Do Not Prepay a 6% Loan While the SDP Pays You 10%

Deployment is when servicemembers get serious about debt. The pay goes up, the spending opportunities vanish, and everyone comes home determined to have killed the car loan.

For a debt the SCRA has capped, that instinct costs you money.

The two rates sitting on your desk

Your debt is at 6%, or lower. You sent the notice letter, your pre-service obligations dropped to the statutory cap, and the interest above it was forgiven, not deferred. Section 3937(a)(3) also required your monthly payment to come down. There is no accrued-interest balloon waiting at the end of your enlistment.

The Savings Deposit Program pays 10%, guaranteed, on up to $10,000, for anyone serving in a combat zone or drawing hostile-fire or imminent-danger pay in a qualifying area for at least 30 consecutive days. It is a Treasury-backed program. The return is not a projection. It is a number in the DoD Financial Management Regulation.

You are borrowing at 6% and being offered 10%, risk-free, by the same government that capped the 6%.

Prepaying the debt in that situation means voluntarily earning 6% instead of 10%.

The break-even, and why it almost never bites

There is exactly one thing that complicates it: SDP interest is taxable. Prepaying debt returns that debt’s rate tax-free.

So set them equal. Ten percent times (one minus your tax rate) equals six percent. Solve for the tax rate:

t = 40%.

Prepaying a 6%-capped debt beats the SDP only if your combined marginal tax rate exceeds about 40%. Run it at real brackets:

Combined marginal rate	SDP after tax	Beats a 6% debt?
0% (deployed, pay excluded, income under the standard deduction)	10.0%	Yes, by 4.0 points
12%	8.8%	Yes, by 2.8 points
22%	7.8%	Yes, by 1.8 points
24%	7.6%	Yes, by 1.6 points
32%	6.8%	Yes, by 0.8 points
32% federal + 9.3% state = 41.3%	5.9%	No. Prepay wins, barely.

To clear 40% you need a 32%-or-higher federal bracket plus a high-tax state of legal residence, which for most servicemembers means a senior officer with a high-earning spouse who never established a no-tax domicile. That is a rounding error of the force.

And the combat-zone exclusion pushes the SDP side further up, not down. Your combat-zone pay is excluded from income, so a deployed member’s taxable income can be near zero, which means the tax on the SDP interest can genuinely be 0%. (The exclusion covers compensation for active service; it does not shelter the SDP interest itself, which is why the regulation says the interest is taxable. Both things are true, and on net you are usually in a very low bracket anyway.)

For essentially every enlisted member and most officers, the SDP wins, and it is not close.

Now the three ways this becomes bad advice

This is the part the milblogs skip, and getting it wrong will cost you more than the trade earns.

1. The cap only reaches PRE-SERVICE debt

Section 3937 caps debt “incurred… before the servicemember enters military service.” Full stop.

The 24% card you opened at the mall outside the gate in your second year? The 14% loan from the dealership on the strip? Those are capped at nothing. They are during-service debt and the SCRA does not touch them.

For uncapped debt, the comparison flips completely. The break-even is the SDP’s after-tax yield:

• At the 22% bracket, SDP nets about 7.8%. Any uncapped debt above that should be paid before you fund the SDP.
• At the 12% bracket, the threshold is about 8.8%.

A 20% credit card is not a close call. Pay it. Then fund the SDP.

So the rule is not “never prepay while the SDP is open.” The rule is:

Kill uncapped debt above ~8%. Then fill the SDP to $10,000. Then attack everything else.

The 6%-capped loan sits at the bottom of that list because it is at 6%, not because the SDP always wins.

2. It is not either/or. The SDP is a $10,000 bucket, and it fills up

The SDP caps out. It does not compete with your debt beyond $10,000, so framing this as “don’t prepay” invites you to sit on a pile of cash doing nothing.

The correct sequence is: the first $10,000 goes to the SDP. Every dollar after that goes at the debt.

