Every Wealth Rule, Explained: Where It Came From and How It Fails

Personal finance travels as rules of thumb: the rule of 72, six months of expenses, ten times your salary, 100 minus your age, 4% a year. Rules of thumb are worth having — they are compressed experience — but every one of them was built for a situation, and each breaks somewhere. This page walks the standard set: where each rule came from, why it works, and the specific situation that breaks it. No rule here should be followed blindly; every rule here should be understood quickly.

The rule of 72: how fast does money double?

Divide 72 by the annual return and you get the approximate doubling time — 8 years at 9%, 12 years at 6%. The trick is commonly attributed to Luca Pacioli's 1494 Summa de arithmetica, and it works because it approximates the logarithm in the compounding formula. Its failure mode is precision: at returns far from the 6–10% range the approximation drifts, and it says nothing about sequence or taxes. It is a street estimate, not a plan — the exact arithmetic lives in any compound-interest calculator.

The mirror rule: inflation halves money too

Run 72 against inflation instead of returns: at 7% inflation — roughly the illustrative rate used in our worked guides — prices double in about a decade, which is another way of saying a rupee held flat halves in ten years. The failure mode is the opposite of the return version: people apply it to everything equally. Food inflation and electronics inflation are not the same 7%; the rule is a blunt but useful sizing tool for "what will this cost at retirement".

50/30/20: the budget skeleton

Half of take-home to needs, 30% to wants, 20% to saving — set out by Elizabeth Warren and Amelia Warren Tyagi in their 2005 book All Your Worth. The mechanism is honest: it caps wants without banning them, which is why it survives. It breaks where fixed costs eat the 50% — metro rent can take that whole half before food — and where income is informal or seasonal. Use it as a diagnostic, not a law: if needs are 65%, the plan is not "save 20% anyway", it is "fix rent or income". Our income-ladder guide shows the sizes at each level.

Emergency fund: three to six months

The recommendation you will find on official investor-education pages, from SEC's investor.gov to Vanguard's guides: hold three to six months of essential expenses in something you can reach instantly. The mechanism is behavioural as much as financial — the fund exists so a bad month never forces you to sell investments or swipe a credit card. It breaks for single incomes and freelancers, whose income variance argues for nine to twelve months, and it breaks silently the other way: money beyond the fund is safety theatre, not investing.

Term cover: ten to fifteen times income

Life insurance worth ten to fifteen times annual income while anyone depends on you is an industry convention rather than a single citable formula, and it works because it roughly replaces a decade of take-home. It breaks when the multiplier ignores the household's actual shape: a ₹1 crore loan already outstanding, three dependents with a twenty-year horizon, or a spouse with strong earnings each move the right number. The convention is a starting quote, not the answer.

100 minus your age: how much equity?

Subtract your age from 100 and hold that percentage in stocks — 70% at 30, 60% at 40. Research papers (including the Social Security Administration's review) describe it as a popular rule of thumb with no rigorous origin; it encodes one real insight — equity share should fall as horizon shortens — and one bad assumption: that everyone at the same age has the same wealth and the same human capital. It breaks for anyone with a pension, a large inheritance, or a long runway after 60. Lifecycle funds automate something like it; they are still not a personal answer.

The 4% rule: how much can you withdraw?

Withdraw 4% of the retirement corpus in year one, then inflation-adjust: William Bengen's October 1994 Journal of Financial Planning analysis of historical US data found no 30-year window that exhausted a balanced portfolio on that schedule. It became the most quoted number in retirement planning, and the most misused: it is calibrated to US markets, a 30-year horizon and a specific 50/50 mix. It breaks for early retirees (longer horizons), for markets with different return-and-inflation paths, and for anyone who treats it as a guarantee rather than a historical observation. Our retirement calculator and FIRE planner let you stress your own assumptions instead.

The cost of delay, and the cost of fees

Two rules we computed rather than inherited. Start the same ₹10,000 monthly SIP at 25 instead of 35, at an assumed 11%, and the age-60 corpus is roughly three times larger — about ₹5 crore versus ₹1.6 crore — because the early decade compounds through every later one. And a 1% annual fee against a 10% return, held for thirty years, leaves the fee-free portfolio about 30% ahead: the fee does not look like much on any single statement and compounds exactly like a negative return. Both numbers are arithmetic on stated assumptions — verify them on the SIP calculator — and both break the same way: they assume uninterrupted plans. A SIP abandoned in a crash makes both rules irrelevant.

What every rule has in common

Each rule compresses a mechanism into a number: compounding (72), adaptation of lifestyle to income (50/30/20), liquidity under stress (emergency fund), dependents' futures (term cover), horizon (100-minus-age), withdrawal sustainability (4%). The number is the shortcut; the mechanism is the asset. Learn the mechanism once, and you will know when the shortcut is lying — which is the only way any rule of thumb is safe to keep.

Educational content, not financial advice. Rule origins verified against the sources listed below on 29 October 2026; the fee-drag and cost-of-delay figures are our own arithmetic on stated assumptions (11% return, 1% fee, 10% return), not research findings.