Compound Interest Calculator — With Monthly Contributions

Interest earning interest

Compounding means earnings are added to the balance and then themselves earn. That makes growth exponential rather than linear, and it accelerates as the balance rises — which is why the last few years of a long period contribute far more than the first few.

Compounding frequency matters less than people assume

You can compound annually, quarterly, monthly or daily. Most people expect that choice to move the result substantially. On 10,000 at 8% for 10 years with no contributions:

• Annually — 21,589.25
• Quarterly — 22,080.40
• Monthly — 22,196.40
• Daily — 22,253.46

The whole spread from annual to daily is 664.21 over a decade, and the annual-to-monthly step — the switch people actually consider — is 607.15. Set against the 11,589.25 of interest earned, that step is under six per cent of the growth.

So frequency is worth understanding and rarely worth moving accounts for. The rate and the time do the work.

The contribution outweighs the starting sum

This is the more useful comparison. Keep everything the same and add 200 a month:

• The original 10,000 grows to 21,589.25
• The contributions grow to 36,589.21
• Total: 58,178.46

Just under two-thirds of the final figure came from the monthly additions rather than from the opening balance — and 200 a month is a modest amount. Over a realistic horizon a consistent habit beats a larger starting sum, which is the opposite of where most attention goes.

Reading the interest figure correctly

Interest is reported as the final total minus everything you put in. With contributions, total contributed is 34,000.00 and interest is 24,178.46.

Worth noticing why that is so much larger than the 11,589.25 of the first case: the rate did not improve. More money was working, for varying lengths of time. A bigger interest number partly reflects a bigger deposit, so compare it against what you paid in rather than reading it as a better return.

One behaviour worth disclosing

The lump sum compounds at the frequency you select. The monthly contribution always compounds monthly, whatever that setting says.

That is defensible — money paid in monthly does begin earning monthly — but it means the two halves of the calculation run on different periods unless you have chosen monthly. If you are comparing this against another calculator and the contribution figures differ slightly, this is very likely why.

Arithmetic, not a forecast

A single fixed rate held for the whole period is an assumption. Real returns vary year to year and can be negative, so treat the output as what would happen at a constant rate rather than as a projection. The value is in comparing scenarios against each other, where the same assumption applies to both.

For regular-contribution planning in rupee terms, the SIP calculator covers that specific case.