Deposit Interest Calculator
Calculate interest and maturity amount for fixed deposits.
Interest type
Total interest
400,000
Maturity amount
10,400,000
Deposit interest calculator
Calculates interest and maturity amount for a fixed deposit.
Enter principal, term in months, and annual rate; choose simple or compound to get interest and amount at maturity.
- Assumes a lump sum deposited until maturity, like a fixed-term deposit.E.g. 10,000,000, 12 months, 3.5% compound → maturity amount is calculated.
- Fixed deposit
- A fixed deposit is a sum placed for a set period; at maturity you receive principal plus interest. Rates often depend on term.
- Annual interest rate
- The annual rate is the interest rate over one year. A monthly rate is often approximated as annual rate ÷ 12.
Simple versus compound interest on deposits
A fixed deposit is straightforward, but how interest compounds, how tax is applied, and what happens at maturity all change what you actually keep.
Simple and compound
Simple interest applies only to the principal. 10,000 at 4% for three years earns 400 a year, so 1,200 in total.
Compound interest also earns interest on the interest. This calculator compounds monthly, so the same deposit earns about 1,273 over three years. The gap is small at first but widens quickly with time.
Time is what makes compounding matter
The same 10,000 at 4% for ten years earns 4,000 simple against about 4,908 compound. Over twenty years it is 8,000 against about 12,226 — a gap of more than 4,200.
Compounding is unremarkable early and dramatic late. For short deposits the rate itself matters far more than whether interest compounds.
Compounding frequency
How often interest is added changes the outcome. 10,000 at 4% compounded annually gives 400 in the first year; compounded monthly it gives about 407.
This is why comparing headline rates alone is unreliable. Look for an effective annual rate, which restates everything on a common basis and lets you compare accounts with different compounding frequencies fairly.
Tax treatment
Interest is taxed differently depending on where you are. Korea withholds 15.4% at source, Japan 20.315%. In the US, interest is ordinary income reported on Form 1099-INT and taxed at your marginal rate rather than withheld.
Some countries add a further layer above a threshold. Korea, for example, aggregates financial income above 20 million won per year with other income at progressive rates. Check the rules that apply to you before assuming the headline return.
What happens at maturity
Once a fixed deposit matures, the agreed rate typically stops and a much lower post-maturity rate applies — often near 0.1%.
Leaving matured money sitting for months earns almost nothing. Set up automatic renewal or a maturity reminder so the balance does not idle at the default rate.
| Term | Simple · compound |
|---|---|
| 6 months | 200 · about 202 |
| 1 year | 400 · about 407 |
| 3 years | 1,200 · about 1,273 |
| 10 years | 4,000 · about 4,908 |
| 20 years | 8,000 · about 12,226 |
Frequently asked questions
- Is compound interest always better?
- Over any period longer than one compounding cycle, yes, and the advantage grows with time. Over three years or less the difference is small enough that the rate itself matters more.
- If the rate is 4% and I deposit for six months, what do I earn?
- Half of it, since quoted rates are annual. On 10,000 that is 200 before tax.
- Does compounding frequency matter?
- Yes. Monthly compounding at 4% returns slightly more than annual compounding at 4%. Compare effective annual rates rather than headline rates.
- How is deposit interest taxed?
- It varies. Korea withholds 15.4%, Japan 20.315%, and the US treats it as ordinary income on Form 1099-INT taxed at your marginal rate.
- What if I leave the money after maturity?
- A much lower post-maturity rate applies, often around 0.1%. Set up automatic renewal or a reminder so the balance does not sit idle.