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How SafeWithdrawls Works

SafeWithdrawls runs a full calculation engine in your browser. There is no server doing the math -- everything happens locally, in real time, as you adjust your inputs. This page walks through what happens behind the scenes when you run a scenario.

The Calculation Pipeline​

When you use the Scenario Builder, five things happen in sequence:

1. You Enter Your Financial Details​

The starting inputs are straightforward:

  • Portfolio size -- How much you have saved for retirement
  • Annual expenses -- How much you need to withdraw each year to cover your spending
  • Current age and retirement horizon -- How many years the portfolio needs to last
  • Asset allocation -- The split between stocks and bonds
  • Inflation assumption -- How fast your expenses are expected to grow

These inputs are shared across every method you select, which is what makes the comparisons meaningful. Every strategy starts from the same financial situation.

2. You Select Methods to Compare​

Choose any combination of the 23 withdrawal strategies. You might compare a conservative approach like the 4% Rule against a dynamic method like Guyton-Klinger, or test a life-expectancy approach like VPW against a valuation-based method like CAPE-Based withdrawals.

There is no limit on how many methods you can run simultaneously.

3. The Engine Generates Return Sequences​

Before any withdrawal calculations can happen, the engine needs a sequence of annual investment returns to project forward. SafeWithdrawls offers three ways to generate these returns:

Historical mode starts from a specific year in the historical record and replays the actual sequence of returns that followed. If you choose 1972, your projection uses the real returns from 1972, 1973, 1974, and so on. This is useful for testing against known periods.

Bootstrap mode builds a return sequence by randomly sampling from 96 years of actual US stock and bond returns (1928--2024). Each year in your projection is drawn independently from this dataset, with replacement -- meaning the same historical year can appear more than once. Because the samples come from real data rather than a statistical model, the full range of market behavior is preserved, including severe crashes that a bell-curve model would treat as nearly impossible.

Crisis presets are a specific form of historical mode that starts from years associated with major market disruptions:

  • 1929 -- Great Depression
  • 1972 -- Stagflation era
  • 2000 -- Dot-com crash, followed by the 2008 financial crisis
  • 2008 -- Global financial crisis
  • 2020 -- COVID-19 pandemic crash

These presets let you stress-test your strategy against the worst periods in modern market history.

For a deeper explanation of why bootstrap sampling produces more realistic projections than Monte Carlo simulation, see the Methodology page.

4. Each Method Runs Year-by-Year Projections​

Once the return sequence is generated, every selected method runs its own calculator against that same sequence. Each calculator implements the specific rules and formulas of its strategy.

For a 30-year projection, the calculator steps through each year in order:

  1. Determine the withdrawal amount -- based on the method's formula and the current portfolio balance
  2. Subtract the withdrawal from the portfolio
  3. Apply the year's investment return to the remaining balance
  4. Adjust for inflation where the method requires it
  5. Record the results -- portfolio balance, withdrawal amount, whether expenses were fully covered

The key point is that every method processes the same return sequence and starts from the same portfolio. The only difference is how each method decides what to withdraw each year. This makes comparisons genuinely apples-to-apples.

5. Results Appear with Scores and Projections​

The output includes:

  • A portfolio trajectory chart showing how each method's balance evolves over time
  • A year-by-year projection table with balances, withdrawals, and returns for every method
  • Success scores summarizing how well each strategy performed

How Success Is Measured​

Traditional retirement calculators give you a single number: the percentage of simulations where your money lasted. SafeWithdrawls takes a more nuanced approach with two complementary scores.

Portfolio Health (0--100)​

Portfolio Health measures longevity risk. The core question: does your portfolio survive the full projection period?

A score of 100 means the portfolio lasted the entire horizon with a healthy ending balance. Lower scores indicate the portfolio was depleted before the end of the projection, with the score reflecting how many years were funded and how steep the decline was.

Need Coverage (0--100)​

Need Coverage measures whether your withdrawals actually met your stated expenses. This is the dimension most calculators ignore entirely.

A strategy might keep your portfolio alive for 30 years, but if it forces you to cut spending by 40% in half of those years, calling it "successful" misrepresents the experience. Need Coverage tracks how often and how severely withdrawals fell short of your target expenses.

A score of 100 means your full expenses were covered every single year. Lower scores indicate periods where the method's formula produced withdrawals below what you needed.

Overall Score​

The Overall Score combines Portfolio Health and Need Coverage into a single number. This gives you a quick way to compare strategies, while the individual scores let you understand the trade-offs. A high Portfolio Health score with a low Need Coverage score tells a very different story than balanced scores in both dimensions.

The 23 Methods​

SafeWithdrawls includes 23 distinct withdrawal strategies, organized into six categories based on their core approach:

  • Fixed-Rate methods withdraw a set dollar amount or percentage each year
  • Guardrail-Dynamic methods adjust withdrawals up or down based on portfolio performance
  • Life-Expectancy methods scale withdrawals to your remaining life expectancy
  • Valuation-Based methods adjust withdrawals based on market valuation metrics like the CAPE ratio
  • Income-Coordinated methods integrate Social Security, annuities, or other income sources
  • Cashflow-Matching methods dedicate specific assets to cover specific expenses

Each method has its own dedicated calculator that implements its published formula. The formulas come from peer-reviewed research, practitioner publications, and established planning frameworks. You can explore each method in detail in the Method Reference.

Transparency by Design​

SafeWithdrawls is built to be the opposite of a black box.

Expand any year in the projection table to see the exact calculations -- what the method withdrew, what return was applied, what the inflation adjustment was, and how the ending balance was computed.

Every formula is documented. Each method's page in the Learn section explains the math behind the calculator, with references to the original research.

All inputs are visible. The return sequence, inflation assumptions, and method parameters are shown alongside the results. Nothing is hidden behind "proprietary models" or opaque scoring systems.

The goal is not just to show you which strategy scores highest -- it is to help you understand why each strategy behaves the way it does, so you can make an informed decision about which approach fits your situation.

Next Steps​