The TSP Mistake Costing Service Members Thousands

The Thrift Savings Plan is one of the most powerful financial tools available to U.S. service members, and one of the most frequently misused. With 2026 contribution limits now in effect, a new Roth catch-up requirement active since January 1, 2026, and a 27-pay-period calendar creating a mid-year trap for percentage-based contributors, the gap between what participants contribute and what they could accumulate over a career can reach tens of thousands of dollars.

This article identifies the four most consequential TSP errors active-duty members under the Blended Retirement System (BRS) may be making right now, using 2026-confirmed figures from TSP.gov and IRS Notice 2025-67.

Mistake 1: Contributing Below 5% and Leaving the Full Match on the Table

Under the Blended Retirement System (BRS), the Department of Defense contributes to your TSP through two mechanisms: an automatic 1% of basic pay, deposited regardless of whether you contribute anything yourself, and a matching contribution of up to 4% additional: dollar-for-dollar on the first 3% you contribute, then 50 cents on the dollar for the next 2% (TSP.gov, Contribution Limits, as of 2026).

The full 5% government contribution is only triggered when you contribute at least 5% of basic pay. Contributing less means forfeiting part of that match.

A concrete illustration: an E-5 with approximately $3,000 in monthly basic pay who contributes only 3% receives a 4% government contribution, leaving 1% of basic pay, roughly $30 per month, uncaptured. Over 12 months, that is $360 in missed contributions before any compounding. Over a 10-year career, the compounded gap grows substantially larger.

Participants may want to verify their current contribution percentage through myPay or their TSP account at tsp.gov and confirm they are at or above the 5% threshold to capture the complete government match (TSP.gov, Contribution Limits, as of 2026).

Mistake 2: Maxing Out Too Early and Losing the Match Mid-Year

The 2026 elective deferral limit for TSP contributions is $24,500, up from $23,500 in 2025 (TSP.gov, Bulletin 25-3, as of November 2025; confirmed effective January 1, 2026). This is the combined annual cap on traditional and Roth employee contributions.

The critical detail: if you contribute a high flat percentage of pay and reach $24,500 before December, your contributions stop, and so does the government match. DoD matching is calculated per pay period. No employee contribution in a given pay period means no matching contribution for that period.

2026 compounds this risk: the calendar produces 27 pay periods rather than the standard 26. A participant who set their election based on a 26-pay-period year may hit the $24,500 limit one pay period early, forfeiting one full period of matching (Military Money Manual, Military TSP Match Max, as of 2026).

One approach: elect a specific dollar amount per pay period rather than a percentage of pay, calibrated to reach, but not exceed, $24,500 in the final pay period of the year. For a 27-pay-period year, that works out to approximately $907 per pay period ($24,500 / 27). Participants may find it useful to recalculate their elections each January rather than carrying over a prior-year percentage.

Mistake 3: Parking Everything in the G Fund and Accepting Capital-Preservation Returns

The G Fund, the Government Securities Investment Fund, is the TSP's capital-preservation option. It invests in a special non-marketable U.S. Treasury security issued exclusively for the TSP and is the only core fund that guarantees return of principal against market fluctuations (TSP.gov, Investment Options, as of 2026).

That guarantee carries a trade-off. As of July 2026, the G Fund's year-to-date return stands at approximately +2.30%, while the C Fund (tracking the S&P 500 Index) has returned approximately +11.35% YTD, the I Fund approximately +15.83% YTD, and the S Fund approximately +16.05% YTD (TSP.gov, C Fund, as of July 2026; fedinfo.org, TSP Fund Performance 2026, as of July 2026). Past performance does not guarantee future results, but the long-run pattern is consistent: the G Fund functions as a capital-preservation vehicle, not a long-term growth engine.

For early- and mid-career service members with decades before retirement, a portfolio concentrated entirely in the G Fund may represent a meaningful opportunity cost over time. Participants may find it useful to review how their current allocation aligns with their time horizon and risk tolerance.

