Compound Interest Formula
The Compound Interest Formula helps you project the future value of an investment, considering both an initial principal and regular contributions, benefiting from interest earning on interest.
Formula
Copy the exact expression or work through it step by step below.
Future Value = Principal x (1 + Rate / Periods) ^ (Periods x Years) + Monthly Contribution x [((1 + Rate / Periods) ^ (Periods x Years) - 1) / (Rate / Periods)] Variables
FV
Future Value
The future value value plugged into the compound interest calculation.
P
Principal
The principal value plugged into the compound interest calculation.
r
Rate
The rate assumption used in the compound interest calculation.
n
Periods
The periods value plugged into the compound interest calculation.
t
Years
The time horizon applied to the compound interest formula.
MC
Monthly Contribution
The recurring amount that changes the compound interest result over time.
Step By Step
- 1
Set the baseline case with the real calculator inputs.
Initial Principal = $25,000, Monthly Contribution = $600, Annual Return Percent = 7.00%, Years = 20
- 2
Translate rates, periods, and cash values onto the same footing before combining them.
Keep the compound interest assumptions consistent instead of mixing monthly and annual views.
- 3
Apply the formula and read the first calculator outputs, not just the headline assumption.
The calculator lands with final balance at $401,264 and total contributions at $169,000.
- 4
Run one changed scenario so the formula is stress-tested before it is trusted.
The compound interest calculator page is the fastest way to compare that second case.
Worked Example
Compound Interest sample case
Initial Principal
$25,000
Monthly Contribution
$600
Annual Return Percent
7.00%
Years
20
Future Value = Principal x (1 + Rate / Periods) ^ (Periods x Years) + Monthly Contribution x [((1 + Rate / Periods) ^ (Periods x Years) - 1) / (Rate / Periods)] using initial principal $25,000, monthly contribution $600, annual return percent 7.00%, years 20.
The calculator lands with final balance at $401,264 and total contributions at $169,000.
Common Variations
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Sources & References
- Compound Interest — Investopedia
- The Power of Compound Interest — Kiplinger
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