Lump-Sum Mortgage Payment: Recast, Prepay, or Invest?

A windfall — a bonus, an inheritance, a home-sale profit, a vested grant — opens up choices most homeowners never think through carefully. Sending it to the mortgage feels responsible, but “send it to the mortgage” actually means at least three different things with very different outcomes. This guide lays them out. Compare all of them on your numbers in the Decision Engine.

Your four real options

1. Prepay (extra principal): Apply the lump sum to principal and keep paying your normal amount. Shortens the term, saves the most interest, payment unchanged.
2. Recast: Apply the lump sum and have the lender re-amortize. Same term and rate, lower monthly payment, less interest saved than prepaying.
3. Invest: Don’t touch the mortgage; invest the money for a potentially higher — but uncertain — return.
4. Refinance with the lump sum: Use the cash to pay down the balance as part of a refinance to a new rate/term. Worth it only if rates are meaningfully lower.

Each optimizes a different thing: interest saved, monthly cash flow, expected wealth, or rate.

Option 1: Prepay — maximum interest saved

Prepaying earns a guaranteed return equal to your mortgage rate and eliminates the interest-heavy tail of your loan. If your goal is to be debt-free sooner and your rate is relatively high, this usually saves the most money overall. The catch: your monthly payment doesn’t change, and the cash is now locked in home equity. Model it in the Extra Payment Calculator.

Option 2: Recast — lower the monthly payment

If your real problem is monthly cash flow, a recast is the tool. The same lump sum buys you a permanently lower required payment, at the cost of saving less total interest. It’s cheap (a flat fee, no closing costs) and keeps your existing rate. Learn the mechanics in what is mortgage recasting, and run it in the Mortgage Recast Calculator.

Option 3: Invest — potentially more wealth

If your mortgage rate is low, investing the lump sum may grow your net worth more than prepaying would save — but the return is probable, not guaranteed, and markets can fall. After adjusting for taxes, compare your mortgage rate to your expected after-tax return. The Decision Engine shows the breakeven return your investments must beat. See pay off mortgage or invest for the full discussion.

Option 4: Refinance with a paydown

If rates have dropped since you took your loan, you might refinance and apply the lump sum to lower the new balance. This can reduce both your rate and your payment — but you’ll pay closing costs, so check the breakeven in the Refinance Breakeven Calculator.

The prerequisites (do these first)

Before committing a windfall to any mortgage strategy:

• Keep an emergency fund (3–6 months of expenses) in liquid savings.
• Clear high-interest debt — paying off a 20%+ credit card beats every mortgage option.
• Capture your employer retirement match — it’s free money.
• Consider taxes if the windfall itself is taxable (e.g., a bonus or sale gain).

Money put into your home is hard to retrieve, so don’t over-commit it.

A simple way to choose

• Want lower payments? → Recast.
• Want to be debt-free fastest and your rate is high? → Prepay.
• Rate is low and you can tolerate risk? → Invest.
• Rates dropped a lot since you borrowed? → Refinance (with a paydown).

Then verify with real numbers, because the “obvious” choice often isn’t once you see the figures.

Where this leaves you

A lump sum can prepay (max interest saved), recast (lower payment), invest (max expected wealth), or fuel a refinance (lower rate) — and the best choice depends entirely on whether you want speed, cash flow, growth, or a better rate. Compare all paths at once, on your own numbers, in the Decision Engine.

Everything above is a back-of-the-envelope estimate, not advice for your specific situation. Confirm the details with your lender before you move real money.

Run the numbers in our calculator →

Frequently asked questions

What's the best thing to do with a lump sum on my mortgage?

It depends on your goal. Prepaying saves the most interest; recasting lowers your monthly payment; investing may build more wealth if your mortgage rate is low. The right choice hinges on whether you want payoff speed, cash flow, or growth.

Should I use a windfall to pay off my mortgage entirely?

Only after you have an emergency fund, no high-interest debt, and your retirement match captured. Putting a windfall into your home converts liquid cash into illiquid equity, so keep enough accessible savings first.

Does a lump sum lower my monthly payment?

Only if you recast. A plain lump-sum prepayment shortens the term and cuts interest but leaves the payment unchanged; a recast re-amortizes the lower balance to reduce the payment.