About this tool
The 15-Year vs 30-Year Mortgage Quiz answers the most common housing-finance question in five major mortgage markets. The 15-year mortgage cuts total interest by roughly 40 to 60 percent, but the monthly EMI is 25 to 35 percent higher than a 30-year on the same principal. The right answer depends on three things that vanilla EMI calculators ignore: your opportunity cost of capital, the mortgage interest deduction available in your country, and the cash-flow shock you can absorb if income drops.
This calculator builds in all three. It computes after-tax interest paid on each track, simulates investing the monthly EMI difference at your chosen real return, and reports the year-30 net worth on each path. Then it sweeps the real-return axis to find the break-even rate at which the two paths tie. For US borrowers, the typical break-even is 4.5 to 5.5 percent real; for India with the Rs 2 lakh Section 24(b) cap, it tends to be lower because the tax shield is small relative to total interest.
How it works
- Pick country. Sets currency, default 15 vs 30 year rates, and the tax-shield rules (US $750K cap, India Section 24(b) Rs 2L cap, UK/CA/AU no deduction).
- Enter loan amount, income, tax rate, expected stock return, and years you plan to stay.
- Read the verdict banner. One of three outcomes: 15-year saves $X over the loan life, 30-year + invest comes out ahead by $Y, or it is a tight call within ~5 percent.
- Read the side-by-side: EMI, total paid, total interest, tax shield, after-tax interest, ownership year.
- Read the sensitivity table: at which real-return level does the 30-year win, and how far you are from break-even.
- Read the cash flow row: 15-year EMI as a percent of gross income, with the 28% / 36% lender thresholds marked.
The EMI formula
Opportunity cost
Country-specific tax shield rules
| Country | Mortgage interest deduction | Effective shield | Notes |
|---|---|---|---|
| 🇺🇸 US | Itemise: interest on first $750,000 of acquisition debt (TCJA 2017 cap) | 0 to 37% of interest | Only if itemised deductions exceed the 2026 standard deduction ($15,750 single, $31,500 joint). Most middle-income borrowers no longer itemise. |
| 🇬🇧 UK | None for owner-occupied (abolished 2000) | 0% | Buy-to-let landlords get a 20% tax credit on mortgage interest, not a full deduction. Owner-occupied: zero shield. |
| 🇨🇦 Canada | None for primary residence; Smith Manoeuvre route is possible but complex | 0% (default) | Capital gains on primary residence are tax-free, which functions as a different kind of shield. Investment property interest is deductible. |
| 🇦🇺 Australia | None for owner-occupied; full deduction for investment property | 0% | Negative-gearing model targets investment properties. Owner-occupied loans get no direct interest shield. |
| 🇮🇳 India | Section 24(b): Rs 2 lakh on interest, Section 80C on principal Rs 1.5 lakh (old regime only) | 10-30% of capped Rs 2L | Cap binds quickly on large loans. Anything above Rs 2L of annual interest is unsheltered. New regime: no deduction. |
The tool applies the country rules automatically. For US borrowers at $750,000+ loans, the shield is computed only on the first $750K; above that, additional interest is treated as unsheltered.
How the opportunity-cost math actually works
The opportunity cost argument has two components. First, the 30-year mortgage frees up monthly cash flow that you can in theory invest. On a $400,000 loan at the default rates, that frees up about $1,100 per month in years 1 to 15. Second, that invested amount compounds for 30 years, while the 15-year mortgage borrower has a paid-off house at year 15 and presumably starts investing the full EMI of about $3,300 per month from year 16 onward.
The naive calculation favours the 30-year. But three reality checks tighten the margin:
- Behavioural drift: empirical data from Vanguard and Fidelity show borrowers invest 60 to 70 percent of the planned EMI difference, not 100. The shortfall pushes break-even higher.
- Sequence risk: stock returns are volatile. The 15-year mortgage compounds at a fixed 5 to 6 percent guaranteed (the rate of interest avoided). The investing track compounds at risky 7 to 10 percent. A bad first decade can make the 30-year + invest underperform even if the long-run average return matches.
