Decision Explorer

How the House-vs-Car Decision Is Evaluated

This calculator compares four ways your household could handle a major purchase decision — buying a house, buying a car, doing both, or saving instead — against your household’s real combined income and debt. Enter your household’s total annual income and monthly debt (carried forward automatically from the Household Profile step, if you’re following the Major Purchase Decision Journey), then the price, down payment, rate, and term you’re considering for a home and/or a car. This calculator computes each option’s monthly payment using the same math as the site’s own Mortgage and Auto Loan calculators, then checks each one — and the combined “Both” option — against your household’s real debt-to-income capacity.

The single most important thing this calculator gets right that a visitor comparing Mortgage and Auto Loan separately could easily miss: a household can often afford a house alone, or a car alone, while still not being able to afford both payments at the same time. “Both” is always evaluated against the combined monthly payment, never treated as automatically affordable just because each half looks affordable on its own.

The Formula

$$\vB{\text{House Payment}} = \text{Mortgage}(\vA{\text{Home Price} - \text{Down Payment}}, \vA{\text{Rate}}, \vA{\text{Term}})$$ $$\vB{\text{Car Payment}} = \text{AutoLoan}(\vA{\text{Car Price} - \text{Down Payment}}, \vA{\text{Rate}}, \vA{\text{Term}})$$ $$\vC{\text{Both Affordable}} = (\vB{\text{House Payment}} + \vB{\text{Car Payment}}) \le \vA{\text{Household Income}} \times 0.36 \div 12 - \vA{\text{Existing Debt}}$$

An option is affordable when its monthly payment fits within the household’s back-end debt-to-income capacity (36% of gross monthly income, minus existing debt) — the same standard ratio the site’s own Mortgage Affordability calculator uses. The “Neither” option instead projects what the household could grow by investing the down payment(s) and continuing to contribute what the House and/or Car payments would have cost.

Worked Example

A household with $90,000 in annual income and no other debt is considering a $400,000 home ($80,000 down, 6.5%, 30 years) and a $25,000 car ($2,500 down, 6.5%, 24 months):

1. House alone: monthly payment ≈ $2,023, within the household’s ~$2,100 front-end limit — affordable. 2. Car alone: monthly payment ≈ $1,002, well within the household’s ~$2,700 back-end limit — affordable. 3. Both together: combined payment ≈ $3,025, which exceeds the household’s ~$2,700 back-end limit — not affordable, even though each option looked fine on its own.

Key Factors to Consider

• The 36% back-end ratio is a common lending guideline, not a personal budget recommendation. Lenders use this threshold to assess default risk, but a household’s own comfortable spending level may be lower — an option flagged as “affordable” by this calculator’s DTI check is a lending-eligibility signal, not a personal-finance recommendation to spend up to that limit.
• This doesn’t include ongoing costs beyond the loan payments themselves. Property taxes, homeowners insurance, HOA dues, car insurance, fuel, and maintenance all add real ongoing cost beyond the House and Car payments modeled here — a combined “Both” option that passes the DTI check can still strain a household’s actual monthly budget once these are added.
• The “Neither” option’s projection depends heavily on the assumed investment return. A higher assumed return makes saving and investing look more attractive relative to buying, and a lower one makes it look less attractive — try a couple of different reasonable return assumptions rather than treating one number as certain.
• This models one snapshot in time, not how a household’s finances might change. Income growth, a future large expense, or a change in household debt could all shift which options are genuinely affordable — revisit this calculator if your household’s real financial picture changes meaningfully from what’s entered here.

Common Mistakes

• Evaluating House and Car only separately, never as a combined “Both” scenario. Each payment can look individually affordable while the combined total quietly exceeds the household’s real debt-to-income capacity — always check the “Both” result specifically, not just the two individual ones.
• Forgetting that “affordable” here means passing a lending-style DTI test, not a comfortable personal budget. The 36% back-end threshold is what many lenders use to gauge default risk, not a target to spend up to — a household may want real breathing room below that limit.
• Ignoring the ongoing costs layered on top of the loan payment itself. Property taxes, insurance, HOA dues, fuel, and maintenance aren’t part of the House/Car payment this calculator checks against DTI, and can turn a technically-passing option into a tight one in practice.

Useful to Know

The reason this calculator exists alongside the site’s own separate Mortgage and Auto Loan calculators is specifically to catch the case those two miss on their own: a household comparing each purchase in isolation can walk away thinking both are individually fine, without ever checking whether the two payments together still fit. Running the comparison side by side — house alone, car alone, both together, and the opportunity cost of neither — is what actually answers the real question a household facing this decision is asking.