Borrowing Power Calculator

DTI: the number that controls your borrowing power

Debt-to-income ratio (DTI) is the single biggest factor lenders use to decide how much you can borrow. The math is simple: total monthly debt payments divided by gross monthly income. If you earn $8,000/month gross and have $2,400 in monthly debt obligations (including the new mortgage), your DTI is 30%.

Lenders set DTI caps to protect themselves — and you — from over-borrowing. The most common caps are 36% (conservative), 43% (US Qualified Mortgage limit), and 50% (FHA / some aggressive programs). Within these caps, lenders also look at “front-end” DTI (housing costs only / income), which is typically capped at 28%. This calculator uses combined back-end DTI for simplicity — the most relevant number for the affordability decision.

The math works backwards from DTI. If your max DTI is 36% and your gross monthly income is $8,000, the maximum total monthly debt the lender will allow is $2,880. Subtract your existing monthly debts ($500 in car loans, say), and you have $2,380 available for housing. At a 6.5% rate over 30 years, $2,380/month corresponds to a loan of about $377,000. That’s your borrowing power.

What counts as “debt” in DTI calculations

Lenders include in DTI:

• Minimum credit card payments (not the full balance — just minimums)
• Auto loan payments
• Student loan payments (sometimes calculated as 1% of balance if in deferment)
• Personal loan payments
• Existing mortgage payments on other properties
• Child support and alimony (court-ordered)
• The new mortgage payment being calculated (P&I, plus PMI, plus property tax and insurance through escrow)

Lenders do NOT include:

• Rent (since you’re replacing it with the new mortgage)
• Utilities (water, electricity, gas, internet, phone)
• Groceries or other discretionary spending
• Insurance premiums outside of escrowed home insurance (health, life, auto are excluded)
• Investment contributions (401k, IRA, taxable brokerage)
• Charitable giving

This is critical because the DTI formula gives a flatteringly high number compared to your real budget. Lenders only see your debt obligations and income — not your full lifestyle. A 43% DTI on paper might leave you stretched once you add in groceries, utilities, kids’ activities, retirement savings, and the unexpected expenses every household has. Don’t confuse “can borrow” with “should borrow.”

Maximum vs comfortable — they’re not the same

A common rookie mistake is treating the lender’s maximum approval as a target. It’s not — it’s a ceiling. The number this calculator gives you is what a lender will let you borrow, not what you’d be wise to borrow.

The gap between “maximum” and “comfortable” is often 25-40%. Here’s the typical pattern: someone qualifies for a $400K mortgage based on DTI. They buy a $380K house thinking they have a $20K cushion. Then they realize:

• Property tax is higher than estimated ($600/month vs $400 estimated)
• Home insurance has a flood/wind rider in their area (+$80/month)
• The yard needs landscaping ($3K)
• The HVAC needs replacement in year 2 ($8K)
• HOA dues went up 15% in year 3
• Their car payment ended but they replaced it with another car
• Daycare costs jumped when the second kid arrived

Each of these is unpredictable individually, but collectively they’re guaranteed — something will hit each year. Borrowing at the maximum leaves no buffer for the inevitable surprises.

The 28% rule (recommended over 36%)

A widely-recommended personal-finance benchmark is to keep housing costs (PITI + HOA) under 28% of gross income — front-end DTI. That’s noticeably tighter than lenders allow but leaves room for everything else in your life. Run this calculator at 28% DTI with $0 existing debts to see your “comfortable” borrowing range. Then compare it against the “maximum” number at 43% DTI with your real debts. The gap is your safety buffer.

Lender DTI rules by country

DTI thresholds vary internationally. The big picture: most developed markets have similar 35-45% caps for prime mortgages, with some flexibility for high-income or low-LTV borrowers.

United States

• Qualified Mortgage (QM) limit: 43% back-end DTI
• FHA: Up to 50% with compensating factors
• VA: No hard cap, but 41% is the residual income guideline trigger
• Conventional: Most lenders cap at 45%, prefer 36%

United Kingdom

• Affordability assessment rather than strict DTI ratio (since 2014 MCD reforms)
• Loan-to-income (LTI) cap: 4.5x salary for most prime mortgages; 4-4.5x typical
• Stress test: Lender models your payment at higher rates (usually +3% over offered rate)

Australia

• Serviceability assessment — lenders use a custom formula including buffer rates and minimum expenses
• Buffer rate: APRA requires lenders to assess at 3% above actual rate
• HEM benchmark for minimum household expenses (postcode-adjusted)

Canada

• GDS (Gross Debt Service) ratio: Housing costs ≤ 39% of income
• TDS (Total Debt Service) ratio: All debts ≤ 44% of income
• Stress test: Qualifying rate of contract rate + 2% or 5.25%, whichever is higher

Germany

• Typically 35-40% of net income on housing
• Strong emphasis on equity (typically 20-30% down required, vs 5-20% in US)
• Longer fixed-rate periods common (10-15 year fixes)

For most countries outside the US, this calculator’s defaults (36% DTI) are conservative. If you’re in a strict-rules market like Germany or the UK, your effective borrowing power will likely be 10-20% lower than US borrowers at the same income.

Borrowing Power Calculator FAQ

Does my credit score affect borrowing power?

Indirectly — through the interest rate. A 740+ credit score gets you the best rates; below 680 you pay 0.5-1.5% more, which significantly reduces your borrowing power at the same DTI. Run the calculator at your expected rate (lower for great credit, higher for marginal credit) to see the difference. A 100-point credit score improvement can increase borrowing power by 8-12%.

What about my down payment savings?

The calculator shows max home price at 5%, 10%, and 20% down — the most common options. Your actual home price ceiling is the lesser of (a) what you can borrow via this calculator and (b) what your savings allow as down payment. If your down payment savings are the binding constraint, increasing them gives you more options. Use our Deposit Savings Calculator to model how long until your target down payment.

Should I pay off debts before applying?

Often yes — but the math depends. Paying off a $300/month car loan frees up $300/month of housing capacity, which translates to about $47K of additional borrowing power at 6.5% over 30 years. If your car loan balance is under $47K, paying it off increases your borrowing power more than dollar-for-dollar. If it’s higher than that, you’re spending real cash to gain marginal housing capacity — usually not worth it unless you have abundant savings.

Why does the calc allow up to 60% DTI when limits are 43-50%?

Because some non-prime lending programs and some international markets allow higher ratios. The slider doesn’t limit you to US conventional norms — you can model any scenario. For most US users, 36-43% is the right range; for “stretch budget” modeling, 50% is the safety ceiling.

How is “income” defined?

Lenders typically use gross income (before tax) — salary plus reliable bonus/commission (often averaged over 2 years for variable comp), plus rental income, plus pension/annuity income. They usually exclude one-time bonuses, unreliable freelance income, capital gains, and unverifiable cash income. Use a conservative gross income number that you can document with W-2s, pay stubs, and tax returns.

Joint applications vs single — does this calculator work for both?

It works for both — just enter total combined household gross income and total combined monthly debts. Lenders will use the lower of the two credit scores when both applicants are on the loan, so factor that into your interest rate estimate.