Compound Interest Calculator

Last updated: 2026-10-01

This compound interest calculator shows what your savings will grow into — starting balance, annual rate, how often interest compounds, time, and any extra deposits you add along the way. It's built for savers and planners who want a straight answer to "where will I be in ten years?"

How to use this savings growth calculator

Everything happens live: change any field and the result updates instantly.

Principal — initial deposit ($): the lump sum you're starting with today. Defaults to $10,000.
Annual interest rate (%): the nominal rate your account pays over a year, not the APY. Defaults to 5%.
Time (years): how long the money sits and grows, up to 70 years.
Compounding: how often earned interest is folded back into your balance — monthly, quarterly, semi-annually, annually, or daily.
Additional contribution ($): what you add each compounding period. If compounding is monthly, a $200 entry means $200 every month, added at the end of the period.

The result panel shows your future balance first, then breaks it down into principal, total contributions, interest earned, and the final balance — so you can see exactly how much of the result is growth versus your own deposits.

Worked examples

Example 1 — no deposits, just growth: You park $10,000 at 5% with monthly compounding for 10 years. The balance grows to $16,470.09. You put in $10,000 and earned $6,470.09 in interest — without lifting a finger after day one.

Example 2 — starting small and adding monthly: You start with $5,000 at 7%, compound monthly, and add $200 every month for 20 years. The future balance is $124,379.03. Your own contributions total $48,000, so interest did the other $71,379 — more than half of the final result.

Example 2 shows the real lesson: steady deposits plus time beat a big starting balance alone.

How the math works

The formula behind this tool is the standard compound interest formula:

A = P(1 + r/n)^nt

A — the final amount you're looking at.
P — the principal, your starting deposit.
r — the annual interest rate as a decimal (5% becomes 0.05).
n — compounding periods per year: 12 for monthly, 4 for quarterly, 365 for daily.
t — the number of years.

Each period, you earn interest on the full balance so far — including last period's interest. That's the compounding part: your growth earns its own growth.

Compounding frequency helps, but only a little. At 5% on $10,000 over 10 years, monthly compounding gives $16,470.09 while daily gives $16,486.65 — a difference of about $17. Rate and time move the needle far more than how often the interest posts. Our compound interest basics guide walks through the formula step by step, and our methodology page explains the assumptions behind every SialTV Money tool.

Common mistakes and tips

Mixing up rate and APY. This calculator uses the nominal annual rate — the headline number your bank quotes. If your bank only lists APY, know that APY already has compounding baked in, so don't add extra compounding on top.
Forgetting contributions. Many calculators show growth on the principal only. If you actually plan to deposit monthly, enter it — as Example 2 shows, deposits can be most of the final balance.
Overestimating safe rates. A high-yield savings account paying 5% today can drop when rates change. Run the numbers at a lower rate too, so your plan survives normal conditions.
Ignoring taxes and fees. Interest earned is usually taxable, and account fees quietly work against you. The calculator shows growth before taxes — treat the result as a gross figure.
Underestimating time. The biggest lever in the formula is t. Starting five years earlier often beats finding a slightly better rate later.

Frequently asked questions

What is the rule of 72?

Divide 72 by your annual rate for a rough idea of how many years it takes your money to double. At 7%, money doubles in about 10.3 years. It's a quick estimate, not an exact formula.

How much difference does daily vs. monthly compounding make?

Very little at normal rates. On $10,000 at 5% for 10 years, monthly compounding yields $16,470.09 and daily yields $16,486.65 — about $17 apart. A 1-point change in the rate, or a few extra years, matters far more.

What's the difference between simple and compound interest?

Simple interest pays only on your original principal. Compound interest pays on the principal and on the interest already added, so the balance grows faster every period. Loans can work the same way against you — which is exactly why credit card balances balloon.

What interest rate is realistic for savings?

High-yield savings accounts move with the market — they've swung between roughly 1% and 5% in recent years. Use your bank's current quoted rate, and re-run the calculation at a lower rate to see a conservative version of the outcome.

Do contributions compound too?

Yes. Each deposit starts earning interest from the period after it lands, and its interest compounds like everything else. Deposits made earlier have more time to grow, which is why starting now beats catching up later.