Geometric Series Sum Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
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Enter Values
Enter your values and press Calculate to see results.
How to use this calculator
- Enter the first term, common ratio and number of terms.
- Read the last term and the sum.
How it works
Each term multiplies by the common ratio. Last term = a·r^(n−1); sum = a·(1 − r^n) / (1 − r) when r ≠ 1.
Worked example
a=3, r=2, n=5. Terms 3, 6, 12, 24, 48 — sum 93.
Common mistakes
- Using a difference instead of a ratio — that's an arithmetic series.
- Expecting a finite sum for |r| ≥ 1 over infinite terms (only |r| < 1 converges).
Frequently asked questions
What if the ratio is 1?
Then every term equals the first, so the sum is simply a × n.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.