| < 1, rⁿ shrinks to 0 as n grows, leaving S∞ = a₁/(1 − r). This is why 0.999… = 0.9/(1 − 0.1) = 1, and why a bouncing ball that returns to a fixed fraction of its height travels a finite total distance.
Frequently asked questions
What is a geometric sequence?
A list of numbers where each term is the previous one multiplied by a fixed number, the common ratio r. For example 2, 6, 18, 54, … has a₁ = 2 and r = 3.
What is the formula for the n-th term?
aₙ = a₁ · r^(n − 1). For 2, 6, 18, … the 8th term is 2 × 3⁷ = 4,374.
How do you find the sum of a geometric sequence?
For r ≠ 1, Sₙ = a₁(1 − rⁿ)/(1 − r). For 2, 6, 18, … the sum of 8 terms is 2 × (1 − 3⁸)/(1 − 3) = 6,560. If r = 1, the sum is simply n · a₁.
When does an infinite geometric series have a sum?
Only when |r| < 1. Then the terms shrink toward 0 and S∞ = a₁/(1 − r). For example 1/2 + 1/4 + 1/8 + … = 0.5/(1 − 0.5) = 1. If |r| ≥ 1 the series diverges.
How do I find the common ratio from two terms?
r = (a_q / aₚ)^(1/(q − p)). With a₂ = 6 and a₅ = 162, r = 27^(1/3) = 3. When q − p is even, −r works as well, and if a_q / aₚ is negative with an even gap there is no real ratio.
How do I find which term a value is?
Solve a₁ · r^(n − 1) = x, giving n = log(x/a₁)/log(r) + 1. The value is a term only if n is a positive whole number. For 2, 6, 18, … the value 1,458 gives n = 7.
How are geometric sequences related to compound interest?
A balance that grows by a fixed percentage each period forms a geometric sequence with r = 1 + rate. At 5% a year, r = 1.05, so after n years the balance is P · 1.05ⁿ.