Prepay the mortgage, or invest?

Spare cash each month: pay down the mortgage faster, or invest it? Prepaying earns a guaranteed return — your after-tax mortgage rate. Investing earns a likely one. This compares your wealth either way and shows the return where the answer flips.

By the SterlingCat editorial team · Last updated 2026-07-20 · Method verified against the closed-form break-even (how we calculate)

The key points

Prepaying is a guaranteed, tax-free return equal to your after-tax mortgage rate. Investing is a risky, expected return.
Prepay wins when that after-tax rate beats what you'd expect to earn investing; invest wins when it doesn't. Equal rates are a wash.
Deducting your mortgage interest lowers the after-tax rate — and shifts the answer toward investing.

Your details

Remember in this browser

Save your figures here to reuse them across the calculators and switch between scenarios. They stay in this browser — never sent anywhere — and you can export or clear them anytime.

Your mortgage today

How much do you still owe?

The balance left on your mortgage.

What rate are you paying?

Your loan's interest rate.

How many years are left?

Remaining term on the loan.

years

The extra money

How much extra could you put in each month?

Toward the mortgage, or invested — that's the question.

$ a month

Compare over how many years?

How far out to compare your wealth.

years

If you invested instead

What return would you expect, after tax?

Your assumption for the invested money — not a guarantee.

Mortgage-interest deduction Most people take the standard deduction — skip unless you itemize.

Do you deduct mortgage interest?

Marginal tax rate

With your numbers

—

If you prepay

Invested: —
Still owe: —

If you invest

Invested: —
Still owe: —

Where the answer flips — wealth gap vs. your return

Prepay wins Invest wins Your return

Recommendation

—

At which returns?

A scenario projection based on stated assumptions — not a forecast or guarantee. Prepaying is a guaranteed return; investing is not.

Computed in your browser — nothing you enter is ever sent anywhere. If you save your details, they stay in this browser only; a share link carries your numbers in the link itself.

How we calculate this

Both choices spend the same money each month — your required payment plus the extra — so the comparison is fair. You keep the home either way, so its value cancels; what differs is how much you've invested and how much you still owe. We run it month by month:

Prepay: the extra goes to principal until the loan is gone, then the whole freed-up payment is invested for the rest of the horizon.
Invest: you pay the mortgage on its normal schedule and invest the extra; once the loan ends, the freed payment is invested too.

Wealth at your horizon is investments minus what you still owe. The one thing that decides the direction is your after-tax mortgage rate versus your expected return — so the tool finds the break-even return (where the two are a wash) and shows how far your assumption sits from it.

The after-tax rate

If you itemize and deduct mortgage interest, prepaying also shrinks that deduction, so the guaranteed return from prepaying is rate × (1 − your marginal rate), not the full rate. If you take the standard deduction (most people), the interest isn't deducted and the guaranteed return is the full rate. Your expected investment return is entered after tax, so both sides are compared after tax.

What's included, and what isn't

Included: full amortization with extra principal; payments stop at payoff; the freed payment reinvested; the mortgage-interest deduction when you itemize.

Declared omissions (so the tool's claims stay honest): investment-gains tax beyond your after-tax return input; tax-advantaged or employer-matched investing (a 401(k) match usually beats both options — a separate question); return variance (a single expected return, not a distribution); PMI (we assume you're past 20% equity); prepayment penalties (assumed none); and liquidity — prepaying locks cash into the home, investing keeps it reachable. The full protocol is in the project's V&V record.

Terms, in plain words

After-tax mortgage rate: Your mortgage rate minus the tax you save by deducting the interest. If you don't itemize, it's just your rate. This is the guaranteed return prepaying earns.
Break-even return: The expected investment return at which prepaying and investing come out exactly even. Above it, investing wins; below it, prepaying wins.
Guaranteed vs. expected: Prepaying's return is certain. Investing's is an average you might beat or miss — so even at equal expected returns, some people prefer the sure thing.

A sensible next step

Before either, check you have an emergency fund and aren't leaving an employer 401(k) match on the table — a match usually beats both prepaying and taxable investing. This tool is for the money left after those.

Common questions

Isn't paying off debt always smart?

Not always the smartest use of a dollar. Prepaying earns your after-tax mortgage rate, guaranteed. If your mortgage is cheap and you can reasonably expect to earn more investing, investing builds more wealth — at the cost of certainty. That trade between a sure thing and a likely-bigger thing is the whole decision.

Why does deducting interest favor investing?

The deduction makes your mortgage cheaper after tax, so the guaranteed return from prepaying is smaller — which lowers the bar your investments have to clear.

What about the peace of mind of no mortgage?

That's real and personal — the tool can't price it. It gives you the dollar difference; if it's small, the guaranteed, sleep-at-night option is easy to justify.

Sources

Grounded in authoritative primary sources:

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