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Interest Calculator

Calculate compound interest on an initial investment plus regular contributions. See the ending balance, total interest, the interest split, and an inflation-adjusted buying power.

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$
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Contribute at the

of each compounding period. Beginning-of-period contributions earn one extra period of interest.

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%

Applied to the interest earned each compounding period.

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Used only to compute the buying power of the ending balance.

Ending Balance

$54,535.20

Total Principal
$45,000.00
Total Contributions
$25,000.00
Total Interest
$9,535.20
Interest on Initial Investment
$5,525.63
Interest on Contributions
$4,009.56
Buying Power After Inflation
$47,042.54

Balance breakdown

Initial investment 37%, Contributions 46%, Interest 17%
  • Initial investment37%
  • Contributions46%
  • Interest17%

Accumulation schedule

PeriodDepositInterestEnding Balance
Year 1$25,000.00$1,250.00$26,250.00
Year 2$5,000.00$1,562.50$32,812.50
Year 3$5,000.00$1,890.63$39,703.13
Year 4$5,000.00$2,235.16$46,938.28
Year 5$5,000.00$2,596.91$54,535.20
Ending Balance$54,535.20View results

Work out how an investment grows with compound interest when you also add to it over time. Enter a starting amount, optional annual and monthly contributions, the rate and how often it compounds, and the term. You get the ending balance, the principal-versus-interest split, the interest earned by your initial amount versus your contributions, an inflation-adjusted buying power, and a year-by-year schedule.

Formula

Each compounding period the balance earns interest, and that interest is added back so it earns interest too. For a balance B, annual rate r, and n compounding periods per year, one period of growth is:

interest = B × (r / n) × (1 − taxRate)
B        = B + contribution + interest

Buying power = Ending balance ÷ (1 + inflationRate)^years

Contributions made at the beginning of a period are added before interest is applied (earning that period's interest); contributions at the end are added after.

Contributing early and often matters as much as the rate — each deposit compounds for the entire remaining term.

Simple vs. compound interest

$1,000 at 10% for 3 years
Simple interest$1,300 ($100 each year)
Compound interest$1,331 ($100, then $110, then $121)

Examples

$20,000 + $5,000/yr · 5% · annually · 5 years

A $20,000 start with $5,000 added at the beginning of each year, compounded annually at 5%, grows to $54,535.20 after 5 years — $45,000 of principal and $9,535.20 of interest. At 3% inflation that balance has the buying power of about $47,043 in today's dollars.

$10,000 + $100/mo · 6% · monthly · 10 years

A $10,000 start with $100 added at the end of each month, compounded monthly at 6%, reaches $34,581.90 after 10 years from $22,000 of principal.

Frequently asked questions

What does this interest calculator do?
It compounds an initial investment together with optional annual and monthly contributions, then reports the ending balance, how much of it is principal versus interest, the interest earned by the initial amount versus the contributions, and the inflation-adjusted buying power of the result.
Does contribution timing really change the result?
Yes. A contribution made at the beginning of a compounding period earns interest for that period, while one made at the end does not — so beginning-of-period contributions give every deposit one extra period of growth. Over many years the difference adds up.
How is the tax rate applied?
The tax rate is applied to the interest earned in each compounding period, reducing the amount that gets reinvested. A higher tax rate therefore lowers both the interest credited and the compounding on that interest.
What is the buying power after inflation?
It is the ending balance restated in today's dollars, dividing by (1 + inflation rate) for each year of the term. It shows what your future balance would actually be worth at today's prices, not how much interest you lose.
What is the difference between simple and compound interest?
Simple interest is paid only on the original principal, so it is the same every period. Compound interest is paid on the principal plus all previously earned interest, so the balance grows faster over time. This calculator uses compound interest.

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