Pay off the mortgage or invest? How to run your own numbers

Step 1: Find your true after-tax mortgage rate

Prepaying a mortgage "earns" your interest rate, risk-free — but only the after-tax version, and here's the detail that flips intuition: mortgage interest only reduces your taxes if you itemize, and only the interest above what the standard deduction would have given you anyway does real work. With the 2026 standard deduction at $32,200 for joint filers and the SALT deduction capped (a $40,400 headline that phases back toward $10,000 above roughly $505K of income), plenty of high-income households itemize barely or not at all — making their after-tax mortgage rate simply… the rate.

So compute it honestly: if you don't itemize, a 6.5% mortgage costs 6.5%. If you do, only the marginal slice of interest that clears the threshold earns a deduction (and only on interest from up to $750K of acquisition debt for post-2017 loans). Households with old sub-3% loans are in a different conversation entirely — a 2.75% guaranteed "return" from prepayment is a low bar that even conservative alternatives may clear.

Step 2: Set up the honest comparison

The choice is: a guaranteed, tax-free return equal to your after-tax mortgage rate (prepayment) versus an assumed, variable, taxable return (investing). Note the asymmetries. The prepayment return is certain; the investment return is whatever you assume — we won't predict markets for you, and neither should anyone else. The prepayment return is effectively tax-free (interest you never pay); investment returns in taxable accounts get haircut by dividend and capital-gains taxes along the way, plus the 3.8% NIIT at higher incomes.

A fair apples-to-apples: after-tax mortgage rate vs. your assumed portfolio return × (1 − your blended tax drag). At a 6.5% effective mortgage rate, the investment side needs roughly 7%+ pre-tax just to tie — with volatility. At 2.75%, the hurdle is a third of that. The spread between your rate and your honest assumption is the whole financial case, and for most households it's smaller than either camp admits.

Step 3: Price the things the return math can't see

Three factors regularly outweigh a one-point return spread:

• Liquidity is one-directional: money invested can become mortgage payoff any Tuesday; money in the walls only comes back out by selling or borrowing (at whatever rates and approval standards exist that day). Prepaying to the point of being house-rich and cash-thin is a classic self-inflicted wound — payoff generally belongs after emergency reserves, not instead of them.
• Debt is also a fixed obligation in bad states of the world: a paid-off house drops your required monthly burn, which is worth the most precisely when income stops or markets fall — the same moments your portfolio is least pleasant to sell. That's a risk benefit no average-return comparison captures.
• The behavioral term is real on both sides: the peace of a paid-off house is worth actual utility to some people and nothing to others, and "I'll invest the difference" only beats prepayment if the difference actually gets invested — automatically, not aspirationally.

The middle paths (which most households actually choose)

This isn't binary. Split the surplus — invest most, add something principal-directed for the guaranteed win and the shrinking term. Recast the mortgage after a lump prepayment to cut the required payment (lowering fixed obligations without losing liquidity to a full payoff). Refinance the decision itself whenever rates move materially. Or target payoff-by-retirement specifically — entering drawdown years without a required mortgage payment reduces sequence-of-returns pressure, a genuinely different argument than the return comparison. Each path prices the trade-offs differently; the right one depends on your rate, horizon, and temperament.

Run it on your real balance sheet

The inputs are all yours: actual rate and remaining term, actual itemization status, actual liquidity beyond reserves, actual proximity to drawdown. Formation puts them in one place — the mortgage alongside every account and entity, cashflow that shows the true surplus, and projections that show each path on your numbers, assumptions stated and estimates labeled. Formation doesn't call market direction and doesn't make the choice — it makes the choice visible. What you decide is yours; what it's worth checking with is your CPA (the itemization math) and, if you use one, your advisor.