Coast FIRE Assumption Calculator

Coast FIRE Assumption Lab

Compare your Coast FIRE number across return, inflation, fee and withdrawal scenarios. See the full planning range, which assumptions matter most, and a two-variable heatmap.

Low, base and high rangesReturn × inflation heatmapRanked sensitivity drivers

Base Financial Profile

Step 1
yrs
yrs
$
$
$

Assumption Ranges

Step 2

Planning Presets

Presets are editable planning ranges. They are not forecasts, recommendations or guarantees.

Nominal Investment Return

The nominal annual growth assumption before inflation and fees are applied.

%
%
%

Inflation

The rate used to model changes in purchasing power across bridge years.

%
%
%

Investment Fees

Recurring costs that reduce the modeled return available for compounding.

%
%
%

Safe Withdrawal Rate

Converts annual spending into the full retirement portfolio target.

%
%
%

Retirement Age

Determines how many years the portfolio has to compound before the target is needed.

Retirement Spending

Annual spending target used to calculate the full retirement portfolio requirement.

$
$
$

Sensitivity Results

Step 3
Scenarios tested: 81Result range: $1,579,399 (339.1%)Most sensitive: Nominal Return
Lower Calculated Target

$134,887

-71.0% vs base

FIRE Target$1,333,333
Real Return6.76%
Nominal Return9.00%
Inflation2.00%
Fees0.10%
SWR4.50%
Retire Age65
Spending$60,000
Base Case

$465,755

FIRE Target$1,500,000
Real Return3.40%
Nominal Return7.00%
Inflation3.00%
Fees0.50%
SWR4.00%
Retire Age65
Spending$60,000
Higher Calculated Target

$1,714,286

+268.1% vs base

FIRE Target$1,714,286
Real Return0.00%
Nominal Return5.00%
Inflation4.00%
Fees1.00%
SWR3.50%
Retire Age65
Spending$60,000

Total Range

$1,579,399

Range %

339.1%

Scenarios

81

These cards show the result range produced by the selected assumptions. They do not identify the most likely future outcome.

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Useful next steps

Explore this result

Advanced Coast FIRE CalculatorModel multi-phase retirement spending, pension income, Social Security, part-time work, and tax adjustments.Scenario ComparisonCompare multiple Coast FIRE strategies, career changes, or retirement spending levels side-by-side.Coast FIRE Date CalculatorFind the estimated month and year your portfolio reaches Coast FIRE based on your contribution rate.Coast FIRE Gap CalculatorCalculate your exact funding shortfall, progress percentage, and lump-sum or monthly savings needed.
The Assumptions That Move the Result

Which Inputs Change Your Coast FIRE Number?

The same financial profile produces very different Coast FIRE estimates when these inputs change.

01

Investment Return

Nominal annual growth before inflation and fees. Changes the modeled purchasing power of compounding.

02

Inflation

Purchasing-power erosion applied between today and retirement. Determines real portfolio growth.

03

Investment Fees

Recurring costs subtracted from the nominal return each year. Reduces compounded real growth.

04

Withdrawal Rate

Converts annual spending into the full retirement portfolio target. Lower rates raise the target.

05

Retirement Age

Determines how many years the portfolio compounds before the full retirement target is needed.

The Math Behind Each Scenario

How Each Assumption Scenario Is Calculated

Coast FIRE Real Return Formula

Real Return = (1 + Nominal − Fees) / (1 + Inflation) − 1

The Fisher equation combines nominal return, fees and inflation into the real return. All Coast FIRE accumulation math uses this real return to discount the retirement target back to today.

1
Nominal Return − Fees
Net nominal return available for compounding
2
÷ (1 + Inflation)
Fisher equation converts to real return
3
= Real Return
Used for all Coast FIRE accumulation math
4
× Spending ÷ SWR = FIRE Target
Full retirement portfolio requirement
5
Discount to today at real return
Coast FIRE number: today's required balance
The engine sends every assumption combination through the same Coast FIRE calculator as the main tool.
Identical inputs must produce the same Coast FIRE number as the main calculator before display rounding.
The grid tests up to 243 combinations (3 values × 5 assumptions). Results update in the browser.
The heatmap and driver ranking hold all non-axis inputs at the base value while varying two at a time.
How to Use It

Run the Sensitivity Lab in Three Steps

  1. 1

    Enter your current age, retirement age, current investments, monthly contributions and annual retirement spending.

