Lump sum vs. regular extra payments: which wins?
Key takeaways
04 · ideas- A $5,000 lump sum in year 1 saves more interest than $100/month for 4 years
- The 'return on prepayment' is always exactly your mortgage rate — equivalent to your loan rate
- Windfalls above 6-month emergency fund are almost often more effective when deployed against high-rate debt
- The psychological benefit of seeing the loan shrink accelerates commitment
Basic level
The essentials · 1 min
Two strategies for using extra money to pay down a loan:
Strategy A: Regular extra payments ($200/month extra, every month)
Strategy B: Lump sum payments (invest $2,400/year in a savings account, then make one payment)
Are these equivalent? Not quite. Strategy A wins — paying earlier means less interest accumulates.
Example: $200,000 mortgage at 6%, 30 years
Strategy A ($200 extra/month):- Saves $73,000 in interest
- Pays off 8 years early
- Saves ~$67,000 (depending on exact timing)
- About 6% less efficient
The difference: money in your savings account while waiting earns less than you'd save by immediately reducing the mortgage principal.
Key insight: For debt above the savings account interest rate, pay immediately — don't hold for a "big payment" later.
Intermediate level
How it applies · 1 min
Using windfalls optimally
Tax refund ($3,500 average US refund):Apply as a lump sum to the mortgage in April. On a $250,000 6% mortgage in year 10:
- $3,500 lump sum → saves approximately $8,400 in interest over remaining life
- "Return on deployment": $8,400 savings ÷ $3,500 paid = 2.4× over remaining loan life
- At 6% mortgage: puts you ahead by months or years in payoff timeline
- Alternative: max 401k first if you get an employer match (50–100% instant return beats mortgage prepayment)
- After maxing 401k match: mortgage prepayment is hard to beat at rates above 5.5%
At $50,000 applied to a 30-year, $300,000 6% mortgage in year 5:
- Remaining balance drops from ~$285,000 to ~$235,000
- Payoff time reduced by ~7 years
- Total interest saved: ~$86,000
- "Return": $86,000 ÷ $50,000 = 172% total return over remaining life
vs. investing $50,000 at 7% for 25 years: $271,000 (5.4× return, but taxable)
The comparison depends on tax treatment and time horizon, but for shorter remaining terms, mortgage prepayment often wins on a risk-adjusted basis.
The emergency fund rule
Never prepay debt at the expense of a 3-6 month emergency fund. A $10,000 prepayment followed by a car breakdown that forces you onto a 24% APR credit card will cost far more than the mortgage interest you saved.
Order of priority:- Emergency fund (3–6 months)
- Employer 401k match (50–100% an equivalent return at your loan rate (without market risk))
- High-rate debt (above 8–10%)
- Mortgage prepayment (if rate above 5.5%)
- Invest remainder (Roth IRA, taxable brokerage)
Advanced level
The math, step by step · 1 min
Optimal lump sum allocation across multiple debts
Given n debts with balances B_i, rates r_i, and a windfall W:
Mathematically optimal: Allocate entirely to highest-rate debt until eliminated, then next highest rate, etc. (the "avalanche" applied to lump sums).
Proof: allocating x to debt i vs. j where r_i > r_j:- Savings from allocating to $i$: $x \cdot r_i \cdot t_i$
- Savings from allocating to $j$: $x \cdot r_j \cdot t_j$
- Since $r_i > r_j$, allocating to $i$ dominates (assuming similar remaining terms)
However: If t_j \gg t_i (debt j has many years remaining), the remaining term matters. The correct metric is:
The rate is the only thing that matters for lump sum allocation. The "snowball" (smallest balance first) is psychologically beneficial but mathematically suboptimal for lump sums.
The mortgage vs. investment NPV for a windfall:
Where T_g = capital gains tax rate. Prepaying may reduce total interest wins when:
At 6% mortgage, 22% gains rate: invest if r_i > 6\%/0.78 = 7.69\% expected return.