The true 30-year cost of a US mortgage at every rate from 3% to 8%

Mortgage payments are computed monthly. Mortgage costs accumulate annually. The gap between the two is where most homeowner intuition goes wrong: a difference of 1% APR sounds small per month and dramatic over the loan’s life. This piece works out, in dollars, exactly how dramatic. All numbers are deterministic and reproducible — plug them into any mortgage calculator and you’ll get the same results.

The setup

Same loan parameters across all scenarios:

• Principal: $400,000 (median US mortgage 2024).
• Term: 30 years (360 monthly payments).
• Type: Fixed-rate fully-amortising. No extra payments.
• Rate variation: 3%, 4%, 5%, 6%, 7%, 8%.

Formula used (standard amortisation):

monthly = P × r × (1+r)ⁿ / ((1+r)ⁿ − 1)
where r = annual_rate / 12 and n = 360

The headline numbers

At 8% APR, lifetime interest exceeds the principal itself by 64%. The borrower pays the bank more than the entire cost of the house in pure interest. At 3% APR, the interest is “only” half the principal.

Per-percentage-point cost

Each additional 1% on the rate adds substantial lifetime cost:

On a $400k loan, a single percentage point on the rate is worth roughly $80,000-100,000 over 30 years. This is the math that justifies shopping around — a 0.25% rate improvement on this loan is $20,000-25,000 in real money. Even small fees worth paying to get a better rate often pay back themselves many times over.

Where every dollar goes

At 7% APR (current 2026 conditions), the $958,036 total breaks down as:

• Principal: $400,000 (41.7%). What you actually borrowed.
• Interest: $558,036 (58.3%). The bank’s revenue.

Over the loan’s life, you pay more in interest than the house cost. Add property taxes (~30% cumulative at typical rates), insurance, maintenance (~30%), HOA, and the all-in 30-year cost of owning a $400k house at 7% APR runs north of $1.4 million in 2026 dollars.

The first-five-years problem

Amortisation is front-loaded with interest. At 7% APR, the first five years of payments break down as:

After 5 years and $160k in payments, only $23,254 of principal has been paid off. The bank received $136k in interest. Selling at year 5 returns essentially the down payment plus any home appreciation — none of the amortisation has built meaningful equity.

The 15-year comparison

For the same $400k principal at 7% APR (15-year mortgages typically price ~0.5-0.75% below 30-year, but holding rate constant for comparison):

The 15-year saves $310,915 in interest for a monthly payment 35% higher. For a household that can absorb the cash-flow difference, this is the highest risk-free return available to most US consumers — see our 15- vs 30-year comparison.

What this means in practice

1. Rate shopping has 5-figure stakes.A 0.25% improvement is worth $20-25k over the loan’s life. Half a day of phone calls to three lenders is well-spent time.
2. Lifetime interest is the right comparison number. Not monthly payment, not APR difference. Total dollars paid in interest is the only metric that captures the full cost.
3. Early-year extra principal pays back the most.An extra $1,000 paid against principal in year 1 of a 7% loan saves about $7,000 in interest over the loan’s life. The same $1,000 paid in year 25 saves only $300.
4. Refinancing has a real break-even. Use the same per-percentage-point math: a 0.5% rate drop is worth ~$45k over 30 years. Closing costs of $4-6k pay back fast if you stay in the loan.

Methodology

All headline figures are computed from the standard fully-amortising fixed-rate mortgage formula presented earlier on this page. The dataset is synthetic — six rate scenarios applied to one principal and one term — chosen to isolate the rate effect from any sample-of-loans bias.

• Sample design: six rate scenarios (3%, 4%, 5%, 6%, 7%, 8% APR) on one fixed principal ($400,000) and one fixed term (360 months).
• Principal anchor:median US 2024 mortgage origination size per the Federal Reserve Bank of New York’s Household Debt and Credit Report (Q4 2024), cross-validated against the National Association of Realtors 2024 Existing Home Sales report.
• Calculation precision: IEEE-754 double-precision; rounded to nearest dollar for display. Cumulative interest figures are the sum of all 360 monthly interest components, not a closed-form approximation.
• Excluded costs:property tax, hazard insurance, PMI, HOA, escrow shortfalls. The all-in $1.4M figure quoted in the body adds these as separate line items but they aren’t part of the lifetime interest calculation.
• 15-year comparison rate: held constant at 7% APR for clean comparison; real 15-year mortgages historically price 50-75 bp below the 30-year (Freddie Mac PMMS 2010-2024).

Key findings

• $80,000-100,000 lifetime interest cost per 1 percentage point on the 30-year rate. The increment grows slightly at higher rates (3→4% adds $80k; 7→8% adds $98k).
• At 7% APR, lifetime interest ($558,036) exceeds principal ($400,000) by 40%. At 8% APR it exceeds principal by 64%.
• Year-1 principal reduction is only ~1% of the loan at 7% APR — $4,025 against $31,932 in payments. The other $27,907 is pure interest.
• 5-year break-even equity build is <6% of principal at 7% APR — selling at year 5 returns essentially the down payment plus any appreciation, with very little amortisation-derived equity.
• 15-year at 7% saves $310,915 in interest vs the 30-year at the same rate, for a monthly payment ~35% higher.
• 0.25% rate improvement = ~$20-25k lifetime savings on a $400k loan — the math behind every shop-around-for-rates recommendation.

Caveats / Sources of bias

• Synthetic scenario, not loan-level data. We deliberately fix principal and term so the rate comparison is clean. Real loan portfolios show additional variation from origination fees, points paid up front, and rate-lock float gains.
• No early-payoff modelling. US mortgages have ~7-9 year median life because of moves and refinances; lifetime-interest figures assume the loan runs to term, which exaggerates the lived experience of most borrowers.
• No tax-shield adjustment. Mortgage interest is partially tax-deductible in the US for itemising taxpayers. Effective cost after deduction is 15-30% lower for higher-income filers; the headline numbers ignore this.
• 2026 dollar terms throughout. No inflation adjustment on the dollar value of future payments — a $2,661 payment in 2056 is real-cheaper than today by whatever the cumulative inflation has been.
• Fixed-rate only.ARM products have different lifetime dynamics; this analysis doesn’t apply.
• Excluded all-in costs vary by location. Property tax alone ranges from ~0.3% to ~2.5% annually across US states — the $1.4M all-in figure assumes a national-average ~1.1%.

Sources and reproducibility

Numbers in this piece are computed from the standard amortisation formula above using $400,000 principal and 360 months. Every result can be reproduced by feeding the same parameters into our mortgage calculator.

Median US mortgage size from the Federal Reserve Bank of New York Household Debt and Credit Report (Q4 2024) and the National Association of Realtors 2024 Existing Home Sales report. Reference for the amortisation formula: CFPB “What is amortization?” (2024); standard textbook derivation in Brealey, Myers, & Allen, Principles of Corporate Finance (13th ed.), §3. Historical 30-year vs 15-year spread from the Freddie Mac Primary Mortgage Market Survey.