---
title: Compound interest — what it means and why it matters | Allocate AI
description: Growth on top of growth: earnings are added to the balance, and then those earnings earn too, so money grows faster the longer it is left alone.
canonical: https://allocateai.io/glossary/compound-interest
site: Allocate AI — https://allocateai.io
---

# Compound interest

Growth on top of growth: earnings are added to the balance, and then those earnings earn too, so money grows faster the longer it is left alone.

Compound interest is what happens when the returns on money are reinvested and start producing returns of their own. Simple interest on $10,000 at 7% is $700 every year forever. Compound interest pays $700 the first year, then 7% of $10,700 the second year, then 7% of $11,449, and so on. The difference is small at first and enormous later.

After 10 years at 7%, $10,000 becomes about $19,700. After 20 years, $38,700. After 30 years, $76,100. After 40, $149,700. The first decade added under $10,000; the fourth added over $73,000. Nothing changed except time.

A quick shortcut is the rule of 72: divide 72 by the annual return to get the number of years to double. At 7%, money doubles roughly every 10 years. At 10%, every 7 years. At 3%, every 24.

The same mechanism works against you with debt. A credit card at 24% doubles an unpaid balance in about three years, which is why paying it off is one of the highest-return moves available.

The common mistake is waiting for a bigger income before starting. $300 a month from age 25 at 7% grows to about $790,000 by 65. Starting at 35, it reaches about $370,000. The ten-year delay costs more than $400,000, far more than the $36,000 that was not contributed. Time is the ingredient that cannot be bought back.

[Try the Compound Interest Calculator →](https://allocateai.io/calculators/compound-interest)

## Related terms

- [Time value of money](https://allocateai.io/glossary/time-value-of-money) — The idea that a dollar today is worth more than a dollar later, because today's dollar can be invested and grow in the meantime.
- [APY vs APR](https://allocateai.io/glossary/apy-vs-apr) — Two ways of quoting a yearly rate: APR ignores compounding and is used for loans, while APY includes it and is used for savings.
- [Index fund](https://allocateai.io/glossary/index-fund) — A mutual fund or ETF that holds every stock in a market index like the S&P 500, so you own a slice of the whole market at very low cost.
- [Dollar-cost averaging](https://allocateai.io/glossary/dollar-cost-averaging) — Investing a fixed amount on a regular schedule regardless of price, which spreads out timing risk and removes the need to guess when to buy.

**Plan in plain language.**

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Allocate AI is a budgeting and financial-planning app: link your accounts, say what you want in plain words, and it writes the plan and stages every change for your approval. Read more at https://allocateai.io or see all pages in https://allocateai.io/llms.txt.