Amortization based withdrawal
Amortization based withdrawal (ABW) uses amortization techniques to calculate portfolio withdrawals during retirement.
Amortization
Understanding amortization
Amortization is the process of converting the value of a lump sum into a series of payments over time. Most people encounter it when they take out a mortgage or car loan. Amortization is used to calculate the monthly payment required to pay back the loan. The value of the monthly payments has to be made equal to the value of the initial loan.
If the interest rate is zero, amortization is just simple division: If $100,000 has to be paid back over 10 years, then the payment is $100,000 ÷ 10 = $10,000 per year.
But if the interest rate is positive, as is usually the case, then the payment in the above example would have to be more than $10,000 per year. That's because $10,000 in the future is worth less than $10,000 today. This is referred to as the time value of money. If the interest is 5% per year, then $10,000 today is worth:
 one year from now
 two years from now
and so on. We will have to adjust for the differences in value. The amortization formula makes the necessary adjustments and tells us that if the interest rate is 5% per year, then the payments will have to be $12,950 per year for the next ten years for the total value of the payments to be equal in value to the $100,000 that was borrowed today.
Amortization based withdrawal
To calculate portfolio withdrawals, enter the following inputs in the ABW calculator (below):
 current portfolio value
 number of years over which withdrawals are to be spread out
 expected return of the portfolio.
Example
A 65 year old retiree with a $1 million portfolio expected to earn 3% per year and seeking to spread out withdrawals over 36 years (ages 65 to 100), would enter . The amortization calculator says that this retiree can withdraw $44,470 per year. This means that if the portfolio actual rate of return remains constant at 3% per year and the retiree withdraws $44,470 every year, then the portfolio will be fully depleted after 36 years.
Real vs nominal rate of return
If a real (inflation adjusted) rate of return is used, the withdrawal strategy becomes automatically adjusted for inflation. In the above example, if the real rate of return is 3%, the retiree will be able to take out $44,470 (inflation adjusted) each year. If instead, the retiree enters a nominal interest rate of 5%, the calculator says that they can take out $60,434 per year. But that's in nominal dollars, so real consumption will decline each year by the inflation rate. In practice, it is strongly recommended to use a real rate of return.
Update and adapt
Withdrawals are recalculated each year with updated information about portfolio value, expected rate of return, and number of years left. This allows withdrawals to continually adapt to new information and mitigates the impact of past incorrect assumptions. Amortization spreads the course correction required across all remaining years, thereby minimizing the impact on any one year.
Example
Consider what happens in our earlier example if, instead of gaining 3% as expected, the portfolio actually loses 10%. The shortfall relative to expectations is . Our retiree had expected to start age 66 with $984,196 in the portfolio. Instead, the portfolio contains only $859,977, which is 12.6% less. Consider what happens in the following two scenarios:
Scenario 1: Expected return stays the same
Suppose that the retiree believes that this market drop has not impacted future expected returns, which in the retiree's opinion, remains at 3%. The retiree enters the new reduced portfolio value, , number of years remaining , and expected real return . The withdrawal is now reduced to $38,857 per year. That is a 12.6% reduction relative to the previously planned withdrawal of $44,470. So the 12.6% portfolio loss is being matched by a 12.6% spending cut across all remaining years.
Scenario 2: Expected return increases
If the market drop was due to a pure valuation change and the underlying earnings of the portfolio are not impacted, then the retiree might expect a higher rate of return going forward.
If expected return is based on an earnings yield model (e.g. 1/CAPE) for stocks and yield to maturity for bonds, a 12.6% reduction in valuation will result in a higher expected real return of . With the higher expected return, the withdrawal becomes $41,181. That is a reduction of only 7.4% relative to the original plan of $44,470. A 12.6% reduction in the portfolio led to only a 7.4% reduction in income, because the higher expected return partially offset the loss in portfolio value.
So to the extent that market volatility is caused by valuation changes and gets incorporated into expected returns, withdrawals from the portfolio will be less volatile than the portfolio itself.
Adding a terminal balance
Instead of fully depleting the portfolio, the amortization can be modified to leave behind a terminal balance. This could be used to fund end of life care or a bequest, or it could simply serve as a general financial cushion.
In the ABW calculator, enter:
 terminal balance
Example
The 65 year old retiree in the earlier example wants to have $200,000 left at the end for a bequest. Enter in the calculator to get a withdrawal of $41,401 per year. If the actual rate of return matches expectations, then after withdrawing $41,401 for 36 years (ages 65 to 100), the retiree will have $200,000 remaining in the portfolio at the end of age 100.
Rising or falling withdrawal schedules
Instead of aiming at a constant withdrawal schedule, some retirees may prefer to aim at a rising or falling withdrawal schedule. Retirees worried about incurring unexpected expenses or generally running out of money in later years may want to use a rising withdrawal schedule  take less early so they can have more later. Those wanting to fund an active early retirement may want to use a declining withdrawal schedule  take more early but have less later. (Note the similarity with savings. This can be viewed as a decision about how much to save in the early retirement years in order to fund late retirement consumption.)
