The Math Behind Your Money: What Your School Skipped

Investing · 21 May 2026 · Team Armor

If you've started a SIP without fully understanding what's happening underneath it, you're in good company. This post walks through the five return concepts every new Indian investor should know - simple interest, compound interest, absolute return, CAGR, and XIRR with worked examples, clean formulas, and one table that answers most of the questions you'll have.

The Math Behind Your Money: What Your School Skipped

9.72 crore Indians now run a monthly SIP. In March 2026, monthly SIP contributions crossed ₹32,000 crore for the first time, hitting ₹32,087 crore (Source: AMFI Monthly Note, March 2026, page 16). And yet, if you asked most of those investors what their XIRR actually measures, or whether their 12% return is genuinely good after inflation, you'd get a pause.

The math behind personal finance was never taught in school. We learnt compound interest but not how compounding actually behaves, what CAGR measures, or why time matters more than rate. This is the blog that fills that gap.

What is simple and compound interest?

Let’s start with the basics. Simple interest is calculated on the original principal only, the base amount never changes for interest purposes. Every year, you earn exactly the same rupee amount in interest regardless of how long the investment runs.

Visual representation of the simple interest formula: Interest equals Principal multiplied by Rate multiplied by Time, with icons representing each variable. — The simplest equation in personal finance, and the slowest one.

So let’s say,

You lend a colleague in Chennai ₹1,00,000 at 10% per year. Under simple interest, you earn ₹10,000 every year and a flat ₹50,000 over 5 years, ₹2,00,000 over 20 years.

ear-by-year breakdown of simple interest on ₹1,00,000 at 10% per year. Each year, interest is charged on the same ₹1,00,000 principal, earning a flat ₹10,000 every year for five years, totalling ₹50,000. — The principal never grows. The interest is flat. Year 5 looks exactly like year 1.

Simple interest is predictable and easy to calculate but it's also slow because the ₹10,000 you earn in year 1 never starts earning for you. It sits idle while the original ₹1,00,000 keeps doing all the work.

On the other hand, compound interest is interest calculated on the principal plus all the interest already earned.

Visual representation of the compound interest formula: Amount equals Principal multiplied by one plus Rate, raised to the power of N (number of years), with icons for each variable. — One small change to the formula. A very large change to the outcome.

Each year, the previous year's interest gets folded into the new starting balance and the next year's interest is calculated on that larger number. Most people understand compounding in theory. They don't feel it.

If we were to consider the same example, under compound interest, each year's interest earns its own interest. After 5 years you've earned ₹61,051. After 20 years you've earned ₹5,72,750. Same money, same rate, same time. Nearly triple.

ear-by-year breakdown of compound interest on ₹1,00,000 at 10% per year. The opening balance grows each year as interest is added: ₹1,00,000 in Year 1 grows to ₹1,61,051 by end of Year 5. Total interest earned is ₹61,051, compared to ₹50,000 under simple interest. — Each year's interest joins the principal. The base grows. The interest grows with it.

Every SIP, every PPF, every long-held mutual fund grows this way by default. You don't have to do anything to compound. You just have to not interrupt it.

Two-panel diagram showing compound interest on ₹1,00,000 at 10% per annum. The left panel illustrates yearly growth from Year 1 to Year 5, with interest earned rising from ₹10,000 to ₹14,641, and corpus growing to ₹1,61,051. The right panel compares 20-year outcomes under simple interest (₹3,00,000 final corpus) versus compound interest (₹6,72,750 final corpus), showing a gap of ₹3,72,750 — nearly 3x. — Same principal, same rate, same time. The only difference is whether the interest is allowed to earn its own interest.

What is Absolute Returns, CAGR and XIRR?

Notice how the mutual fund apps show you a return number?

You feel good seeing it but the number you're reading often misrepresents what your money is actually doing.

There are three return figures every individual who is investing should be able to tell apart.

1. Absolute Return

Absolute return is the total percentage gain on an investment from start to end, with no adjustment for how long it took. It is the right measure only for investments held for less than one year.

isual comparison of a mutual fund advertising 80% returns over 8 years against a bank fixed deposit at 7% per year. ₹100 invested grows to ₹180 in the fund (an annualised return of approximately 7.7%) and ₹176 in the FD over the same period. The FD nearly matches the fund's growth with none of the volatility, illustrating how absolute return numbers hide time. — An 80% return over 8 years is only 7.7% annualised. A bank FD nearly matches it, with none of the volatility. Absolute return is the most misleading number on your dashboard.

