Free calculator · Personal Finance
Compound Interest
Calculates the future value of a starting balance plus monthly contributions with interest compounded monthly, and splits the ending balance into the dollars contributed and the growth earned on top of them. Inputs are the starting principal, the monthly contribution, the annual rate, and the number of years.
By Devon Coombs, CPA, MBA · Teaching Professor of Finance, Santa Clara University · Reviewed August 2026
Interactive Tool
Compound Interest
Future value
$170,619
after 20 years, compounded monthly · about $104,124 in today's dollars at 2.50% inflation
The gap between the lines is compounding; it widens fastest in the late years.
Total contributed
$70,000
principal + every deposit
Growth
$100,619
59% of the ending balance
Real value today
$104,124
deflated at 2.50%/yr
From contributions
$130,232
deposit stream grown
FV = $10,000 × (1 + 7.0%/12)^240 + $250 × ((1 + 7.0%/12)^240 − 1) ÷ (7.0%/12) = $170,619
Figures are before taxes and fees and assume the rate holds every month. The real-value line converts the ending balance to today's purchasing power, which is usually the number that matters for a goal decades out.
Learn the concept
This calculator comes from the free Corporate Finance: The 10 Laws of Finance course, where the concept is taught with readings, worked examples, and practice questions.
Precomputed reference tables
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Built by Devon Coombs, CPA, MBA, Teaching Professor of Finance at Santa Clara University.