Mortgage Prepayment vs Investing: A Decision Framework

The math is straightforward: a 4% mortgage costs you 4% on every extra dollar of principal. If your investments earn 8% (historically typical for stocks), you're 4% better off investing.

But this ignores three critical factors:

1. Investment returns aren't guaranteed — your mortgage rate is locked in
2. Tax deductions on mortgage interest lower your effective rate
3. The psychological value of being debt-free is real and personal

Neither answer is universally "right." The best choice depends on your complete financial picture.

Step 1: Effective mortgage cost

If your mortgage rate is 5% and you're in the 24% tax bracket and itemize deductions:

Effective rate = 5% × (1 − 0.24) = 3.8%

If you take the standard deduction (most people post-2018), your effective rate is the full 5%.

Step 2: Expected investment return

Historical S&P 500: ~10% nominal, ~7% after inflation

After capital gains tax (15–20%): ~8–8.5% nominal

Step 3: Compare

8.5% investment return vs 5% mortgage cost = 3.5% spread favoring investing

8.5% vs 3.8% (with deduction) = 4.7% spread

The larger the spread, the stronger the case for investing. When the spread is small (<2%), the guaranteed "return" of mortgage payoff becomes more attractive.

Strong cases for prepayment:

1. High mortgage rate (6%+): The guaranteed "return" is hard to beat risk-adjusted
2. Approaching retirement: Eliminating your largest monthly expense provides security
3. Risk aversion: If market volatility causes you stress, a guaranteed 5% "return" beats a volatile 8% average
4. Already maxing tax-advantaged accounts: If your 401(k), IRA, and HSA are maxed, the tax advantage of investing shrinks
5. Short remaining term: If you have 5–7 years left, prepayment frees cash flow soon

The emotional benefit of owning your home outright is worth more to some people than the mathematical edge of investing.

Strong cases for investing:

1. Low mortgage rate (under 4%): Locked in during 2020–2021? That's essentially free money after inflation
2. Tax-advantaged space available: 401(k) match is an instant 50–100% return — always take it before extra mortgage payments
3. Long time horizon: 20+ years lets compounding work and smooths out volatility
4. Emergency fund in place: Never prepay a mortgage if it means no cash reserves
5. High income growth expected: Your mortgage payment becomes proportionally smaller over time

Priority order: employer match → high-interest debt → emergency fund → max tax-advantaged accounts → then decide between mortgage prepayment and taxable investing.