What is a Reverse FIRE calculation?

Traditional retirement calculators ask how much you save to tell you when you can retire. A Reverse FIRE calculator fixes your desired retirement age as a constraint and calculates the exact maximum spending budget you can sustain today while still achieving financial independence on schedule.

Coast FIRE is the milestone where your existing invested portfolio is large enough that compound interest alone will grow it to your full retirement number by your target age, requiring zero additional savings contributions.

[2026] Reverse FIRE Calculator | Early Retirement Budget & Coast FIRE Engine [Free]

Monthly Net Take-Home Calculate Net Salary →

Current Invested Portfolio

Current Age: 30

Target Retirement Age: 50

⚙️ Advanced Market & Withdrawal Assumptions (ROI, Inflation, SWR)

Pre-Retirement Nominal ROI: 7.0%

Expected Long-Term Inflation: 3.0%

Safe Withdrawal Rate (SWR): 4.0%

Maximum Allowed Monthly Spending Budget

$0

Requires saving $0 /mo (0% of net pay)

Accumulation Phase: 20 Years Target Portfolio: $0 Real Compound ROI: 3.88%

Your 2026 FIRE Milestones Breakdown

Key portfolio thresholds based on your target retirement lifestyle and empirical withdrawal multiples:

Traditional FIRE (100%)

$0

Full targeted lifestyle

Lean FIRE (75%)

$0

Basic core essentials

Fat FIRE (150%)

$0

Luxury travel & flexibility

Coast FIRE Target

$0

Needed today at age 30

Barista FIRE (60%)

$0

With part-time income

Mathematical Modeling & Empirical Research Foundations

The Reverse FIRE calculation determines the required monthly savings ($S_{req}$) by equating total future portfolio growth against the safe withdrawal multiple ($M_{ret} = \frac{12}{SWR}$). Your sustainable spending budget is computed as $\text{Net Pay} - S_{req}$.

Unlike traditional forward-looking planners that leave retirement timing ambiguous, reverse modeling fixes your retirement age as a binding boundary condition. By compounding real returns (derived via Fisher's equation $r = \frac{1 + i}{1 + \pi} - 1$) and evaluating historical 30- to 50-year survival rates from Bengen (1994) and the Trinity Study, the calculator ensures purchasing power is preserved against inflation indefinitely.