3. The SDP is smaller than you have been told

Three details from the regulation that almost every article gets wrong:

• The $10,000 ceiling counts principal AND accrued interest together. Once the account touches $10,000 it stops compounding and pays simple interest on the $10,000.
• Interest is paid on the average quarterly balance, so a member who ramps up over a deployment earns far less than a full year at 10%.
• You cannot front-load it. Deposits are limited to your unallotted current pay and allowances, and the regulation expressly bars accumulating back pay before you deploy in order to dump it in. That means roughly one month’s pay per month, and a junior member with allotments may never reach $10,000 at all.

A realistic 9-to-12-month deployment nets a few hundred dollars to about $800. Not “a free $1,000.” It is still the best risk-free return available to any American, and it is still worth taking. Just size it honestly.

Also: SDP money is not liquid. Withdrawals require an emergency and your commander’s approval. Never fund it with your emergency fund.

The trap on the way home

Interest stops accruing 90 days after you leave the zone. If you have not withdrawn by 120 days after your qualifying duty ends, DFAS automatically transfers the whole balance into your military pay account.

So somewhere around the four-month mark after redeployment, $10,000-plus lands in your checking account while you are home, decompressing, and feeling like you earned something.

That lump sum is the single most reliably squandered money in military personal finance. The trade only works if it actually gets redeployed at the debt. Calendar the date now. Send it the day it arrives.

Where the math changes

Student loans in a hostile-fire area are at 0%, not 6%. Under 20 U.S.C. § 1087e(o), Direct Loans first disbursed on or after October 1, 2008 accrue no interest at all for up to 60 months while you serve in an area of hostilities qualifying for special pay. Prepaying a 0% loan returns you exactly nothing. The argument here is not just stronger, it is absolute. (Note the limits: Direct Loans only, so FFEL and Perkins do not qualify, and neither do Direct Loans disbursed before that 2008 date.)

Pursuing PSLF? Prepaying is actively harmful, for reasons that have nothing to do with the SDP. Do not do it.

Mortgage close to 80% LTV? If prepaying gets you there and cancels PMI, the effective return on those specific dollars can blow well past 10%. That is a genuine exception. Take it.

Debt in default, or accruing penalty rates and collection costs? The SCRA cap does not fix fees or cure a default. Cure the debt first, then optimize.

Separating soon? For non-mortgage debt, the cap ends the day your military service ends. That capped card snaps back to its contract rate at separation. Point the returning SDP lump sum straight at it.

One more reason prepaying is the wrong instinct here

People prepay to free up cash flow. On a capped debt, that already happened.

Section 3937(a)(3) required your lender to reduce the periodic payment by the amount of interest forgiven. Your payment came down when the cap applied. Prepaying an installment loan on top of that typically just shortens the term. It does not cut the monthly payment again.

So the cash-flow argument for prepaying, which is the real reason most people do it, does not even apply.

The deployment payoff order

1. Confirm you qualify for the SDP: combat zone, or hostile-fire / imminent-danger pay in a qualifying area, for at least 30 consecutive days. Enroll through your finance office.
2. First: kill any uncapped debt above roughly 8%. That means during-service debt the SCRA never touched. This is where your money is actually leaking.
3. Second: fund the SDP to the $10,000 ceiling, as fast as your unallotted pay allows. You cannot front-load it, so start month one.
4. Third: everything after that goes at the debt, worst rate first.
5. Leave the 6%-capped debt alone. Pay it on schedule. It is the cheapest money you will ever hold.
6. Calendar 120 days after your qualifying duty ends. DFAS will push the SDP balance into your pay account. Send it at the debt the day it lands.
7. If you separate soon after, remember the non-mortgage cap dies at separation. That is the debt to finish off with the SDP money.

The full deployment picture, including the tax-free Roth angle and the 0% student loan benefit, is in the deployment money stack. And the reason your debt is at 6% in the first place is the pre-service window.

The law behind this: 50 U.S.C. § 3937(a)(2)

Forgiveness of interest in excess of 6 percent: the excess is forgiven, not deferred, and the periodic payment must be reduced accordingly: read the statute.