The FIRE Net-Return Model: Combine Nominal Returns, Inflation, Taxes, and Fees

Bottom line: most FIRE models fail by mixing units, not by misusing arithmetic

If your projected portfolio is expressed in future nominal dollars while your spending target remains in today's dollars, the two numbers cannot be compared directly.

A complete net-return model must use one consistent system:

1. Nominal model: both assets and spending are projected in future dollars.
2. Real model: both assets and spending are expressed in today's purchasing power.

Only then should you account for fund expenses, trading costs, taxes, and cash drag. A practical sequence is to estimate the after-friction nominal return first, then convert it to a real return with the exact inflation formula.

For the foundation, see Same Return, Different FIRE Speed and How Much Do You Need to Save for FIRE?.

Which return belongs in a FIRE plan?

Define:

Rg = gross nominal return
F  = fund, advisory, and platform expenses
T  = trading costs and spread drag
X  = estimated annual tax drag
C  = excess-cash drag
I  = inflation

First estimate the net nominal return:

Rn = Rg - F - T - X - C

Then convert it to a real net return:

Rreal = (1 + Rn) / (1 + I) - 1

At a 6% net nominal return and 2.5% inflation:

(1.06 / 1.025) - 1 = 3.41%

Subtracting inflation is acceptable for a quick estimate. The ratio is better for a long-horizon model.

Tax drag is account- and household-specific

U.S. investors do not have one universal investment tax rate. The result depends on account type, holding period, ordinary versus qualified dividends, realized gains, state rules, and the household's wider taxable income.

Do not subtract a marginal income-tax bracket from the entire portfolio return. Instead, estimate:

Annual tax drag =
tax attributable to portfolio cash flows / average invested assets

Keep taxable brokerage, traditional retirement, and Roth assets separate. A distribution from a traditional 401(k) or IRA, a qualified Roth distribution, and a taxable-account sale can produce very different spendable cash.

If the estimate is uncertain, use scenarios such as 0.2%, 0.6%, and 1.0%. The After-Tax FIRE Withdrawal Template provides the cash-flow version of this model.

U.S. example: what remains from a 7% headline return?

Assume a household has 20 years before reducing work:

• current investable assets: $250,000
• end-of-year contributions: $24,000
• gross nominal return: 7.0%
• weighted fund expense: 0.15%
• advisory and platform costs: 0.15%
• trading and spread drag: 0.05%
• estimated tax drag: 0.35%
• excess-cash drag: 0.10%
• inflation assumption: 2.5%

Net nominal return:

7.00% - 0.15% - 0.15% - 0.05% - 0.35% - 0.10% = 6.20%

Real net return:

(1.062 / 1.025) - 1 = 3.61%

The household can now choose one of two consistent methods.

Method A: future nominal dollars

Compound assets at 6.20%, increase future contributions and spending with their chosen inflation assumptions, and compare the result with a future-dollar FIRE target.

If today's core spending is $72,000 and the planning withdrawal rate is 3.5$2.06 million. At 2.5% inflation for 20 years, the equivalent nominal target is about $3.37 million.

Method B: today's purchasing power

Compound at the 3.61% real rate, express contributions in constant purchasing-power dollars, and compare the result with the $2.06 million real target.

The two approaches should be close. A large gap usually means inflation was applied to only one side.

A spreadsheet structure you can reproduce

Create these columns for each year:

The recurrence is:

P(t+1) = P(t) × (1 + Rn) + Contribution(t) - Withdrawal(t)

For a real model, replace Rn with Rreal and keep contributions and withdrawals in constant dollars.

Build the baseline in the Fire Path FIRE Calculator, then document your model choices using the methodology page.

Run combined sensitivity tests, not isolated ones

At minimum, compare:

Lower returns and higher inflation can occur together. Testing each one separately can miss the scenario that matters most. See Annual FIRE Recalculation and Sensitivity Examples for the broader process.

Avoid double counting

• Fund and advisory expenses can be deducted from the gross return estimate.
• Inflation should be converted with the ratio formula for precision.
• Tax drag should reflect account activity, not a tax bracket applied to all assets.
• Cash drag applies only to cash beyond the reserve's intended job.
• Lifestyle creep raises the spending target; it is not also a return deduction.
• If a total-return record is already net of fund expenses, do not subtract the expense ratio again.

These boundaries matter more than adding another decimal place.

A 30-minute annual calibration

1. Gather beginning and ending balances plus net cash flows.
2. Calculate a cash-flow-adjusted portfolio return.
3. Express fund, advisory, trading, and tax costs as a share of average assets.
4. Start with BLS CPI, then adjust for the household's housing, healthcare, and education mix.
5. Run baseline, defensive, and high-inflation cases.
6. Record why assumptions changed; do not rewrite long-run returns merely because of one good or bad year.

A FIRE model is not a forecast. It is a consistent accounting system for uncertainty.

References

• U.S. Bureau of Labor Statistics: Consumer Price Index
• SEC Investor.gov: How Fees and Expenses Affect Your Investment Portfolio
• IRS Publication 550: Investment Income and Expenses
• IRS: 401(k) Plans
• SEC Investor.gov: Compound Interest Calculator

Scope and freshness

• Scope: U.S. households modeling FIRE accumulation, transition, and portfolio withdrawals.
• Units: Use either nominal future dollars or real today-dollars, but never mix them. All numbers are educational examples.
• Last reviewed: 2026-05-29.
• Limits: Actual federal and state taxes depend on filing status, account type, income, holding period, and current law.
• This article is educational and is not investment, tax, legal, or individualized retirement advice.

Next step: build a baseline in the Fire Path FIRE Calculator, then connect it to the after-tax withdrawal template and annual recalculation framework.