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15 vs. 30-Year Mortgage Calculator
15 vs. 30-Year Mortgage Calculator
15-Year Lifetime Interest Savings
$273,247.35
Term Analysis
The 15-year term costs $694.2 more per month but saves $273,247.35 in lifetime interest. Investing that monthly difference at 8% would grow to $240,218.77 over 15 years — less than the interest saved, so the shorter term builds more wealth at these rates while also leaving you mortgage-free 15 years earlier.
*Mortgage rates shown are inputs, not quotes. This estimate excludes mortgage interest deductions, PMI, and closing costs, and is educational rather than lending or tax advice.
Why the 15 vs. 30-Year Decision Matters
Choosing between a 15-year and a 30-year fixed mortgage is one of the biggest leverage decisions an American household will ever make. The 30-year mortgage dominates US homeownership — roughly 80% of purchase loans — because it keeps monthly payments low and preserves cash for other goals. The 15-year loan typically carries a rate about 0.5 to 1 percentage point lower and builds equity dramatically faster, but demands a much larger monthly payment. The difference compounds for decades either way: hundreds of thousands of dollars in interest, or hundreds of thousands in investment growth. For investors, the question is not just 'which payment can I afford' but 'which term makes me wealthier.' A 15-year term guarantees a return equal to your mortgage rate on every extra dollar paid, tax considerations aside. A 30-year term lets you invest the monthly difference, potentially earning more than the rate you pay — at the cost of discipline and volatility. With US mortgage rates now normalizing after the pandemic-era lows, the spread between terms is meaningful, and the math deserves a fresh look for every purchase or refinance.
Amortization and Opportunity Cost Math
Both terms use the standard fixed-rate amortization formula: Payment = P × r / (1 − (1 + r)^−n), where P is the loan amount, r is the monthly interest rate (annual rate divided by 12), and n is the number of monthly payments — 180 for 15 years, 360 for 30 years. Total interest is simply the sum of all payments minus the principal. Because the 30-year loan spreads repayment over twice as many months, far more of each early payment goes to interest, which is why its lifetime interest is typically two to three times the 15-year figure even at similar rates. The opportunity-cost leg compares the two strategies honestly. The monthly payment gap between the 15-year and 30-year loans is treated as a stream invested each month over the 15 years the shorter loan runs, growing with the future value of an annuity formula: FV = PMT × [((1 + r)^n − 1) / r]. If that invested stream ends up larger than the interest saved, the 30-year term plus investing wins on wealth; if not, the 15-year term wins — and it also eliminates the debt 15 years sooner, a guaranteed, volatility-free outcome.
Expert Insights
The Guaranteed Return of the 15-Year Term
Every extra principal dollar on a 15-year mortgage earns exactly your loan rate, risk-free and tax-efficient. If your quote is 5.75%, that is a better guaranteed return than CDs or Treasuries offered in most environments. Only a truly consistent equity investor should expect to beat it with the 30-year difference — and consistency is the hard part.
Mortgage Interest Deduction Rarely Changes the Answer
Since the 2017 standard deduction nearly doubled, only about 10% of US filers itemize, and those who do deduct only the interest above the standard deduction. The after-tax advantage of a 30-year loan is a fraction of its headline rate for most households. Run this calculator on pre-tax terms; the deduction almost never flips the decision.
Payment Flexibility Is an Underpriced Option
A disciplined borrower can take the 30-year loan and make 15-year-level payments, keeping the extra cash as a buffer if income drops. This replicates most of the interest savings with a safety valve — at the price of a slightly higher rate. This hybrid only works if the extra payments actually happen when times are good.
Actionable Tips
- 1
Get Both Quotes for the Same Week
The entire comparison hinges on the spread between the two rates, which changes daily with the bond market. Ask a lender or online marketplace for the 15-year and 30-year rates on the same day with points held constant, then plug both into this calculator before the spread moves.
- 2
Stress-Test the 15-Year Payment
Before choosing the shorter term, confirm the payment stays below about 25% of gross monthly income even if one salary disappears. The math on this page assumes the payment is made for 180 straight months; a forced sale or refinance during hardship erases most of the interest savings you chose the term for.
- 3
Automate the Difference If You Choose 30
If the 30-year wins your scenario by the invested-difference test, set an automatic transfer of exactly the monthly gap into a brokerage account on payday. Without automation, the difference quietly disappears into lifestyle spending, and you will end up with the 30-year loan and none of the offsetting investments.
Real-World Examples
Priya Chose Certainty Before Kids' College
Priya, a project manager in Ohio, faced a $400,000 loan with a 0.75-point rate spread favoring the 15-year. The calculator showed about $190,000 in interest savings for a $1,100 higher monthly payment. With two kids heading to college within 12 years, she valued the guaranteed savings over a hypothetical portfolio and took the 15-year, paying off the home the year her youngest started school.
Marcus Invested the Difference and Won
Marcus, a software engineer with a strong equity record, used the 30-year term when rates hovered near 6.5%. The $900 monthly gap went into a 60/40 portfolio every month for 15 years. At an average return around 8%, the account outgrew his interest savings by a meaningful margin, and he kept the liquidity for a job loss he actually experienced in year four — when the 15-year payment would have hurt.
The Nguyens Split the Difference
The Nguyens took the 30-year loan for safety after a business downturn, then set up automatic extra principal payments equal to the 15-year payment. When revenue dipped twice, they paused the extras without renegotiating the loan. Over 20 years they paid roughly $120,000 less interest than a straight 30-year schedule while keeping a term that flexed with their income.
Glossary of Terms
- Amortization
- The process of paying off a loan through fixed monthly payments that shift gradually from mostly interest to mostly principal.
- Opportunity Cost
- The return you give up by using money for one purpose instead of investing it elsewhere.
- Loan Term
- The total length of a mortgage, most commonly 15 or 30 years for US fixed-rate loans.
Frequently Asked Questions
Everything you need to know about this topic.
Ivy Sinclair-Wren
Financial Chaos Analyst
Ivy Sinclair-Wren is a Financial Chaos Analyst covering investing, AI, wealth psychology, and the emotional consequences of opening finance apps during market crashes. Based in Melbourne, she specializes in demystifying the US tax code and helping users navigate the intersection of spreadsheet logic and human irrationality.