Model it yourself

Calculators & models

Every model here runs in JavaScript in your browser. Nothing you type is transmitted, logged or stored, there is no signup, and each one shows the outputs that make products look worse as prominently as the ones that flatter them.

5Interactive models
0Fields sent to a server
55Embedded mini-models sitewide
Why these five

The decisions worth modelling

Most financial choices do not need a spreadsheet. These five do, because the outcome is dominated by compounding, amortisation or an ordering effect — three things human intuition consistently gets wrong.

  1. Compounding is non-linearYour instinct underestimates it badly over long horizons, and underestimates fee drag even more.
  2. Amortisation is front-loadedThe share of an early mortgage payment going to interest surprises almost everyone who sees the schedule for the first time.
  3. Payoff order mattersThe same money applied in a different sequence changes both the total cost and the finish date.
  4. Withdrawal rates are not intuitiveThe difference between 4% and 3% is a third more capital — or several more working years.
  5. Coverage rules of thumb fail at the edges"Ten times salary" is wildly wrong for both young families and near-retirees, in opposite directions.

Private by construction

There is no endpoint to send your data to. Close the tab and the inputs are gone — that is a property of how the tools are built, not a policy promise.

Every assumption is editable

If you disagree with our return, inflation or fee assumption, change it. A model you cannot argue with is marketing.

Limits stated openly

Each tool lists what it does not model. Deterministic projections are useful; pretending they are forecasts is not.

Context for the numbers

Read these alongside the models

A calculator gives you a figure. These guides tell you whether it is the right figure to be optimising.