Yes, an extra $100 is worth bothering with. On a $300,000 balance at 6.5% with 25 years remaining, $100 a month pays the loan off 32 months early and saves $38,719 in interest — more than most people save in a year of coupon-clipping, from less than the cost of a weekly takeout dinner.
The ladder shape is the insight: each additional $100 of monthly extra does more than the one before it, not less, because it lands on an ever-smaller balance and compounds for the remaining months. That is why the jump from $250 to $500 saves more than the jump from $50 to $250. Prepayment rewards consistency more than size.
Find your rung by working backwards from your budget, not forwards from an ideal. The right extra is the largest number you can sustain through a bad month — an autopay you never have to re-decide. Any rung on this table beats standing on the ground floor paying the contractual schedule.
How the extra payment comparison works
Every row of the table is an independent month-by-month simulation of your loan with exactly one variable changed: the monthly extra. The base row (your current payment, no extra) is the yardstick; each rung shows the payoff date, total months, months sooner, and interest saved against that yardstick.
Because all rows share the same balance, rate, and start date, the differences are purely the effect of the extra payment — no other assumption leaks into the comparison.
interest_m = balance_m × r balance_m+1 = balance_m − payment + extra − interest_m
The "custom" field adds your own amount to the ladder so you can test the number your budget actually produced, not just the round numbers.
Worked example. Balance $300,000 at 6.5%, 25 years remaining. Baseline payoff Oct 2051, interest $307,686.45. $100/mo: Feb 2049, saves $38,719.02. $250/mo: Feb 2046, saves $80,244.60. $500/mo: Sep 2042, saves $125,588.47. The last $250 buys $45,343.87 more savings than the jump from $100 to $250 — the ladder steepens.
Terms & definitions
Amortization
The process of paying off a loan with scheduled payments that cover both interest and principal. Early payments are mostly interest; later payments are mostly principal.
Principal
The amount you borrowed and still owe. Extra payments reduce principal directly, which shrinks every future interest charge.
APR
Annual Percentage Rate — the yearly cost of the loan including fees. Your monthly interest charge uses the note rate, which is usually a touch lower.
Term
The number of years the loan is scheduled to last. A 30-year term has lower payments but roughly double the total interest of a 15-year term at the same rate.
Frequently asked questions
Is even $50 a month worth it?
On the example loan ($300,000 at 6.5%, 25 years left), $50 a month saves $20,819 and pays off 17 months early. It is not life-changing, but it is money you would otherwise hand the bank for nothing. The right framing is not "is $50 enough?" but "what does $50 cost me not to do?"
Lump sum or monthly extra — which is better?
A dollar is a dollar to your balance regardless of when it arrives, but a dollar earlier saves more interest than a dollar later. If you hold $5,000 waiting for the "right time," you are paying interest on it in the meantime. The usual optimum: lump what you have (keep a safety cushion), then autopay the monthly habit.
Should I invest instead of prepaying?
Compare guaranteed vs expected: prepaying a 6.5% loan is a guaranteed 6.5% return; investing hopes for more but can lose. Most balanced plans do both — capture any employer 401(k) match first (instant 100% return), keep an emergency fund, then split surplus between investing and prepaying. See the
full analysis.
Do I need to inform my bank about extra payments?
You need to label them. Use your servicer's "additional principal" field online, or write "apply to principal" with a mailed payment. Unlabeled extras are often held as "future payments" — applied to interest you have not been charged yet, which saves you nothing.
References
- CFPB — Can I prepay my mortgage without penalty?
- CFPB — What is an amortization schedule?
Educational content only — not financial, tax, or legal advice. Your loan terms govern; confirm details with your servicer.