08/26/2026
Prepay a 6.6% mortgage early and your amortization schedule spits out a jaw-dropping "interest saved" number. It's the most seductive figure in personal finance. And the one I'd trust least.
Here's why it flatters prepayment so much.
On a young, large mortgage, almost every early payment is interest and barely any is principal. So throwing a lump sum at the balance early looks like it wipes out a fortune in future interest, sometimes hundreds of thousands over the life of the loan.
That total is real. But it's a nominal number spread across three decades, and the actual rate you earn by prepaying is just your mortgage rate. Nothing more.
And that rate probably isn't what you think it is.
If you itemize, your mortgage interest is deductible at the margin. That can quietly knock a couple of points off the rate that actually matters. A loan with a 6.6% sticker rate might really be costing something closer to the mid-4s after tax.
Now the comparison looks different. You're not weighing a scary six-figure interest total against zero. You're weighing a mid-4s after-tax return against whatever else that cash could do: superfunding a 529 or maxing out retirement contributions.
Prepaying is still certain. No market risk, no timing. That's worth something, and for some people it wins.
Just don't let the "interest saved" headline make the decision for you. Price out the true after-tax rate on the debt, then compare it honestly to the alternative. That's the only version of this math that's apples to apples.