Compound Interest Calculator

See what your savings become: starting amount, optional monthly deposits, rate and years — future value and total interest earned, in any currency.

Why compounding feels slow, then sudden

Compound interest pays you on your original money and on the interest you've already earned. The formula for a lump sum is FV = P × (1 + r/n)^n×t — but the intuition is simpler: growth accelerates because the base keeps growing. 100,000 at 10% earns 10,000 in year one but over 25,900 in year ten. The famous rule of 72 estimates doubling time: 72 ÷ rate. At 8%, money doubles roughly every 9 years.

Monthly deposits are the quiet hero: each deposit starts compounding the day it lands, so over long periods regular saving usually beats a bigger starting amount. Run the calculator once with deposits and once without — the gap is the strongest argument for starting now, even small.

Simple vs compound interest

Simple interest is paid only on your original amount, so it grows in a straight line. Compound interest is paid on your original amount and on the interest already earned, so it curves upward and accelerates over time. Over long periods the difference is enormous — which is why compounding is often called the most powerful force in saving.

The cost of waiting

Starting early beats saving more later. Because each year of compounding builds on the last, money invested in your twenties can outgrow larger sums invested in your forties. Running the calculator with a small monthly deposit over a long horizon shows why "start now, even small" is the single best savings habit.

How compounding frequency changes the result

The same headline rate produces different outcomes depending on how often interest is added. The formula is A = P(1 + r/n)^(nt), where n is the number of compounding periods a year.

On 100,000 at 10% for 10 years: annual compounding gives 259,374, quarterly 268,506, monthly 270,704 and daily 271,791. More frequent is better, but with sharply diminishing returns — the jump from annual to monthly is worth far more than monthly to daily.

This is why lenders and banks quote two numbers. The nominal rate is the headline; the effective annual rate (EAR) includes compounding and is what you actually earn or pay. A 12% nominal rate compounded monthly is an effective 12.68%. When comparing accounts or loans, always compare EAR to EAR — comparing a nominal rate against an effective one makes the wrong product look better.

The Rule of 72

A quick way to estimate doubling time without a calculator: divide 72 by the annual rate. At 6%, money doubles in about 12 years; at 9%, about 8 years; at 12%, about 6.

It works in reverse too, and this is where it stings — inflation halves purchasing power on the same schedule. At 8% inflation, money left in a non-earning account loses half its value in about nine years. That reframes the real question: not "how much will I have" but "how much will it buy". A 6% return during 8% inflation is a real return of roughly −2%, however healthy the nominal figure looks.

Regular deposits change everything

A lump sum compounds, but steady contributions compound and keep adding new principal, which is what actually builds most people's savings.

Compare over 20 years at 8%: a single 100,000 deposit grows to about 466,000. Depositing 1,000 a month instead — 240,000 contributed in total — reaches roughly 589,000. Doing both gets you past a million.

Two things follow. First, consistency beats size: regular modest deposits usually outperform waiting to invest a large amount later. Second, time beats rate — starting five years earlier typically does more than finding an extra percentage point of return, and it is far easier to control.

Frequently asked questions