Interest Calculator
Interest Settings
Calculation Results
| Ending Balance | — |
|---|---|
| Total Principal (Invested) | — |
| Total Interest Earned | — |
How to Use This Interest Calculator
Manually comparing simple vs compound growth on a napkin gets messy fast—especially when you add recurring deposits and pick a compounding schedule. I reach for this layout when a client asks whether their savings account quote is simple or compounded, or when we model a fixed-rate investment with monthly top-ups.
- Enter starting principal and annual rate. Use the quoted nominal rate (% per year) from your account or loan terms.
- Set the time period. Fill in years, months, or both—the calculator totals them into one duration.
- Choose compounding frequency. Pick monthly, daily, continuous, or Simple Interest (No Compounding) to compare methods side by side.
- Add optional contributions. Enter a periodic deposit amount, contribution frequency, and whether deposits land at the beginning or end of each period.
- Press Calculate. Review ending balance, total principal invested, interest earned, and the year-by-year growth schedule.
For inflation-adjusted portfolio planning, try the Investment Calculator. Need pure compound math without the simple-interest toggle? See the Compound Interest Calculator.
Interest Formulas and Practical Applications
Think of simple interest like rent on a fixed deposit box—you pay (or earn) the same dollar amount every period based only on what you started with. Compound interest is more like a snowball rolling downhill: each period's earned interest becomes part of the balance that earns again next period.
Simple interest
I = P × r × t
Where P is principal, r is the annual rate as a decimal, and t is time in years. On $10,000 at 6% for 5 years, simple interest totals $3,000, ending at $13,000.
Compound interest
FV = P(1 + r/n)nt
n is compounding periods per year. The same $10,000 at 6% compounded monthly for 5 years grows to about $13,488 without additional deposits—roughly $488 more than simple interest because prior interest keeps earning.
Contributions and timing
Recurring deposits use standard annuity logic. Beginning-of-period timing gives each deposit one extra compounding interval compared to end-of-period timing—a difference that adds up over long horizons with monthly contributions.
Frequently Asked Questions
How do I switch between simple and compound interest?
Select Simple Interest (No Compounding) from the Compounding Frequency dropdown. All other frequency options use compound interest on the chosen schedule.
How does compounding frequency affect my ending balance?
More frequent compounding earns interest on interest sooner, so daily or monthly compounding produces a higher ending balance than annual compounding at the same nominal rate.
What is the difference between end and beginning contribution timing?
End of period means each contribution is added after interest is calculated for that period. Beginning of period adds the deposit first, so it earns interest for the full period.
Can I enter both years and months for the time period?
Yes. The calculator combines years and months into one total duration before applying interest.
What does the growth schedule show?
The schedule breaks down yearly contributions, interest earned that year, cumulative interest, and ending balance so you can see how growth accelerates over time.