ToolVaults

Compound Interest Calculator

Project long-term investment growth with compound interest and regular contributions. Choose your compounding frequency and contribution timing.

Runs 100% in your browser — nothing is uploaded.
Final balance
$300,851
Total contributions
$130,000
Total interest earned
$170,851
Year-by-year
YearBalanceContributionsInterest
0$10,000$10,000$0
1$16,919$16,000$919
2$24,339$22,000$2,339
3$32,294$28,000$4,294
4$40,825$34,000$6,825
5$49,973$40,000$9,973
6$59,782$46,000$13,782
7$70,299$52,000$18,299
8$81,578$58,000$23,578
9$93,671$64,000$29,671
10$106,639$70,000$36,639
11$120,544$76,000$44,544
12$135,455$82,000$53,455
13$151,443$88,000$63,443
14$168,587$94,000$74,587
15$186,971$100,000$86,971
16$206,683$106,000$100,683
17$227,820$112,000$115,820
18$250,486$118,000$132,486
19$274,790$124,000$150,790
20$300,851$130,000$170,851

How to use

  1. 1
    Enter starting amount

    The lump sum you begin with, if any.

  2. 2
    Enter monthly contribution

    What you'll add every month. Even a small amount compounds meaningfully over decades.

  3. 3
    Enter annual rate and years

    Rate is the expected return (7% is a common long-term stock market assumption). Years is the horizon.

  4. 4
    Read the projected balance

    Final value + how much of it was contributions vs interest earned.

Examples

$0 start · $500/mo · 7% · 30 years
Input
Start $0 · $500/mo · 7% · 30 years
Output
Final: $566,764 · Contributed: $180,000 · Interest: $386,764
Classic retirement savings scenario. Two-thirds of the final balance is interest, not contributions.
$10,000 start · $0/mo · 5% · 20 years
Input
Start $10,000 · $0/mo · 5% · 20 years
Output
Final: $26,533 · Interest: $16,533
A one-time deposit left alone. Rule-of-72 shortcut: at 5%, money doubles every ~14 years.
Start early: $200/mo for 40 years
Input
Start $0 · $200/mo · 7% · 40 years
Output
Final: $525,454 · Contributed: $96,000 · Interest: $429,454
Fewer dollars in, more dollars out — because time in the market beats dollar amount.

Frequently asked

What is compound interest?

Interest earned on both the original principal AND on the interest previously earned. It's why "time in the market" beats "timing the market" — small differences in early years compound into large differences over decades.

Does compounding frequency really matter?

Marginally. Going from annual to monthly compounding on 7% for 30 years changes the final balance by less than 1%. Daily vs monthly is basically noise. Focus on the rate and contribution rate instead.

Start-of-period vs end-of-period contributions?

Start-of-period contributes at the beginning of each compounding interval (called "annuity due"), so it earns interest for one extra period. Slightly higher final balance. Most tools default to end-of-period.

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