Guide · Financial Calculator
Compound interest calculator — free online
Compound interest is the most powerful force in personal finance. Unlike simple interest, which is calculated only on your original principal, compound interest is calculated on both your principal and the interest already earned. Over time, this creates a snowball effect where your money grows at an accelerating rate.
Simple interest vs compound interest
With simple interest, a $10,000 deposit at 6% earns $600 every year — always the same amount, always on the original $10,000. After 10 years you have $16,000.
With compound interest, that first year's $600 becomes part of your balance. Year two, you earn 6% on $10,600. Year three, on $11,236. Each year the base grows, so each year's interest is larger. After 10 years you have $17,908 — nearly $2,000 more, without doing anything differently.
How to use Pebble as a free compound interest calculator
Switch Pebble to Financial mode (tap the mode button and select 💰). For a straightforward compound interest calculation — no monthly contributions, just a lump sum growing over time — set PMT to 0 and fill in the other four fields:
- N — Number of periods (years for annual compounding, months for monthly)
- I/Y — Annual interest rate as a percentage
- PV — Your starting principal (enter as a negative number — money you put in)
- PMT — 0 (no regular contributions)
- FV — What you're solving for: the future value of your investment
Worked example: $10,000 at 6% for 10 years
You invest $10,000 in an account earning 6% annual interest, compounded yearly. What will your balance be after 10 years?
Enter these values, then tap CPT → FV
Your $10,000 grew to $17,908 — a gain of $7,908. Of that, only $6,000 would have been earned with simple interest. The extra $1,908 is pure compounding: interest earning interest, year after year.
Annual vs monthly compounding
The more frequently interest compounds, the faster your money grows. To calculate monthly compounding on the same investment, convert everything to months: N becomes 120 (10 years × 12), and the calculator divides I/Y by 12 internally.
Monthly compounding adds an extra $286 with no additional effort. Over longer time horizons and larger amounts, this difference becomes significant — another reason to check the compounding frequency on any account you open.
The most important variable: time
Compound interest rewards patience above all else. Consider two investors who each put $5,000 into an account earning 7% annually. One invests for 30 years, the other for 20 years. The difference a single decade makes is striking:
Ten extra years nearly doubles the final balance — without investing a single extra dollar. That's compounding at work. The earlier you start, the less you need to contribute to reach the same goal.