Quick take
- A dollar of principal in year two avoids 28 years of interest; the same dollar in year 25 avoids five.
- Prepaying shortens the loan but doesn't lower your payment — a recast does that.
- At a 3% rate, prepaying is a weak use of capital. At 7%, it's a strong one.
- Build the reserve first. Money sent to principal is hard to get back exactly when you'd need it.
Why a little extra goes further than it looks
Every dollar you send beyond your required payment goes entirely to principal — none of it is split off to interest the way your regular payment is. That shrinks the balance interest gets calculated on for every remaining month of the loan, which is why modest monthly amounts compound into large lifetime savings.
Try the one-time lump sum field too: extra money applied earlier in the loan (when the balance is largest) saves meaningfully more than the same amount applied later.
Read the full guide: How Extra Mortgage Payments Save You Six Figures in Interest →
Why an extra dollar today beats an extra dollar later
Extra principal payments are unusually powerful early in a loan and progressively less so as it matures. The reason is structural. Interest each month is charged on the outstanding balance, so a dollar of principal removed in year two avoids interest on that dollar for the remaining twenty-eight years. The same dollar in year twenty-five avoids interest for five.
This is why the headline results look implausible until you work through them. Modest, consistent extra payments starting immediately can remove several years and a substantial interest total from a 30-year loan — not because the amounts are large, but because each early dollar cancels decades of compounding.
The practical implication runs against instinct. People often plan to pay extra "once things settle down," which is usually the point at which extra payments do the least. If the choice is a small amount now or a large amount in fifteen years, the small amount now frequently wins.
Prepaying doesn't lower your payment — unless you recast
Sending extra principal shortens your loan. It does not reduce your required monthly payment. You'll owe the same amount next month, and every month after, until the balance reaches zero years ahead of schedule.
That surprises people who were prepaying specifically to relieve monthly pressure. The tool for that is a recast: after a lump-sum principal payment, the servicer recalculates the required payment against the new, lower balance using your existing rate and remaining term. Same loan, same rate, smaller payment.
Recasting is rarely advertised, partly because it's unprofitable to promote compared to a refinance. If you have a rate you're happy with and want a lower payment, it's often the cheaper answer by an order of magnitude — no new loan, no closing costs, no reset amortization schedule.
Whether the mortgage is the right target at all
Extra principal is a guaranteed return equal to your mortgage rate. That's a real and riskless return, which makes it a reasonable benchmark — and also makes the comparison to alternatives fairly clean.
A rough order of operations that holds up for most households: cover an employer retirement match first, since that's an immediate return no mortgage rate approaches. Then retire any debt costing more than your mortgage — credit cards at 20%+ are not a close call. Then build an emergency reserve, because home equity is illiquid precisely when you'd need it. Extra mortgage principal comes after those, and competes against long-horizon investing on roughly even terms depending on your rate.
At a 3% mortgage rate, prepaying is a weak use of capital compared to almost anything else. At 7%, it's a strong one. The same behavior is right or wrong depending entirely on the rate you hold, which is why blanket advice about paying off your mortgage early tends to be unhelpful.
The liquidity trade nobody mentions
Money sent to principal is difficult to retrieve. Getting it back means a cash-out refinance or a HELOC, both of which require qualifying — and lenders are least willing to extend credit at exactly the moments you'd most need it, like a job loss.
A borrower who prepaid aggressively for four years and then loses income is in a worse position than one who kept the equivalent in savings, despite being objectively "ahead" on the loan. The mortgage doesn't care that you're eleven payments ahead; next month's payment is still due.
This isn't an argument against prepaying. It's an argument for sequencing: reserve first, principal second. The interest saved by prepaying instead of holding cash is small. The cost of having no cash during a disruption is not.