In the ABW calculator, enter:
 rate of growth of the withdrawal schedule
generates a rising withdrawal schedule, generates a declining withdrawal schedule, and generates a constant withdrawal schedule.
Example
If the 65 year old retiree in the earlier example wants withdrawals to grow at 1% per year, set to get a withdrawal of $38,349. Withdrawals are scheduled to grow 1% per year, rising to $54,325 at age 100.
If instead the retiree wishes to spend more in early retirement and wants withdrawals to fall by 1% per year, set to get a withdrawal of $51,118. Withdrawals are scheduled to fall 1% per year, declining to $35,959 at age 100.
Asset allocation
Fixed asset allocation
The amortization formula assumes an expected rate of return on the portfolio which stays consistent over the retirement period. This is easier to justify if the retiree plans to maintain a fixed asset allocation (AA). If instead the retiree is planning to change the AA over time (e.g. glide down from 60/40 to 30/70 over the course of the retirement), then the expected return will display more variations over time, skewing the amortization computation. The calculator provided here is not designed for such case. Entering the average expected return in the calculator may provide a rough estimate, but due to the potential for significant errors, more precise modeling is recommended.
Fixed asset allocation with subportfolios
The portfolio can be divided into multiple subportfolios and a different fixed allocation applied to each subportfolio.
Suppose the retiree is more risk averse over certain categories of consumption (e.g. food and housing), and less risk averse over other categories (e.g. vacations). The portfolio can be subdivided into a safer portfolio to fund food and housing and a riskier portfolio to fund vacations. The safe portfolio would get a safe fixed AA like 30/70, and the riskier portfolio would get a riskier fixed AA like 80/20. Multiple subportfolios could be created to fund different consumption goals, each with a different AA that matches the riskreward considerations for that goal. Withdrawals would be calculated separately for each subportfolio.
An example of a strategy that uses subportfolios is the Liability Matched Portfolio (LMP) strategy. The portfolio is divided into a "liability matched portfolio" and a "risk portfolio". The liability matched portfolio has an AA of 0/100 and is used to fund nonnegotiable consumption. The risk portfolio is given a riskier AA and is used to fund discretionary "nice to have" consumption.
Estimating the expected return
The expected return is the amount of profit or loss an investor can anticipate receiving on an investment. An expected return is calculated by multiplying potential outcomes by the odds of them occurring and then totaling these results.^{[1]} The expected return can be expanded to analyze a portfolio containing many investments. If the expected return for each investment is known, the portfolio's overall expected return is a weighted average of the expected returns of its components.^{[1]}
The expected return is usually based on historical data and is therefore not guaranteed into the future; however, it does often set reasonable expectations.^{[1]} Some models rely more on current data (e.g. market price).
The ABW calculation requires the user to input the expected rate of return of the portfolio. ABW does not recommend a specific model for this purpose. The user is free to use whatever model of expected return they prefer. Regardless of the method, the user must be aware if the estimate is in real (inflationadjusted) or nominal (not inflationadjusted) terms. Examples of models that can be used include:
Models based on historical returns
This model uses a fixed value equal to the longterm weighted average of historical results. The weights are the same as the components of the portfolio's asset allocation, applied to historical averages of each market segment of interest.
For example, the Credit Suisse Global Investment Returns Yearbook provides 120 years of data on stocks, bonds, bills, inflation and currency for 23 national markets and for the world as a whole^{[2]}. As a rule of thumb, in real (inflationadjusted) terms, stocks returned approximately 5% worldwide and bonds returned approximately 2%. If one's portfolio uses a 60/40 asset allocation, then the expected return would be 5% x 60% + 2% x 40% = 3.8%.
Models based on current yields
This model uses a value computed on a periodic basis (e.g. annual) derived from recent yields metrics. Such variable models have historically displayed useful smoothing properties when used in the context of an amortization based withdrawal method, as illustrated in scenario 2 above.
For stocks, the earnings yield E/P (the inverse of a price to earnings ratio P/E) can be used. Given earnings volatility, it is typically preferred to average earnings over a longer time period than just one year. The inverse of the Shiller PE10 ratio (aka CAPE) for a given stock market gives an earnings yield that averages earnings over the last 10 years. For the US, multpl.com provides the latest CAPE. For international markets, Star Capital provides a monthly update. Stocks earning yields are real (inflationadjusted) values.
For bonds, the SEC Yield can be used. Yields for regular bonds are nominal values and need to be adjusted for expected inflation (e.g. the US Federal Reserve 2% target) to obtain a real value. For TIPS, the SEC Yield is a real value. As bonds yields tend to provide short to mid term expected returns (e.g. a decade at most), it might be beneficial to compute the average of historical returns and current yields.
Models based on experts estimates
Multiple wellknown expert sources provide expected returns for various market segments, while regularly updating corresponding models. Care should be exercised about the exact nature of such estimates (e.g. real vs. nominal, midterm vs. longterm, etc.)
