How this interest calculator works
This calculator estimates both interest earned and ending balance over time. It supports an initial lump sum, annual and monthly contributions, beginning or end-of-period timing (aligned with each compounding period), nominal annual rate with several compounding choices (including a daily model using 365 days per year mapped to monthly steps), investment length in years and months, optional tax on each interest accrual, and optional inflation to translate the ending balance into approximate today's purchasing power.
Interest allocated to "initial investment" vs "contributions" uses a pro-rata split each time interest is applied, based on how much of the balance came from the original principal versus accumulated contributions. Totals reconcile to the overall ending balance.
Simple interest vs compound interest
Simple interest is earned only on the original principal. Each period adds the same dollar amount of interest. Formula: Interest = Principal × Rate × Time.
Compound interest is earned on principal and on interest that has already been credited. That is why growth can accelerate over long horizons. A common form is A = P(1 + r/n)^(nt) for a fixed rate and compounding frequency.
How compounding frequency affects growth
Holding the quoted annual rate constant, more frequent compounding generally produces a slightly higher ending balance on savings (and a slightly higher effective cost on loans). The gap widens over many years. This tool uses the standard nominal-rate convention: each compounding step applies r/n per period when there are n periods per year, and approximates daily compounding with a monthly equivalent rate for practical schedule display.
The Rule of 72
A quick estimate for doubling time: divide 72 by the interest rate expressed as a percent (for example, 72 ÷ 8 ≈ 9 years at 8%). It is a mental shortcut, not a substitute for exact compounding math, and it is most accurate for moderate rates.
Periodic contributions
Regular deposits increase both the balance that earns interest and the share of growth attributable to new money. Whether contributions are modeled at the beginning or end of each compounding period changes how many accrual steps apply to each deposit — end-of-period contributions typically receive one fewer accrual within that period than beginning-of-period contributions.
Tax rate on interest
Many interest-bearing accounts produce taxable ordinary income in the U.S. The tax field here reduces each interest accrual by that percentage before it is reinvested, which is a simplified way to illustrate how taxes can lower effective compounding versus a tax-free account. Real outcomes depend on account type, timing, brackets, and rules — consult a tax professional for your situation.
Inflation and real buying power
Inflation means future dollars usually buy less than today's dollars. The calculator divides the nominal ending balance by (1 + inflation)^(years), where years come from your total months ÷ 12, to show an approximate inflation-adjusted value. This is an educational estimate, not a forecast of actual future inflation.
Fixed vs variable rates
This calculator assumes a fixed nominal rate for the whole horizon. Many real products float with benchmarks or reset over time; modeling those requires scenario analysis or different tools.