The 50/30/20 rule
Split your take-home pay three ways: 50% to needs, 30% to wants, 20% to saving or extra debt payments. It's a decent default when you've never budgeted before. It breaks in high-rent cities, where housing alone can swallow the whole needs half. The 50/30/20 calculator splits your actual pay for you.
The 4% rule, or 25x
Withdraw 4% of a diversified portfolio in year one, adjust for inflation each year after, and historically the money has lasted 30 years or more. Flip it round and your target is 25 times your annual spending. Spend $50,000 a year and you need $1,250,000. The retirement calculator works out how long yours takes to build.
The rule of 72
Divide 72 by a growth rate and you get roughly the years it takes money to double. At 8%, about 9 years. It cuts both ways: at 3% inflation, prices double in about 24 years. The compound interest calculator does the growing and the inflation calculator does the shrinking.
The 28/36 rule
Lenders' shorthand for what you can afford: housing costs under 28% of gross monthly income, and all debt payments combined under 36%. Go past those lines and approvals get harder and life gets tighter. Check yourself against both with the home affordability calculator and the debt-to-income calculator.
The 40x rent rule
Landlords often want your annual income to be 40 times the monthly rent. A $1,625 flat implies a $65,000 salary. It's their screening rule, not your budget, so treat it as a ceiling rather than a target. The rent affordability calculator shows what's comfortable, not just what's approvable.
Three to six months of essentials
That's the standard emergency fund: enough to cover three to six months of essential spending, not total spending. Steady job, lean towards three. Variable income or a single earner, lean towards six. The emergency fund calculator puts a number on yours.
20% down avoids PMI
Put 20% down on a house and you skip private mortgage insurance, which typically runs roughly 0.3% to 1.5% of the loan per year for nothing you keep. Sometimes buying sooner with less down still wins, but know the cost first. See the down payment calculator and my guide to what PMI actually is.
Avalanche beats snowball, on paper
Paying the highest-rate debt first always minimises total interest. That's just arithmetic. But the gap is sometimes small enough that the morale of clearing small debts first wins in practice. The debt payoff planner shows both side by side, so you can see what the motivation costs.
Real return, not nominal
Subtracting inflation from your return is close but wrong. Divide (1 + return) by (1 + inflation) instead: 8% nominal with 3% inflation is 4.85% real, not 5%. Small gap, but it compounds for decades. The real return calculator does it properly.
Fees compound like returns do
A 1% annual fee sounds like almost nothing. Over 30 years, versus a 0.05% fund, it can consume roughly 18% of your final pot. That's years of your own contributions gone to someone else. The fee impact calculator shows the damage on your numbers.
Save half of every raise
You never had the money, so it doesn't hurt to bank it. Half your lifestyle still improves, and the saved half compounds: a $5,000 raise part-saved can outgrow the raise's own face value over a career. The raise value calculator shows what yours is really worth.
Your savings rate beats your return
How much you save matters far more than what you earn on it. On the same income, going from saving 10% to 50% cuts the years to financial independence from 52 to 17, at a 5% real return. No fund manager can do that for you. Run yours through the savings rate calculator.
Pay the statement balance in full
Do that every month and credit cards are free, sometimes better than free with rewards. Carry a balance and they're among the most expensive money there is. There's no middle ground worth living in. If you're carrying one now, the credit card payoff calculator maps the way out.
Longer terms feel cheaper and cost more
Stretching a loan drops the monthly payment and raises the total you hand over, because you rent the money for longer. Compare total cost, not just the payment. The loan comparison calculator puts two offers side by side and shows the whole bill.
Rules of thumb are starting points
Every rule on this page bends somewhere. 50/30/20 assumes your rent is reasonable, the 4% rule assumes history repeats, and 28/36 was written for lenders, not for you. That's why each one links to the calculator that does the real arithmetic. Use the rule to get moving, then use the tool to get it right for your numbers.