That fund is annualising at 7.7% but an FD at 7% would have nearly matched it, with none of the volatility. Absolute return hides time, which is precisely what makes long-term investing different from short-term.

2. CAGR (Compounded Annual Growth Rate)

CAGR (Compounded Annual Growth Rate) is the single annual rate at which your investment grew, smoothed out across all the ups and downs in between. It is the standard measure for any investment held for more than one year in India.

Visual representation of the CAGR formula: CAGR equals End Value divided by Start Value, raised to the power of one over N (number of years), minus 1. Icons illustrate the End Value, Start Value, time period, and the subtraction step. — The one formula that lets you compare any two multi-year investments on the same scale.

Let’s say, a 34-year-old in Mumbai invested ₹1,00,000 in an equity mutual fund in 2020. In 2025, it's worth ₹1,61,051.

CAGR = (1,61,051 / 1,00,000)^(1/5) − 1 = 10% per year.

Why CAGR matters when comparing funds?

Fund A shows 80% returns over 8 years. Fund B shows 65% over 5 years. Which is better? You cannot compare these with absolute return. CAGR tells you Fund A earned 7.7% per year and Fund B earned 10.5% per year. Fund B is the stronger performer by a wide margin.

Side-by-side comparison of two mutual funds. Fund A delivers 80% returns over 8 years, which is a CAGR of 7.7%. Fund B delivers 65% returns over 5 years, which is a CAGR of 10.5%. The graphic shows that despite Fund A's higher absolute return, Fund B is meaningfully better when measured on an annualised basis. — Fund A looks bigger. Fund B is actually better. CAGR is what reveals which is which.

Both numbers are real. Only one tells you which fund actually grew faster. For any lumpsum investment held more than a year, CAGR is the right measure.

3. XIRR (Extended Internal Rate of Return)

This is where it gets interesting for you, because you don't invest in lumpsums. You put it as SIPs. Every month, ₹10,000 (or whatever your amount is) leaves your account and buys mutual fund units at that month's NAV. Each of those monthly investments have been compounding for a different length of time.

CAGR can't handle that. It assumes one entry point and one exit point. XIRR can. It calculates the annualised return across a series of irregular cash flows, which is exactly what a SIP is.

This is the number on your CAS statement (the consolidated statement CAMS or KFintech sends you, as mandated by SEBI). It's also the most honest measure of how your SIP is actually performing. A fund showing 14% CAGR over 5 years might show a 12% XIRR for your specific SIP because of when you invested and at what NAVs.

The rule: if you invested once, check CAGR. If you invest every month, check XIRR. The absolute return number on your app dashboard is almost never the one you should rely on for long-term decisions.

The Full Picture: Simple Interest vs Compound Interest vs Absolute Return vs CAGR

Here is every return concept you'll encounter as an investor in India and exactly what each one means for your money.

Comparison table covering four return concepts: Simple Interest (interest on original principal only, formula P × R × T, used for informal loans and old FD structures, turns ₹1 lakh into ₹3 lakh over 20 years at 10%); Compound Interest (interest on principal plus accumulated interest, formula P × (1+R)^N, used for FDs, bonds, mutual funds, SIPs, turns ₹1 lakh into ₹6.72 lakh over 20 years at 10%); Absolute Return (total percentage gain with no time adjustment, formula (End − Start)/Start × 100, only useful for investments held under one year); CAGR (smoothed yearly growth rate across multiple years, formula (End/Start)^(1/N) − 1, used for comparing mutual funds and evaluating SIP returns, equals 10% per year in the example). — Four return concepts, one table. Print this, save it, or screenshot it — it answers most return-related questions a new investor will have.

The 20-year column makes the stakes clear. Simple interest turns ₹1 lakh into ₹3 lakh. Compound interest turns it into ₹6.72 lakh. The ₹3.72 lakh difference came entirely from interest earning interest. You invested the same amount, waited the same time — the math did the rest.

Time is the only input you can't buy back

Now we come to the most expensive lesson in personal finance.

Look at the example illustrated below:

Side-by-side timeline comparison of two SIP investors. Both invest ₹15,000 per month in the same equity mutual fund earning 12% CAGR. The investor who starts at age 25 invests for 35 years, contributing ₹63 lakh of own money, and ends with ₹9.7 crore at age 60. The investor who starts at age 30 invests for 30 years, contributing ₹54 lakh of own money, and ends with ₹5.3 crore. A five-year delay results in a ₹4.4 crore gap in final corpus, despite only a ₹9 lakh difference in invested capital — nearly half the corpus lost to a five-year delay. — Two investors. Same SIP. Same fund. Same return. Five years apart. The one who started at 25 has nearly double the corpus at 60.