  2. 2

    Select a preset or enter low, base and high values for return, inflation, fees, withdrawal rate, retirement age and spending.

  3. 3

    Review the result range, heatmap and driver ranking to see which assumptions have the greatest effect.

What It Does Not Model

Calculator Limitations to Review

  • Does not predict future returns, inflation or market conditions.
  • Does not assign probabilities to any scenario.
  • Does not model sequence-of-returns risk or variable market performance.
  • Does not calculate personal income taxes or withdrawal penalties.
  • Does not model pension, Social Security or other income sources.
  • A planning range is not investment, tax, legal or retirement advice.
When to Use This Calculator

Assumption Scenarios This Tool Handles

You want a planning range

Use it when one Coast FIRE result feels too precise for a long-term projection.

You are unsure about return

Compare lower, base and higher nominal-return assumptions side by side.

You want to test inflation

See how higher purchasing-power erosion changes the amount required today.

You are comparing fees

Estimate how recurring investment costs affect long-term projections.

You are reviewing withdrawal rates

Compare how different withdrawal-rate assumptions change the full retirement target.

You want the main result driver

The ranking shows which selected assumption range creates the largest change.

Avoid False Precision

Common Assumption-Analysis Mistakes

These pitfalls can silently distort an otherwise careful planning range.

Treating a preset as a forecast

Preset values are editable planning examples, not predictions of future markets or inflation.

Picking the lowest number as best

A lower calculated Coast FIRE number may require more optimistic return or inflation assumptions.

Mixing nominal and real returns

Nominal return, inflation and fees are combined via the Fisher equation — not added separately.

Ignoring investment fees

Fees reduce the net return used throughout the Coast FIRE projection.

Treating sensitivity as probability

The grid compares scenarios but does not assign a probability to any result.

Using extreme ranges

Very wide or unrealistic ranges produce mathematically valid but less useful planning scenarios.

Assumption Lab FAQs

What is the Coast FIRE Assumption Calculator?

It compares your Coast FIRE number across different return, inflation, fee, withdrawal-rate, retirement-age and spending assumptions using the same financial profile. You enter low, base and high values for each assumption and the tool tests every combination.

Why calculate a range instead of one number?

A range shows how sensitive the result is to uncertain long-term assumptions. It does not identify which scenario will occur, but reveals how much the target changes when inputs move.

What is the base case?

The base case is the central set of assumptions selected by the user. All ranges, percentage changes and driver rankings are compared with this result.

How does the sensitivity heatmap work?

The heatmap varies two selected assumptions while holding every other input at its base value. Each cell shows the resulting Coast FIRE number and its difference from the base case.

What does the driver ranking show?

It measures the Coast FIRE result spread between the low and high values for each assumption while other inputs remain fixed. The assumption with the largest spread ranks first.

Does this calculator predict future returns or inflation?

No. It compares user-selected planning scenarios and does not forecast market performance, inflation or the probability of any result.

Why do fees affect the Coast FIRE number?

Investment fees reduce the modeled return available for compounding. A lower net return generally increases the amount required today under the same spending and retirement assumptions.

Does the base case match the main Coast FIRE Calculator?

Yes. Identical inputs must produce the same Coast FIRE number in both tools. The Assumption Lab uses the same calculation engine as the main calculator — not a simplified substitute.

Financial disclaimer: This calculator is an educational planning tool. It uses simplified assumptions and is not investment, tax, legal, pension or financial advice. Review official sources and consult a qualified professional before making financial decisions.

Last reviewed: July 2026.Review the Coast FIRE calculation methodology →

Methodology and Trust

How This Sensitivity Engine Works

  • Methodology version: 1.0.0. Last reviewed: July 2026.
  • Results recalculate in the browser as inputs change. No data leaves the device.
  • The engine uses the same Coast FIRE calculation as the main calculator — not a simplified substitute.
  • Identical inputs produce the same base-case result as the main calculator before display rounding.
  • The Fisher equation combines nominal return, fees and inflation into the real return used for all math.
  • The grid tests up to 243 scenarios (3 low/base/high values × 5 assumption dimensions).
  • The heatmap holds non-axis inputs at base values while varying two assumptions at a time.
  • This is an educational planning tool — not investment, tax, legal or financial advice.
MethodologyData sourcesCalculator changelogFinancial disclaimer
Run your numbers

Coast FIRE Assumption Calculator

Enter your profile, adjust the assumption ranges and review the result range, heatmap and driver ranking before relying on any single number.

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