How the numbers are worked out

A deterministic engine produces every number

Every figure you see comes from a single, versioned calculation engine. The same inputs and the same engine version always produce the same result — there is no randomness and no Monte Carlo simulation. Money is handled with exact decimal arithmetic, not floating-point approximations, so the figures add up.

Your freedom age

WhatIf projects your investable savings forward, month by month, growing them at a return assumption you can see and change. Your freedom age is the point at which those savings could cover your essential spending using a safe-withdrawal rate — the share of a portfolio a household might draw each year. Everything is expressed in today’s dollars, adjusted for inflation, so the age means the same thing from one year to the next. Your home is counted toward your net worth but never toward the savings that fund freedom, because you cannot spend the house you live in.

New Zealand tax and KiwiSaver rules

When a decision involves a change in pay, WhatIf estimates take-home income using individual income tax rates, the ACC earners’ levy, and KiwiSaver contribution rates transcribed from Inland Revenue’s own published pages. Each rule is stored with the date it takes effect, so a projection uses the rate that applies on the relevant date — not a single hard-coded figure that would already be wrong after a scheduled change.

These figures are illustrative and are being carried through independent review before they inform anything more than a scenario. The estimate is annual-bracket-based and deliberately excludes things like student loan repayments, secondary tax codes, and Working for Families — so if any of those apply to you, treat the take-home figure as a rough guide, or enter your own known take-home pay instead.

Assumptions are yours to see and change

Investment returns, the safe-withdrawal rate, and inflation are assumptions, not facts, and WhatIf shows them next to the result rather than hiding them. You can change them, and where an answer depends heavily on an uncertain return, WhatIf shows a range — how the picture shifts if the return is higher or lower — instead of a single confident number. It never gives a green “yes” or red “no”: it shows the trade-offs and the guardrails and leaves the decision to you.

What the AI does, and what it never does

WhatIf can read a plain-English question to work out which scenario you mean and pull out the figures you have given. It can also put an engine result into words. It never performs, invents, corrects, or alters a calculation — every number is produced by the deterministic engine, and an explanation that mentioned a figure the engine did not produce is rejected, not shown.