As an example, Vanguard and Research Affiliates are often cited as credible sources.
Extra income and expenses
Besides their savings portfolio, retirees may have other sources of present or future income such as social security, pensions, annuities, rental income, parttime wages and lump sum payouts ("extra income"). Retirees may also plan for exceptional expenses such as college tuition, a lingering home mortgage or longterm care expenses that may require funds beyond what is provided by regular budgeting ("extra expenses"). Such future extra income and expenses can be amortized by computing the present value of such future sums (see Wiki article on calculating present value by discounting future cash flows: Comparing Investments) and converting in an additional series of payments, beyond the regular payment derived from the savings portfolio's balance. It is strongly recommended to perform such computations in real (inflationadjusted) terms. Funding such additional payments in the shortterm can be achieved in different ways, e.g. structuring financial assets according to the retiree's personal risks/rewards goals. ABW is neutral and does not take a position on how best to do it. Below are some of the strategies which can be used:
Separate subportfolio focused approach
The retiree can choose to keep funds for some or all of the extra income and expenses outside the regular amortization process. They can allocate extra income and budget for extra expenses, separate from the ABW calculation. This is effectively a partitioned portfolio approach where extra income and expenses are being handled in a separate safe subportfolio.
A retiree formally pursuing the separate portfolio approach described earlier may find it convenient to use conservative estimates of social security and pensions to fund the safe subportfolios. For more complex cases, sizing the subportfolio can be achieved by a present value computation of future extra income and expenses (e.g. when using a bonds ladder, use the bonds interest rate to discount future income/expenses). The income streams are typically duration matched. Gaps remaining can be filled with duration matched bonds. Depending on the asset allocation, there may be little to no rebalancing needed in safe subportfolios.
Savings portfolio focused approach
The retiree funds the additional payments through additional withdrawals from the savings portfolio, while maintaining a fixed asset allocation on the savings portfolio. The discounting and amortization rate used should then be the savings portfolio's expected rate of return. Care should be exercised about the additional burden on one's savings portfolio and possible consequences of market crises before steady future income (e.g. SS/Pensions) materializes.
Calculate the extra net income (extra income minus extra expenses) for each year of the planned withdrawal period, using conservative estimates of extra income and exceptional expenses. Calculate its aggregate present value using the portfolio's expected rate of return ^{[3]}. Calculate amortized additional withdrawals based on such present value, in the same manner as the savings portfolio balance is amortized. Such computation is independent from the portfolio's rebalancing procedure and doesn't change the expected return computation. More information on this approach is available here: Early Retirement and Time Value of Money, a blog article which comes with a companion spreadsheet.
Total portfolio focused approach
The retiree adds the present value of safe extra net income (extra income minus extra expenses) to the savings portfolio to arrive at the value of the total portfolio. Asset allocation and withdrawal calculations are done on the total portfolio, counting the value of the safe extra net income as a bond. When the withdrawal scheduled for a given year exceeds the extra net income for that year, the difference is withdrawn from the savings portfolio. Conversely, when the withdrawal scheduled for a given year is less than the extra net income for that year, the difference is contributed to the savings portfolio.
Calculate the extra net income (extra income minus extra expenses) for each year of the planned withdrawal period, using conservative estimates of extra income and exceptional expenses. Calculate its aggregate present value using bond interest rates. Add this value to the savings portfolio for all calculations, counting it as a bond. Include it when calculating withdrawals. Include it when calculating the asset allocation of the total portfolio (for rebalancing) and the expected return of the total portfolio (to enter into the amortized payment computation). More information on this approach is available here: Total portfolio allocation and withdrawal.
ABW calculator
Basic calculator
The Excel spreadsheet to calculate withdrawals in on Google Drive: ABW Calculator
Amortization based withdrawal formulas describes the formulas used in the calculator.
Advanced calculators
These calculators expand the basic functionality to include other considerations.
An Excel spreadsheet that incorporates extra income and expenses using the "total portfolio focused approach" is available on Google Drive: Total portfolio allocation and withdrawal.
A Google Sheets spreadsheet that incorporates extra income and expenses using the "savings portfolio focused approach" is available here (please make a copy): Amortized Spending Budget  Savings Portfolio Approach. This spreadsheet also provides various backtesting capabilities.
Support
Ongoing discussion and support is in Bogleheads® forum topic: Amortization Based Withdrawal (ABW).
See also
 Comparing investments  Tutorial on Time Value of Money equations
 How Much Can I Afford to Spend in Retirement  Blog and spreadsheets from Ken Steiner (actuarial approach for determining reasonable spending budgets)
References
 ↑ ^{1.0} ^{1.1} ^{1.2} Expected Return  Investopedia
 ↑ Credit Suisse Global Investment Returns Yearbook  2020 edition
 ↑ Choosing An Appropriate Discount Rate For Retirement Planning Strategies Michael Kitces, July 2017

