Coast FIRE Calculator: Use Present and Future Value to Check the Threshold

Bottom line: a Coast FIRE calculator needs seven inputs, all in consistent units

A reproducible calculation needs:

1. current age;
2. target retirement age;
3. annual retirement spending;
4. reliable retirement income;
5. planning withdrawal rate;
6. real net return;
7. current long-term retirement assets.

The most common error is using today's spending with a nominal return that has not been adjusted for inflation. This article uses today's purchasing power throughout: spending, income, targets, and contributions are real amounts, and the return is a real net return.

Coast FIRE only checks whether long-term retirement assets may grow to the target. It does not mean retirement is possible today. Reduced work still needs to fund current life. See When Coast FIRE Fits Better Than Full-Time Work.

Step 1: calculate the retirement portfolio gap

Retirement portfolio spending gap S =
annual retirement spending
- reliable after-tax income during the same years

Reliable income can include reviewed Social Security, pension, or annuity estimates. Watch the timing:

• Social Security may start after the planned work exit;
• the estimate depends on earnings and claiming age;
• a bridge period must be funded separately;
• uncertain income should be discounted or excluded from the baseline.

If work stops at 55 and Social Security starts at 67, the 12-year bridge cannot be erased from the model.

Step 2: calculate the retirement target

Retirement target F = S / w

At a $72,000 annual portfolio gap:

A lower planning rate creates a higher target. Four percent is a historical framework, not a guarantee for a very long FIRE horizon. See Why the 4% Rule Is Not for Everyone.

Step 3: discount the target to today

n = target age - current age
r = real net return

Coast threshold PV = F / (1 + r)^n

Real net return should reflect:

gross nominal return
- fees, trading friction, and estimated tax drag
then converted for inflation

See The FIRE Net-Return Model.

Complete U.S. example

Assume:

• age 35 now;
• target age 60, so n = 25;
• retirement spending of $72,000 in today's dollars;
• Social Security excluded until the estimate and claim strategy are reviewed;
• withdrawal rate 3.5%;
• real net return 4%;
• current long-term retirement assets $650,000.

Target

F = $72,000 / 3.5% ≈ $2.057 million

Coast threshold

PV = $2.057 million / (1.04 ^ 25) ≈ $772,000

Gap

Coast gap = current assets - threshold
          = $650,000 - $772,000
          = -$122,000

The household is below the baseline threshold. That does not mean contributing $122,000 today; 25 years remain for ongoing contributions.

How much annual contribution closes the gap?

For equal real end-of-year contributions C:

C =
[F - P × (1 + r)^n]
× r / [(1 + r)^n - 1]

P is current retirement assets. In the example:

C ≈ $7,788 per year

That is about $649 per month in today's purchasing power. Nominal payroll contributions would need to rise over time if real contribution purchasing power is held constant.

If r = 0, use:

C = (F - P) / n

This avoids a division-by-zero spreadsheet error.

What changes when Social Security is included?

Suppose the household reviews its SSA estimate and claim timing and uses $18,000 of real annual after-tax income in the period where it actually applies, with the bridge funded separately:

S = $72,000 - $18,000 = $54,000
F = $54,000 / 3.5% ≈ $1.543 million
PV = $1.543 million / (1.04 ^ 25) ≈ $579,000

Current assets of $650,000 would exceed that threshold. The conclusion now depends on benefit amount, start date, taxes, and bridge funding. Keep a “no Social Security” baseline and a “reviewed estimate” case rather than selecting only the better answer.

Return sensitivity

Excluding Social Security and holding other inputs constant:

If only the 5% real-return row passes, do not immediately declare Coast. Ask whether the household can hold the required allocation and whether fees, taxes, and inflation are correctly reflected.

The calculator should return four answers

Do not show only “pass/fail.” Return:

1. real retirement target F;
2. today's Coast threshold PV;
3. current asset surplus or gap;
4. required annual or monthly real contribution when below.

Add at least three scenarios:

Coast is a long-horizon decision. One precise number creates false confidence.

What should not be counted as current retirement assets?

• Home equity without a documented sale, rental, or downsizing plan and costs.
• Emergency reserves.
• College, home-purchase, or caregiving funds needed within three to five years.
• Unvested benefits or insurance values that cannot be confirmed.
• Social Security, pension, and account balances counted twice.
• A concentrated asset the household cannot hold through stress.

Countable Coast assets =
assets that can remain invested
without being used before retirement

Passing the threshold is not enough

Current cash flow still needs to pass:

after-tax reduced-work income
- current essential spending
- health coverage and benefit replacement
- emergency-fund maintenance
- near-term goals
>= 0

If it is negative, retirement assets will be interrupted and the Coast equation no longer applies. Use the Fire Path Calculator for the complete FIRE path and treat Coast as one career-decision metric.

References

• SEC Investor.gov: Compound Interest Calculator
• Social Security Administration: Retirement Benefits
• Social Security Administration: Life Expectancy Calculator
• BLS: Consumer Price Index
• IRS: 401(k) Plans

Scope and freshness

• Scope: U.S. households using today's dollars to estimate a Coast FIRE retirement-asset threshold.
• Last reviewed: 2026-07-10.
• Limits: The formula simplifies returns into a constant real rate. Actual returns have sequence risk, while benefits, taxes, healthcare, spending, and family duties change.
• This article is educational and is not investment, tax, insurance, employment, legal, or individualized retirement advice.

Next step: build the full scenario in the Fire Path Calculator, then return to the Coast FIRE Readiness Check for current cash flow and benefits.