Inflation and investment returns vary widely by country, currency, and time period — the numbers above are neutral starting points, not a prediction. Replace them with your own expectations.
At a 3.9% real return, you would need about $226,657 today to coast to your retirement target with no further contributions. At your current contribution rate, you are not projected to reach Coast FI before age 65 — you would likely need to keep contributing all the way through.
Spending is not assumed flat forever. Research on real retiree spending finds it runs high through the active early years, eases through the seventies, then lifts again later as care costs arrive — a shape usually called the spending smile. Across ages 65–95 that averages about 86% of the figure you entered, which is why the target above sits roughly $140,000 below the $1,000,000 a flat-spending assumption would give. OptiAI uses the same model, so this number should match what you see in the app for the same inputs.
Educational estimate, not financial advice. Every figure is conditional on the assumptions you entered — if they hold. Uses the Fisher real return (nominal return adjusted for inflation) and a life-stage spending curve; it does not model taxes, sequence-of-returns risk, healthcare costs rising faster than general inflation, or a safety margin. The app lets you edit the spending stages; this tool uses the standard three.
How it works
The calculator estimates the portfolio you would need today — your "coast number" — that, growing untouched at your expected real (inflation-adjusted) rate of return, reaches your retirement target by your chosen retirement age with no further contributions. It compares that coast number to your current portfolio, projects your balance at retirement if you stop contributing versus if you keep contributing, and estimates the age at which coasting becomes possible given your current pace.
The math uses the Fisher real return, r = (1+nominal)/(1+inflation) − 1, rather than the simpler nominal − inflation shortcut. The shortcut looks close over a year or two, but the gap compounds — over a multi-decade horizon it can overstate your projected number by several percent, always in the flattering direction.
Spending is not assumed flat forever
The retirement target is not simply your spending divided by your withdrawal rate. That calculation assumes you spend the same amount, in today's money, every year until you die. Studies of actual retiree spending find a different shape: high through the active early years, easing through the seventies as travel and discretionary spending wind down, then lifting again at the end as care costs arrive — the spending smile.
The calculator models that as three stages (100% of your figure from 55, 80% from 71, 85% from 83) and scales the target by the average across your horizon. It shows the flat-spending target alongside, so the difference is visible rather than hidden. This is the same model OptiAI uses in the app, so the number will not shift on you when you sign up — the app simply lets you edit the stages and add a safety margin, which this tool does not.
Worth saying plainly: assuming flat spending is not the conservative choice, it is just a different and larger estimate. If you want a genuine cushion in your plan, it is better to add one deliberately and know how big it is.
Every assumption is yours to change
There are no country-specific or hidden defaults here. Inflation and investment returns vary widely by country, currency, and over time — the starting numbers are neutral placeholders, not a forecast. Enter your own expectations for a result that means something for your situation.
How OptiAI helps
OptiAI keeps your Coast FI number alongside your real net worth, goals, and the rest of your financial picture, so you can ask an AI assistant how you are tracking instead of redoing this math by hand every year.