Financial Projection

Every projection, from a simple retirement calculator to a company’s five-year model, runs the same loop. Each year starts with last year’s ending balances, adds income and investment growth, subtracts spending and taxes, and hands the result to the next year. The loop is easy. The judgment lies in the inputs, because a small error repeats every year and compounds over decades.

A household projection is only as complete as its ledger. If it tracks balances but not cash flow, it cannot tell you whether a year’s spending was actually funded. If it ignores taxes, a pre-tax 401(k) balance looks larger than the spending it can support. To build one:

A projection can report results in nominal dollars, the amounts that will actually appear on future statements, or in constant dollars at today’s prices, which strip out Inflation so a future sum can be compared with prices now. Both are valid; mixing them is the error. A $1 million balance 25 years from now sounds larger than it is: at 3% inflation it buys roughly what $478,000 buys today.

The Consumer Price Index for All Urban Consumers rose 3.4% in the 12 months to August 2026, with the core index up 2.4%. A projection’s inflation rate is a long-run assumption, not last year’s reading, but it should be one you can defend.

Today’s-dollar models have one catch. Taxes are charged in nominal dollars, and some thresholds never rise with prices. The 3.8% net investment income tax starts at $200,000 of modified adjusted gross income for single filers and $250,000 for joint filers, and the IRS notes those amounts are not indexed for inflation. Over decades, inflation alone pushes more households over them. A sound projection works out each year’s tax in that year’s own dollars, then converts the results for display.

The simplest projection assumes the same return every year, such as a steady 6%. That is fine for estimating a savings target, but real markets deliver their average unevenly. While you are adding money, the order of returns matters less. Once you are withdrawing, a few bad years at the start can do lasting damage, a problem called sequence of returns risk, and a straight line cannot show it.

A single line also looks more precise than it is. A balance projected to the dollar 30 years out is one result from one set of assumptions, so read it as the middle of a range. Three tools supply the range: a Monte Carlo simulation runs many random return paths, a historical backtest replays real past sequences in order, and scenario planning changes several assumptions together to describe distinct futures. All three rest on the same year-by-year engine, so a sound deterministic projection remains the foundation.

In business, a financial projection usually means a set of forecast statements. The Small Business Administration’s business plan guidance asks for forecasted income statements, balance sheets, cash flow statements and capital expenditure budgets covering the next five years, with quarterly or even monthly detail for the first year. Lenders and investors use them to judge whether the business can repay a loan or grow.

A personal projection runs much longer, often from today to the end of the longest plausible lifespan, and it has to handle problems businesses rarely face: accounts with different tax rules, contribution limits, required minimum distributions, Social Security claiming, and a household whose paychecks stop at retirement. It serves financial planning rather than a loan application, so its job is to compare choices, not to impress a reader. The discipline is the same, though: state every assumption openly, and update the numbers when actual results arrive.

Most projection errors are not arithmetic mistakes; they are assumptions nobody examined. Because each year feeds the next, a small error in year one grows with every year that follows. A useful habit is to write each assumption next to the source or reasoning behind it, then revisit the list every year when you replace projected balances with actual ones. Assumptions that looked reasonable five years ago may not look reasonable now. Before trusting a result, check for these: