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Deposit Interest Calculator

Enter the deposit amount (or monthly amount), rate and term to get interest income and maturity value.

Deposit interest looks too small to bother with, which is exactly why few people check it: a lump sum held to maturity and a monthly savings plan are calculated in completely different ways, and breaking a term deposit early usually drops the rate to the sight-deposit level — a difference of ten times or more. Both plans are handled on separate tabs, with the interest and the maturity value stated outright.

This is simple interest: principal × annual rate × years, with no interest on the interest. Money withdrawn before maturity is normally paid at the sight-deposit rate, which is why a partial withdrawal is often better than closing the whole balance. Rates here are for reference — banks apply their own, and promotional products that pay more usually require a much larger minimum deposit.

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How to use

  1. Choose the plan: a lump sum held to maturity, or a fixed amount added each month.
  2. Enter the amount (or the monthly deposit), the annual rate and the term.
  3. Read the interest earned and the maturity value.
  4. Check the early-withdrawal note before breaking a term deposit early.

How it works

Time-deposit interest formula

Lump-sum time deposit: interest = principal x annual rate x term (months) / 12. For example 100,000 for 1 year at 1.95% gives 1,950 interest, totaling 101,950 at maturity. Longer terms usually carry higher rates, but early withdrawal generally earns the demand-deposit rate.

Small monthly savings and accumulated month factors

Small monthly savings: deposit a fixed amount each month; interest = monthly deposit x accumulated month factor x monthly rate, where the accumulated month factor = term months x (term months + 1) / 2. For 1 year the factor is 78; depositing 1,000 monthly at 1.35% gives about 87.75 interest. Because deposits are made monthly, average principal used is only about half the total, so interest is naturally lower than a lump sum of the same amount.

Why banks use 360 days

For easier interest calculation, banks commonly agree on "a year as 360 days and a month as 30 days," a historical convention from the central bank. So a lump-sum deposit converts the annual rate by "term months / 12," slightly differing from a real-day calculation — follow the bank system's convention at the counter.

Deposit rates are set by each bank rather than centrally, so this tool simply compounds the annual rate you enter; the rate actually posted by your bank at the time of deposit is what determines your return.

Code example

JavaScript Simple interest: a lump sum and a monthly deposit 

function lumpSum(principal, rate, years) {       // lump-sum deposit
  const interest = principal * rate * years;
  return { interest, total: principal + interest };
}

function monthlyDeposit(monthly, rate, years) {  // monthly deposit
  // Each instalment sits for (years*12 - k + 0.5)/12 years (bank convention)
  const months = years * 12;
  const interest = monthly * rate *
    (months * (months + 1) / 2) / 12;
  return { interest, total: monthly * months + interest };
}

Python The same formulas in Python 

def lump_sum(principal, rate, years):
    interest = principal * rate * years
    return interest, principal + interest

def monthly_deposit(monthly, rate, years):
    months = years * 12
    interest = monthly * rate * (months * (months + 1) / 2) / 12
    return interest, monthly * months + interest

lump_sum(100000, 0.0155, 3)   # interest 4650, total 104650

FAQ

Which pays more interest, lump-sum or small monthly savings?

Lump-sum, for the same term. Small monthly savings deposits month by month, so only the last deposit stays the full term, with average principal about half the total, giving clearly less interest. If funds are available at once, prefer the lump-sum time deposit.

What happens on early withdrawal of a time deposit?

Most banks pay the demand rate as of the withdrawal date on the withdrawn portion, a sizable interest loss; some support "one partial withdrawal," keeping the original time-deposit rate on the remainder. When you need cash urgently, first confirm whether partial withdrawal is allowed, then decide how much to take.

Do deposit rates change?

Yes. Banks set deposit rates autonomously on the central bank's benchmark, varying by bank, term and even minimum amount (like large-denomination certificates). Within a deposit term the rate is generally locked; renewals use the rate posted at that time.

Is auto-renewal worthwhile?

Auto-renewal is convenient, but applies the rate posted on the renewal date, which may be below current new-product rates. Before maturity, compare the latest rates across banks and, if needed, manually withdraw and re-deposit.

How do large-denomination certificates differ from ordinary time deposits?

They require a high minimum (often 200,000), usually pay more than ordinary time deposits of the same term, and most support transfer or tiered early withdrawal for better liquidity. With large funds, compare the rates and early-withdrawal rules, computing maturity interest on each with this tool.

Which is better, small monthly savings or monthly money-market fund investing?

Small monthly savings has a fixed rate, a single lump-sum withdrawal at maturity and deposit-insurance protection (up to 500,000 principal and interest per bank), but poor liquidity and demand-rate early withdrawal; a money-market fund is withdrawable anytime with market-driven returns and no deposit insurance. Choose small monthly savings for certainty, a money-market fund for liquidity.

Is deposit interest subject to income tax?

Personal savings deposit interest is currently temporarily exempt from personal income tax, so the principal and interest at maturity are after-tax; for corporate deposits, interest income is included in taxable income and taxed lawfully — different conventions.

Is the deposit amount uploaded or used otherwise?

No. The deposit amount, rate and term are used only locally in the browser; the page sends no data to a server, with no account binding or data return. History stays local and is clearable anytime — for full privacy, use an incognito window, gone on close.