Paying down your mortgage is more attractive when its after-tax cost exceeds the return you can earn at an acceptable risk and you will still have enough cash left. Keeping the cash can make more sense when the mortgage rate is low or you need flexibility for a move, repairs, or uneven income. Compare after-tax dollars first, then decide what access to the money is worth to you.
Compare after-tax amounts
Suppose you have $100,000 available, a 3% mortgage, and a savings account yielding 4%. Assume mortgage interest provides no additional deduction and savings interest is taxed at 35%.
| Use of $100,000 | Approximate first-year effect |
|---|---|
| Pay down the mortgage | $3,000 less interest |
| Keep it at a 4% yield | $4,000 interest before tax |
| Tax on savings interest | −$1,400 |
| Savings interest after tax | $2,600 |
Paying down the mortgage comes out about $400 ahead for the year. The higher advertised savings rate loses its advantage after tax. Keeping access to $100,000 may still be worth that $400 difference.
You can also work backward. With a 35% tax rate on savings interest, the savings account would need to yield about 4.62% before tax to match the 3% interest avoided: 3% ÷ (1 − 35%). That assumes the mortgage interest creates no additional deduction.
These are simplified one-year comparisons with constant rates and balances. Mortgage amortization, changing savings yields, and fees affect actual results. Stocks can earn more or lose value, so an expected investment return is not as certain as the interest avoided by paying down debt.
How much does the mortgage deduction help?
Count only the deduction that reduces your tax. If you take the standard deduction, paying mortgage interest may provide no additional tax benefit. Even if you itemize, the benefit depends on deduction limits and how far your itemized deductions exceed the standard deduction.
The IRS mortgage-interest guide explains the limits. Many loans incurred after December 15, 2017 are subject to a combined $750,000 acquisition-debt limit, or $375,000 for married filing separately. Older qualifying debt and refinancing can follow different rules.
Home equity interest generally qualifies as home-mortgage interest only when the loan proceeds buy, build, or substantially improve the home securing the loan, subject to other limits. Borrowing against the house to invest requires a separate interest-deduction analysis.
What will the cash need to do?
Before making a large payment, give the cash a job. List emergency reserves, known tax bills, repairs, a possible move, and any period when income could stop. Keep the money needed for those jobs accessible.
Paying principal moves money into home equity. Retrieving it generally requires selling the home or qualifying for new borrowing. A credit line can help, but its availability and rate can change; it is not the same as cash already in your account.
If you keep the mortgage and invest the money, test a bad year: the portfolio falls, income drops, and the mortgage payment still arrives. The strategy has to work without being forced to sell investments at an unfavorable time.
Prepayment, recasting, and refinancing are different
| Action | What changes | What to check |
|---|---|---|
| Extra principal payment | Reduces the balance and future interest; usually shortens payoff time | How the servicer applies the payment and whether a prepayment penalty applies |
| Recast | Recalculates the payment after a lump-sum principal reduction | Whether the lender permits it, required payment size, and fees |
| Refinance | Replaces the existing mortgage | New rate, closing costs, loan term, and break-even period |
A lump-sum payment usually does not lower the required monthly payment by itself. If lower monthly spending is the goal, ask the lender about a recast before sending the money. Check the agreement for a prepayment penalty too, especially before a large lump-sum payment.
For a refinance, compare both the payment and the payoff date. A lower payment from restarting a 30-year term can come with more years of interest.
Make the decision with four numbers
Write down:
- Cash to keep: money for emergencies and known near-term expenses.
- Interest avoided: the benefit of the proposed principal payment, after any tax deduction you would lose.
- Alternative return: what the same money might earn after tax, with its risk made explicit.
- Required monthly payment: what you still owe after the proposed change.
If the comparison is close, a partial payment can split the difference. You can reduce interest while keeping enough cash to avoid borrowing again for the next surprise. The objective is a mortgage decision that fits the household’s finances, not winning a rate comparison by a fraction of a percent.