Understand the numbers behind your plan.

Durendo projects your finances year by year, including detailed tax calculations. See how your numbers and assumptions shape the results, how we check the calculations, and what the results mean.

Your finances, calculated together.

Income, spending, investments and taxes affect one another. Durendo calculates them together to show how your plan develops over time.

Each year builds on the previous one. What you earn, spend, withdraw and keep invested changes the resources available in later years.
Your household changes over time. Retirement, Social Security, healthcare, property decisions and survivor years all shape the projection.

Detailed tax calculations, every year.

Durendo calculates taxes for each year of your plan. Coverage includes federal taxes, state taxes across all 50 states and the District of Columbia, and selected local taxes.

Details that affect what you pay. Calculations account for tax brackets, deductions, capital gains, Social Security taxation and required withdrawals, as well as how income affects healthcare credits and Medicare surcharges.
Taxes respond to your choices. A withdrawal or Roth conversion can change the tax you owe. Durendo calculates that effect, including any additional withdrawals needed to pay the tax.
Compare what you keep after tax. See how choices affect what you can spend, keep invested or leave to others, rather than judging a choice by the total tax alone.

Assumptions shape your results.

Your balances describe where you stand today. Assumptions about returns, inflation, spending and timing help you explore what could happen next.

Use defaults as a starting point. Review the assumptions in your plan and adjust them to reflect your circumstances and expectations.
Test what makes a difference. Compare alternatives to see which assumptions and decisions have the greatest effect on your plan.

Explore uncertainty in different ways.

Understanding how robust your plan is means looking beyond a single projection. These approaches answer different questions.

Scenarios and what-ifs explore specific changes. Compare a different retirement date, spending level or investment allocation to see how it affects your plan.
Monte Carlo explores different market outcomes. Simulated investment returns produce a range of possible results, showing how often your plan works without adjustments under the model’s assumptions.
Past sequences explore past market conditions. Run your plan through past sequences of returns and inflation. These are historical comparisons, not probabilities of future outcomes.

Calculations are checked and maintained.

Durendo’s checks cover individual calculations and how they work together across a plan.

Compare with reference calculations. Selected tax, Social Security and withdrawal scenarios are checked against external models, published formulas and research benchmarks.
Check for unintended changes. Automated tests cover calculations and workflows. Separate checks flag tax rules and data due for review.

Understand what the results mean.

Use the results to understand whether your plan works or needs adjusting, and identify actions that could improve your financial outcomes.

Look at the whole journey. Funding means meeting spending throughout the plan. A positive balance at the end does not, on its own, show that every earlier year was funded.
Distinguish wealth from money available to spend. Property contributes to net worth, but using its value to fund spending requires a choice about how to access it.

Projections depend on your inputs, assumptions and the model’s coverage. They do not predict future markets or tax law, and detailed tax calculations are planning estimates, not a tax return. The reference notes below explain important limits.

Explore the calculation details.

Read more about the methods, coverage and sources behind the projection.

Projection timing, household changes and dollar values

Timing. The projection starts from the plan’s balance date and runs to its planning horizon. The first year can be partial; dated income and life events affect the relevant part of the year.
Dollar values. Calculations use future dollars. Views in today’s dollars adjust for the plan’s inflation assumptions, making amounts at different ages easier to compare.
Household changes. Social Security claiming, required withdrawals, survivor spending and filing status affect the calculation. Healthcare combines coverage timing, entered costs and income-related adjustments.
Property and debt. Loan balances and property values contribute to net worth. A planned sale or an explicit home-equity choice changes what is available to fund spending.
Unmet spending. A funding gap is carried forward in real terms without adding interest. This simplification does not model the cost of borrowing to cover it.

Tax coverage and important limits

Durendo covers state income taxes in all 50 states and the District of Columbia. We review the rules against official state tax sources and maintain references and review dates for each jurisdiction.

Federal calculations. The model includes ordinary income and capital gains, deductions, Social Security taxation, net investment income tax, and required withdrawals. Income also affects modeled Marketplace credits and Medicare surcharges.
State and local calculations. Calculations cover all 50 states and the District of Columbia, including differences in brackets, deductions and the treatment of retirement income and Social Security. Local coverage includes selected city, county and school-district income taxes; some locations require an entered rate.
Roth conversions and withdrawals. Conversions change taxable income and account balances. Withdrawals needed to pay spending and taxes are calculated together, so the projection reflects the tax cost of raising the money.
Estate estimates. What remains for heirs depends on account types, modeled estate taxes and assumptions about tax on inherited accounts. This is a separate measure from what is available for your own spending.
Coverage has limits. Federal alternative minimum tax is not modeled. Property and sales taxes are entered as expenses rather than calculated as bills. Estate estimates do not cover every inheritance or lifetime gifting situation.
Future rules are assumptions. Projections use the rules and scheduled changes represented in the tax data. They cannot anticipate future legislation or replace a calculation of your actual tax liability.

Simulations, historical comparisons and outcome groups

Simulated markets. Monte Carlo generates different return paths using historical blocks or a statistical return model. Taxes, withdrawals and balances respond to each path. The chosen method and assumptions affect the results.
Inflation. In the simulation, inflation is drawn independently of investment returns. The model does not capture every relationship between inflation and markets.
Historical comparisons. Historical scenarios use past return and inflation sequences over the plan’s horizon. Available history limits the periods and investment categories that can be represented.
Outcome groups. Plenty left over, Comfortable and Just enough describe funded outcomes according to the liquid resources remaining relative to spending. Needs adjustment means at least one year’s spending was not fully met under the plan’s rules.
With real estate. When shown separately, this percentage is the share of all simulated outcomes that still need adjustment after allowing for usable real estate equity. It is additional context, not a fifth group to add to the outcome mix or an automatic property sale.

Reference checks and what they establish

Tax calculations. Selected federal and state cases are compared with saved results from NBER’s TAXSIM model. The comparisons have defined tax years, household cases and documented differences; they do not cover every tax situation.
Social Security. Reference checks use published benefit formulas and the Social Security Administration’s own detailed calculator for selected benefit calculations.
Withdrawal research. Historical and simulated withdrawal tests compare selected scenarios with published Trinity-study results. Differences in return assumptions and modeling mean this is a benchmark comparison, not an exact reproduction of every result.
Scope of assurance. Reference comparisons and regression tests help identify errors. They are not independent certification of Durendo or a guarantee of any financial outcome.

Have a question about a calculation or a result? Contact us.

Last reviewed: October 2026.