Money mental models I wish I knew at 25

In 2008 I watched 70% of my investments evaporate. Not over a year. Over weeks. I had put money into things I didn’t really understand, with no cushion if they went wrong — and when the recession came, it went wrong all at once. I remember the specific, hollow feeling of refreshing a screen and watching a number I’d worked years for get smaller. That loss taught me more than any profit ever did.

Here’s the strange part: I was good with money. At eight years old I was the unofficial accountant for my grandmother’s money-lending business in a small town in Northeast India, keeping the ledgers in my careful schoolboy handwriting. I grew up around interest, principal, and “trust but verify” before I knew those were words. And I still blew it at 30 because I’d never learned a handful of mental models that fit on a napkin.

So this is the note I’d hand my 25-year-old self. Three tools. No jargon. Each one took me a loss or a decade to actually believe.

1. Pay yourself first (before the money has opinions)

Most of us run our money like this: income comes in, we pay the bills, we spend on life, and we save whatever is left. The problem is that “whatever is left” is almost always nothing. There’s always one more thing — a phone, a trip, a wedding gift, an EMI. Money expands to fill the room you give it.

Pay yourself first flips the order. The moment income lands, a fixed slice — say 20% — moves into savings and investments before you’ve paid a single bill or bought a single thing. You live on the rest. You’re not saving what’s left after spending; you’re spending what’s left after saving.

The reason this works isn’t financial. It’s behavioural. When saving depends on your willpower at the end of the month, you lose — tired, stressed people don’t make disciplined choices. So you remove the choice entirely. Set up an automatic transfer the day your salary hits. Now saving costs you zero effort and zero decisions, while not saving would require you to actively log in and stop it.

The goal isn’t to be more disciplined with money. It’s to design a system where the disciplined thing happens whether you’re disciplined or not.

That’s the spine of everything I write about — moving from effort to effortless. A saving habit that leans on monthly willpower is high-effort, so you’ll quietly abandon it. An automatic transfer is effortless, so it survives the bad months. Make the good behaviour the lazy one, and you never have to be a hero.

2. The Rule of 72 (why starting at 25 beats starting at 35)

Here’s a piece of mental arithmetic that should be taught in school and isn’t.

Divide 72 by your annual rate of return, and you get the number of years it takes your money to double.

Earning 8% a year? 72 ÷ 8 = 9 years to double. Getting 12%? Six years. That’s the whole formula. You can do it in your head while someone’s still opening their calculator.

Why does this matter so much? Because it makes compounding visible, and compounding is the most underrated force in personal finance. The Rule of 72 was the thing that finally made me understand, in my gut, the power of starting early.

Run it forward. Say you invest a lump sum at 25 and it doubles every nine years. By 34 it’s doubled once. By 43, twice. By 52, four times. By 61, eight times the original. Your friend who starts at 34 instead of 25 doesn’t lose 9 years of saving — they lose the biggest doubling, the one at the very end where the numbers are largest. The last double is worth more than all the earlier ones combined.

This is why “I’ll start investing once I earn more” is such an expensive sentence. The most valuable rupee you will ever invest is the one you put in today, because it has the most time to double, and double again. You can’t buy that time back later with a bigger salary. As the manuscript I wrote puts it: start early, start small, and let consistency do the heavy lifting — slow and steady genuinely wins this race.

3. Margin of safety (the one that cost me 70%)

Back to 2008.

The lesson I should have learned at 25 — and only learned by losing money at 30 — is a concept called margin of safety. Popularised by Benjamin Graham, the investor who taught Warren Buffett, it’s deceptively simple: never make a financial decision that only works if everything goes right. Build in a cushion for the day things go wrong. Because some day, they will.

In investing terms, it means buying things for meaningfully less than you think they’re worth, so that even if you’re wrong, you don’t get wiped out. But the idea is far bigger than the stock market. Margin of safety is:

• An emergency fund of three to six months’ expenses, so one bad month doesn’t become one bad decade.
• Not borrowing right up to the maximum the bank will lend you — leaving room for the interest rate to rise or the income to dip.
• Diversifying, so no single failure can take everything. Don’t carry all your eggs in one basket; if the basket falls, you’ve lost the lot.

I learned this the painful way. I had no cushion. I’d concentrated my money, assumed the good times would continue, and left myself no room for the world to surprise me. The world always surprises you. My father’s generation understood this instinctively — they spent against past income, money already earned and in hand. My generation spends against future income, on credit cards and buy-now-pay-later, betting the future will cooperate. Margin of safety is just the humility to admit it might not.

The quiet thing all three share

Notice what these tools have in common. None of them is about predicting markets, picking winners, or being clever. Pay yourself first is about removing a decision. The Rule of 72 is about respecting time. Margin of safety is about assuming you’ll be wrong sometimes.

They’re all bets on the boring truth that small, consistent, well-designed choices compound — in your favour if you set them up right, against you if you don’t. The reason they belong together is that each one quietly turns a high-effort financial life into a low-effort one. You automate the saving so you don’t have to remember it. You start early so time does the work instead of your salary. You build a cushion so a single bad event doesn’t demand heroic recovery.

What’s effortless to do, though, is just as effortless to neglect. Setting up one automatic transfer takes ten minutes and then runs for thirty years. Not setting it up also takes no effort — and that’s exactly the trap. The easy path in finance is usually the one that quietly costs you the most, because nothing forces your hand until it’s too late.

I’m not a financial advisor, and none of this is personalised advice — for that, find someone good and, as my grandmother would say, trust but verify; it’s your money on the line, not theirs. I’m just a co-learner who paid tuition the hard way and wishes someone had drawn these three things on a napkin for me at 25.

If you want the trackers and templates I actually use to run this stuff, they’re over at the resources page. The longer version — all eight spheres of life, finances included — lives in the book. And if you’re curious why an eight-year-old grandmother’s-ledger kid ended up writing about change, that story’s on the about page.

You can’t get the 2008s of life to stop coming. You can only decide, today, whether your future self meets them with a cushion — or with a screen full of shrinking numbers.