The TSP's Lifecycle (L) Funds offer a professionally designed, diversified mix of all five individual funds, automatically rebalanced as the target date approaches, an option worth understanding for participants who prefer not to manage individual fund elections (TSP.gov, Investment Options, as of 2026). Individual circumstances vary. Allocation decisions depend on personal factors including time horizon, financial goals, and risk tolerance. Participants may consider speaking with a qualified financial advisor before changing fund elections.

Mistake 4: Taking TSP Loans or Early Withdrawals Without Understanding the Full Cost

TSP loans and in-service withdrawals are available, but the financial consequences are frequently underestimated.

When you take a TSP loan, the borrowed dollars are removed from your invested funds and no longer earn market returns for the duration of the loan. You repay with interest, but that interest returns to your own account at the G Fund rate, not the rate your invested funds might otherwise have earned (TSP.gov, TSP Loans, as of 2026).

More critically: if you separate from service with an outstanding TSP loan and do not repay it within the required window, the IRS treats the unpaid balance as a taxable distribution. If you are under age 59-1/2, a 10% early withdrawal penalty applies on top of ordinary income tax (TSP.gov, Information for TSP Participants Leaving Federal Employment, TSPFS29, as of February 2025).

Financial hardship in-service withdrawals carry the same tax exposure: 10% federal withholding on the taxable portion, plus potential state income tax, plus the 10% early withdrawal penalty for participants under 59-1/2 (TSP.gov, Financial Hardship, as of 2026). Unlike a loan, a hardship withdrawal is permanent: the funds cannot be returned to the account.

Participants facing financial pressure may find it useful to evaluate all available options, including the SCRA interest rate cap on pre-service debts and military-specific relief programs, before accessing TSP funds.

2026 Rule Change: Roth Catch-Up Requirement for Higher Earners

A SECURE 2.0 Act provision (Section 603) took effect January 1, 2026, and applies to TSP participants who are age 50 or older and whose 2025 Social Security wages from a TSP-eligible position exceeded $150,000.

For those participants, all catch-up contributions above the $24,500 base limit must now go to Roth TSP, not traditional pre-tax TSP. This applies automatically once contributions reach the elective deferral limit; the payroll system handles the re-designation. Regular contributions up to $24,500 may still be directed to traditional or Roth TSP at the participant's election (TSP.gov, Contribution Limits, as of 2026; IRS Notice 2025-67, as of November 2025).

The 2026 catch-up limits, confirmed by TSP Bulletin 25-3:

Participants in this income range may want to verify with their payroll office that the Roth designation is being applied correctly, particularly if they do not already have a Roth TSP balance established.

Vesting: What Belongs to You, and When

Under BRS, participants are always vested in their own contributions and earnings. Government matching contributions vest immediately upon receipt. Only the automatic 1% contribution requires completing two years of qualifying service to vest (FINRED, Vesting Counselor Checklist, DoD Financial Readiness, as of 2026).

For members who entered service on or after January 1, 2018: the automatic 1% contribution begins at 60 days of service, but matching contributions do not begin until after two years of service. Separating before the two-year mark means forfeiting both the automatic 1% contributions and the associated earnings. Participants approaching a separation decision may find it useful to confirm their vesting status before finalizing a timeline.

By the Numbers: 2026 TSP at a Glance

Source: TSP.gov Bulletin 25-3; IRS Notice 2025-67, as of November 2025. All figures effective January 1, 2026.

Considerations for Service Members

Verify your current TSP contribution percentage in myPay. Participants covered by BRS may want to confirm they are at or above 5% of basic pay to capture the complete government match.

Recalculate your per-pay-period contribution election each January. 2026 has 27 pay periods, and a prior-year percentage election may not produce the same year-end result.

Review your fund elections relative to your expected separation or retirement year. The G Fund may warrant a different weighting for long-horizon participants than for those approaching transition.

If you are age 50 or older and your 2025 wages exceeded $150,000, participants may want to confirm with their payroll office that the Roth catch-up designation is being applied correctly under SECURE 2.0 § 603, effective January 1, 2026.

Before taking a TSP loan or hardship withdrawal, evaluate the full tax and opportunity cost, particularly if separation from service is possible in the near term.

Financial readiness is mission readiness. Siebert's Valor program supports active military and veterans with resources designed for the realities of military financial life. Learn more at siebert.com/programs/siebert-valor.