The headline math on $400K
| 15-year at 5.5% | 30-year at 6.5% |
|---|---|
| EMI: $3,267/month | EMI: $2,528/month |
| Total interest: $188K | Total interest: $510K |
| Total paid: $588K | Total paid: $910K |
Gap: 30-yr pays $322K more interest. But the cash flow advantage of 30-yr is $739/month for 30 years.
If invested at 7% real return: $739/month x 30 years = $904K terminal value.
Net: 30-yr "loses" $322K to extra interest but "gains" $904K in invested differential. Net advantage to 30-yr: $582K.
This is why most financial planners recommend the 30-yr in low-rate environments. The key assumption is that you actually invest the savings rather than spend them.
Country-specific tax shields change the math
| US (TCJA + post-2018) | India (old regime) | UK | Canada | Australia |
|---|---|---|---|---|
| Mortgage interest deductible up to $750K loan (single + MFJ) | Section 24(b): up to Rs 2 lakh interest deductible (self-occupied), unlimited for let-out | Mortgage interest NOT deductible for owner-occupier since 2017 (was tapered out) | Mortgage interest NOT deductible on owner-occupier | Mortgage interest NOT deductible on owner-occupier |
| Effective rate reduction: 22-37% of marginal bracket | Section 80EEA: extra Rs 1.5L for first-time buyers (eligibility cap on property value) | Buy-to-let: deductible only at basic rate via tax credit | Smith Maneuver workaround: convert mortgage to investment loan over time | Negative gearing only on investment property |
| A $20K/year mortgage interest deduction at 32% bracket = $6,400/yr tax saved | Combined effective rate reduction ~30% at top slab |
For US and India borrowers, the tax shield meaningfully tilts toward 30-yr because the interest you save by going 15-yr would have been partly deductible anyway.
| Term | Rate | EMI/mo | Total interest |
|---|---|---|---|
| 15-year | 5.5% | $3,267 | $188,000 |
| 30-year | 6.5% | $2,528 | $510,000 |
| Gap | -1.0% | +$739/mo | +$322,000 |
5-country break-even comparison
Same $400K loan equivalent, 15-yr vs 30-yr, with country-specific tax shields applied.
| US (effective 22% tax bracket itemized) | UK (no deduction) | India (old regime, Rs 2L deduction) | Canada (Smith Maneuver) | Australia (negative gearing only on investment) |
|---|---|---|---|---|
| 15-yr saves $322K interest (after deduction: $251K) | 15-yr saves the full $322K | Effective deduction on first Rs 2L/yr of interest | Can convert mortgage to investment loan over time | For owner-occupier: same as UK |
| 30-yr invest difference + tax-deferred = $904K | 30-yr invest difference at 7% real = $904K | 30-yr advantage smaller for high-loan amounts | 30-yr advantage growing if Smith Maneuver applied | Break-even: 5.0% real |
| Net 30-yr advantage: $653K | Net 30-yr advantage: $582K | Break-even: 5.0% real for Rs 50L loan | ||
| Break-even market return: 4.2% real | Break-even market return: 4.8% real |
When 15-year actually wins
Despite the math favoring 30-yr in most scenarios, 15-yr makes sense in specific situations:
- **High discipline failure risk**: You will NOT actually invest the $739/month difference. The 15-yr forces wealth building via principal paydown.
- **Already at retirement portfolio capacity**: Maxed 401k, IRA, HSA. Need taxable account anyway. 15-yr equity in home is effectively forced savings.
- **Career uncertainty**: 15-yr ends mortgage faster, removing biggest fixed cost. Worth optionality.
- **Late starter**: Less than 15 years to retirement. 15-yr aligns mortgage payoff with retirement timeline.
- **Real-rate environment**: When mortgage rate equals or exceeds expected real stock return (6%+ vs 7%), the gap narrows. 15-yr at 4% vs 30-yr at 6% in a 5% real-return market: 15-yr wins.
- **Estate planning**: Paid-off home transfers cleanly. Less complex than mortgage outstanding.
Common mistakes
- **Refinance to 15-yr when you cannot afford it**. Job loss + 15-yr EMI = foreclosure. Stick with 30-yr for affordability buffer.
- **Not investing the difference**. The math only works if you actually invest the cash flow gap.
- **Ignoring inflation**. 30-year horizon means inflation erodes nominal mortgage. Real cost is lower than headline numbers suggest.
- **Forgetting refinance optionality**. 30-yr can refinance to 15-yr later if rates drop or you accumulate wealth. 15-yr cannot easily "unwind" to 30-yr without refinance costs.
- **Ignoring tax shields**. Country-specific deductions reduce 30-yr effective rate; many analyses skip this.
Run the math for your situation
Use our 🇺🇸 United States calculator to plug in your own numbers.