Pay down to the $750k cap

This one is the jumbo-loan sequel to our early-payoff question: above $750k, the interest math changes in kind, not just in size. The IRS only lets interest on the first $750k of acquisition debt be deducted ($1M for loans from on or before Dec 15, 2017), so the slice of balance above the cap is carried entirely with after-tax dollars - retiring it earns the full mortgage rate, guaranteed, while every dollar below the cap effectively costs less once the deduction clears. That asymmetry suggests a very specific move: pay down to the cap and stop there - unless the same lump would simply earn more staying invested.

This calculator races those two choices fairly. Both scenarios spend the same cash every month: the stay-invested side holds the lump in a taxable brokerage - dividends taxed yearly, gains taxed at liquidation - while the paydown side retires the excess and invests its freed payments, each keeping whatever mortgage-interest deduction actually clears the standard-deduction hurdle. It reports the never-deductible interest avoided, the after-tax cost of borrowing on either side of the cap, the net-worth gap at your horizon, and the breakeven pre-tax return the portfolio must beat for staying invested to win.