Decision tools
These answer questions that have no official answer. A tax calculator can be checked against a published schedule; whether you should rent or buy depends on what you believe about house prices and investment returns. So instead of hiding those assumptions in constants, every one of them is an input you can change — and every number on the page traces back to the ones you set.
- Car total cost of ownership The true cost of owning a car is its depreciation plus finance interest, fuel or electricity, insurance, servicing, tyres, tax and the incidentals — and depreciation is usually the largest line, which is why both the sticker price and the monthly payment understate it.
- Rent vs buy Buying beats renting once the equity you build plus house price growth outweighs the return the same money would have earned invested — which depends on how long you stay, and typically takes well over a decade at ordinary appreciation and investment returns.
- Retirement and FIRE planner You are financially independent once your portfolio reaches your annual retirement spending divided by your withdrawal rate — 25 times your spending at the 4% rule — and the age you get there is driven far more by your savings rate than by your investment return.
- Debt payoff planner The avalanche method — paying the highest interest rate first — always costs less in interest than the snowball method, which pays the smallest balance first; the snowball’s advantage is motivational, and this model prices exactly what that motivation costs.