Calculator Methodologies & Math
Every number our calculators produce is grounded in a specific formula or assumption. These pages document exactly how the math works, what inputs drive the outputs, and where we have made deliberate choices about modeling. Use them to verify our logic, adapt the formulas for your own models, or simply understand why the calculator says what it says.
Why Methodology Transparency Matters
A rent vs buy calculator is only as trustworthy as the assumptions baked into it. Many calculators on the web omit opportunity cost entirely, ignore transaction costs on exit, or apply a single national appreciation rate to every market. Those omissions systematically bias results toward buying. Our methodology documentation shows you exactly what we include and why, so you can judge whether our model reflects your situation.
If a formula produces an output that surprises you, the methodology pages give you enough detail to trace the result back to its source. Understanding the math also helps you stress-test your own situation by changing key assumptions and observing how sensitive the conclusion is to each variable.
Calculator Methodologies
Each of these pages documents the complete mathematical model behind one of our calculators: every variable, formula, and assumption, explained in plain language alongside the equations.
Rent vs Buy Calculator Methodology
The complete explanation of every assumption, variable, and formula inside the Rent vs Buy Calculator. Covers how we model break-even timelines, opportunity cost of the down payment, appreciation rates, maintenance costs, and the total unrecoverable cost comparison across renting and owning.
Mortgage Calculator Methodology
How we handle amortization schedules, monthly payment calculations, total interest paid over the loan term, and the mathematical relationship between rate, term, and principal. Includes the exact formulas used and a plain-English explanation of each component.
Home Affordability Calculator Methodology
The debt-to-income formulas, algebraic home price derivation, and model limitations behind our affordability calculator. Explains how we work backward from income and debt obligations to a maximum safe purchase price, and where lender guidelines diverge from conservative personal finance advice.
How the Math Connects to the Rent vs Buy Decision
The rent vs buy decision is not one equation. It is a system of interacting formulas: mortgage amortization determines how quickly equity builds, opportunity cost models determine what the down payment could have earned elsewhere, transaction cost accounting determines how long you must hold the home before selling makes financial sense, and maintenance cost modeling determines the true monthly cost of ownership above the mortgage payment.
Each calculator isolates one part of that system. The Rent vs Buy Calculator integrates all of them into a unified break-even model. The Mortgage Calculator handles the amortization layer. The Affordability Calculator works in the other direction, solving for the maximum purchase price given income and debt constraints. Together, these tools answer the full question.
Understanding the methodology behind each tool tells you which inputs to take seriously and which are approximations. Appreciation rate, for example, is the single variable with the most uncertainty in any long-horizon model. The methodology pages document how we handle that uncertainty and what range of outcomes different assumptions produce.
Foundational Concepts
These guides explain the mathematical concepts that underpin our calculators, without requiring you to read the full methodology documentation first.
Put the Math to Work
Now that you understand how the model works, run your own numbers. The Rent vs Buy Calculator applies everything documented in the methodology pages to your specific inputs and shows you the break-even timeline for your market.
Open the Rent vs Buy CalculatorThese methodology pages are for general informational and educational purposes only and do not constitute financial, tax, or legal advice.