Same SIP. Same return. A five-year delay. The corpus difference is over ₹4 crore. This is what happens when you give compounding more time. The final years are where the absolute rupee gains explode, because the base is so much larger. Year 30 of an SIP adds more rupees than years 1 through 10 combined.

Money you don't invest at 25 isn't money you can simply invest harder later. The corpus that early SIP builds in the final decade cannot be replicated by a larger SIP started later, because there isn't enough time left for the math to do its work.

The single most powerful thing you can do for your future portfolio is start, even small, today. ₹2,000 a month from age 25 will outperform ₹5,000 a month from age 35 over a 35-year horizon. Time, not amount, is the lever.

What does this means for you?

A few takeaways, in the form you can act on this week:

Stop reading absolute returns on multi-year investments. If your fund app shows you a 5-year absolute number, mentally convert it to CAGR. Roughly: divide by years, then add a small bump for compounding.
Use CAGR when comparing funds. Two funds with different time periods cannot be compared on absolute return. CAGR puts them on the same scale.
Find your XIRR. It's the most honest measure of how your specific SIP is actually performing. CAMS and KFintech both display it on the CAS statement, and most fund apps show it under portfolio summary.
Don't wait until you "have enough" to start. The smallest SIP started today beats the largest SIP started in five years, on a long-enough horizon. ₹2,000 a month from age 25 outperforms ₹5,000 a month from age 35 over a 35-year window. Start now, scale later.

Armor's Scenario Sandbox takes your current portfolio and projects your wealth at 10, 20, and 30 years. It shows the compounding curve specific to your numbers, not a generic example. It also models decisions: what does your trajectory look like if you increase your SIP by ₹5,000/month? What if you extend your holding period by 5 years? On a ₹15 lakh portfolio, either change can shift your final corpus by ₹40–80 lakh. You can see that number before you decide.

Sources

AMFI Monthly Note, March 2026 — amfiindia.com
SEBI Investor Education — sebi.gov.in/investors

Frequently Asked Questions (FAQs)

What is the difference between simple and compound interest?

Simple interest is paid only on the original principal. Compound interest is paid on the principal plus all interest already earned. On ₹1 lakh at 10% over 20 years, simple interest gives you a final corpus of ₹3 lakh. Compound interest gives you ₹6.72 lakh.

What is the formula for compound interest?

Final Amount = Principal × (1 + Rate)^Number of Years. So ₹1,00,000 invested at 10% for 5 years becomes ₹1,00,000 × (1.10)^5 = ₹1,61,051.

What is absolute return in mutual funds?

Absolute return is the total percentage gain on an investment from start to end, with no adjustment for how long it took. It's the right measure only for investments held for less than one year. For longer-term investments, it overstates performance because it hides time.

What is CAGR and why does it matter?

CAGR (Compounded Annual Growth Rate) is the single annual rate at which your investment grew, smoothed across all the ups and downs. It is the standard measure for any investment held longer than one year in India. A fund showing 80% over 8 years and another showing 65% over 5 years look incomparable on absolute return. On CAGR, they're 7.7% and 10.5% and the second fund is clearly the stronger performer.

What is XIRR and how is it different from CAGR?

CAGR assumes a single lump sum invested at the start and a single exit at the end. XIRR (Extended Internal Rate of Return) handles a series of irregular cash flows, like the monthly investments in a SIP. Each SIP installment compounds for a different length of time, which CAGR cannot capture. For SIPs, XIRR is the right measure.

Where can I see my XIRR?

On the Consolidated Account Statement (CAS) that CAMS or KFintech sends to your registered email every month, as mandated by SEBI. You can also check it out in the Armor app.

Why does starting an SIP early matter so much?

Compounding rewards the years furthest in the future, where the base is largest. The final decade of a 35-year SIP adds more rupees than the first 10 years combined. A five-year delay on a ₹15,000 monthly SIP at 12% reduces the final corpus from roughly ₹9.7 crore to ₹5.3 crore, nearly half, despite investing only ₹9 lakh less of your own money.

Is it better to start with a small SIP now or wait until I can afford a bigger one?

Start small now. ₹2,000 a month from age 25 outperforms ₹5,000 a month from age 35 over a 35-year horizon. The first decade of compounding builds the base everything else multiplies on. You can always scale a SIP up later. You cannot get the years back.