- Tax drag on investments: even tax-advantaged accounts have some friction. A taxable brokerage with 15 percent LTCG drag turns a 7.0 percent real into 5.95 percent net.
The tool models the first effect implicitly (you pick your real return), and reports the second via the sensitivity sweep so you can see how break-even moves with your assumed return.
What changes when rates are nearly equal
In the rate-cutting cycles, 15-year and 30-year rates compress. UK and Canadian fixed-term mortgages reset every 5 years anyway, so the 15-vs-30 distinction is less binding than in the US. India home loans are typically floating-rate (repo-linked) regardless of original tenure. The cleanest 15-vs-30 decision is the US fixed-rate market, where you commit to one rate for the full term.
When the 15-year is clearly the right call
- Late starter on retirement saving and you want a paid-off house by 60-65.
- Income stability is high (tenured professor, doctor, civil servant) and you have a 6+ month emergency fund.
- You already max out tax-advantaged retirement accounts and have no other high-priority investing vehicle.
- You expect to inherit or otherwise receive a windfall that would make the higher EMI easy to absorb.
When the 30-year + invest is clearly the right call
- You have not yet maxed out a 401(k), ISA, RRSP, super, or PPF and the EMI difference would fund it.
- Income is volatile (sales, freelance, equity-heavy comp) and you need optionality.
- You are young (under 35) and likely to refinance into a 15-year later as income grows.
- Expected real return on liquid investments is above the break-even and you have the temperament to invest the difference consistently.
Frequently asked questions
Is a 15-year mortgage always better than a 30-year?
Pure interest cost - yes, the 15-year saves 40 to 60 percent of total interest. Total wealth - not always. If you reliably invest the EMI difference at a real return above the break-even (typically 4.5 to 5.5 percent real, depending on country), the 30-year + invest path overtakes the 15-year by year 25 to 30.
What if my income drops on a 15-year mortgage?
The 15-year EMI is contractually higher and inflexible. A 30-year with extra principal can revert to the lower required payment whenever cash flow tightens. This optionality is worth real money - roughly 50 to 100 basis points of equivalent return on the borrowed amount.
Can I take a 30-year and prepay to mimic a 15-year?
Yes, but only if your 30-year rate is not materially higher than the 15-year rate. In the US, 15-year rates are usually 50 to 100 bps lower. If you take the 30-year at the higher rate then prepay, you pay more interest than you would have on the 15-year. The trade-off is optionality vs cost.
Are 7/1 ARMs better than either fixed term?
For borrowers planning to move or refinance within 7 years, a 7/1 ARM at 50 to 75 bps below the 30-year fixed can save substantial interest in the fixed-rate window. After year 7 the rate floats and resets annually. This tool focuses on fixed-rate comparisons; for ARMs, set the years-you-plan-to-stay to your expected exit and compare.
Should I refinance later if rates drop?
Refinancing is essentially a free option you hold. If rates drop 75 bps or more and you plan to stay another 5+ years, a no-closing-cost refinance can save tens of thousands. Refinancing biases the answer towards the 30-year today: you can refinance to a 15-year later if your income grows, but you cannot easily move from a 15-year to a 30-year.
How big is the US mortgage interest deduction in 2026?
Interest on the first $750K SALT-era acquisition debt is deductible under Schedule A. You must itemise. The 2026 standard deduction is $15,750 single / $31,500 joint. If your itemised total (mortgage interest + state/local tax + charity) is under standard, you get zero shield. About 90 percent of US tax filers take the standard deduction post-TCJA.
How does India Section 24(b) cap work?
Section 24b caps the home loan interest deduction at Rs 2 lakh per year for a self-occupied property under the old tax regime. At a 30 percent marginal rate that is a Rs 60,000 annual cash benefit. Anything above Rs 2L of yearly interest is unsheltered. The new regime offers no deduction at all.
What is NPV and why does the tool use it?
The NPV (net present value) comparison discounts future cash flows back to today's dollars at the stock real return. It is the canonical academic framework for comparing two payment streams of different lengths. The tool reports both nominal total-paid figures and year-30 net worth, which capture the same intuition without the discount-rate sensitivity.