TSP Loan Payment Calculator and Repayment Formula

A TSP loan payment calculator estimates the periodic payment and total repayment for a loan from your Thrift Savings Plan account. Enter the amount, interest rate, and repayment term to see the estimated monthly payment and total interest; the calculation below uses standard fixed-payment loan math, not TSP-specific repayment rules.

Formula

The calculator estimates a level monthly payment using the standard amortizing-loan formula:

Monthly payment = round(P × r ÷ [1 − (1 + r)⁻ⁿ], 2)

• P is the amount borrowed, in dollars.
• r is the monthly rate, calculated as APR ÷ 1,200. APR is entered as a percentage per year.
• n is the number of monthly payments, calculated as years × 12.
• round(…, 2) rounds the payment to the nearest cent.

Estimated total interest is monthly payment × n − P. This follows the calculator’s rounded monthly payment, so its result reflects multiplying that displayed payment by the number of payments and subtracting the original amount. The inputs represent a simplified fixed-rate loan. They do not reproduce every detail of an actual TSP account loan.

Worked Example

Using the calculator’s default inputs—$20,000 borrowed, 8% APR, and five years—the term is 5 × 12 = 60 monthly payments. The monthly rate is 8 ÷ 1,200 = 0.0066667.

Substitute those values into the formula:

Payment = round(20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰], 2) = $405.53

Total repayment using the rounded payment is $405.53 × 60 = $24,331.80. Subtracting the $20,000 principal gives estimated total interest of $4,331.80. The actual amount and schedule for a TSP loan can differ because this illustration does not apply plan-specific terms or payment timing.

Assumptions

• The interest rate stays fixed throughout the illustrated term.
• Payments are equal monthly payments, with no additional payment-timing adjustments.
• Interest is modelled using monthly compounding at APR divided by 12.
• Fees, taxes, insurance, PMI, escrow, and TSP plan-specific charges or rules are not included.

How to Use the Estimate for a TSP Loan

A TSP loan is a loan from your own account balance under the Thrift Savings Plan, rather than an ordinary loan from an outside lender. The plan’s loan terms, available loan types, limits, repayment requirements, and administrative details are governed by applicable plan materials. This calculator does not determine eligibility or reproduce those terms. Use it to understand the arithmetic of a chosen amount, rate, and duration, then check the actual figures and schedule with the plan or its official loan information.

1. Enter the amount you are considering borrowing. It must be positive; the minimum and maximum in this calculator are technical input limits, not a statement of TSP loan limits.
2. Enter the APR supplied for the specific estimate you want to model. Do not treat the default 8% as a current, typical, or TSP-offered rate.
3. Enter the term in years. Convert a term stated in months to years by dividing by 12, or use the calculator’s annual input as a decimal where appropriate.
4. Read the monthly payment and total-interest estimate together. Compare them with the official repayment schedule, which may use different conventions or details.

“TSP loan payment calculator” can refer to a general payment estimate or an official plan calculation. The formula here answers the first question: what would equal payments look like under these stated assumptions? It does not calculate the amount you may borrow, confirm that a loan is available, or account for how borrowing affects your account’s investment allocation or future balance.

What the Results Mean—and Do Not Mean

The monthly payment is the estimated amount required each month to amortize the stated principal over the entered term at the entered rate. Total interest is the difference between the total of those rounded payments and the amount borrowed. It is not a separate fee quote, nor does it include charges or changes that are outside the formula.

For a real TSP loan, check the official estimate for the payment amount, payment frequency, interest calculation, start date, and any plan-specific conditions. A payment schedule can also be affected by payroll arrangements and account circumstances. The calculator cannot account for missed or changed payments, early payoff, or a loan that does not follow equal monthly payments.

One common misconception is that borrowing from one’s own retirement account means there is no cost. The calculator’s interest figure describes interest within this simplified repayment model; it does not measure investment growth that might have occurred if the amount had remained invested, nor does it quantify broader financial consequences. Those issues are outside the payment formula and require information not entered here.

Common Input and Interpretation Mistakes

• Entering 8 instead of 0.08: APR is entered as a percentage, so enter 8 for 8%, not 0.08.
• Using months as years: Enter 5 for a five-year term, not 60. The formula converts years to months.
• Treating the default as a quote: The displayed default is only an example input. Obtain the applicable rate and terms from the plan’s official information.
• Reading rounding as an exact schedule: The payment is rounded to cents. Actual payment schedules may handle rounding and final payments differently.
• Assuming the result includes every cost: Fees and plan-specific terms are not inputs, so they cannot appear in the calculated result.

Because the interest input must be above zero for this formula, the calculator does not model a zero-interest loan. The method also assumes a fixed rate and regular monthly payments; it is not suitable for variable payments or a changing rate without recalculating separate periods.

Frequently Asked Questions

How do I calculate a TSP loan payment?
For a fixed rate and equal monthly payments, convert APR to a monthly rate by dividing by 1,200, convert the term to months, and apply the amortizing-payment formula shown above. Confirm the actual payment with the plan’s official estimate.

Does this calculator show the amount I can borrow from my TSP?
No. It estimates payments for an amount you enter. It does not check plan eligibility, account details, or any applicable borrowing limit.

Is the 8% rate in the example the current TSP loan rate?
No. It is a neutral example input for demonstrating the calculation, not a statement about a current or available rate. Use the rate shown in the official information for the loan you are modelling.

Why might my official payment differ?
The official calculation may apply plan-specific terms, payment frequency, dates, rounding, or charges that are not represented here. Check the repayment schedule provided for the specific loan.

Does the interest estimate show the full financial cost of borrowing?
No. It is only the interest implied by the simplified fixed-payment formula. It does not measure investment returns forgone, taxes, fees, or other consequences.

This result is an estimate for educational purposes, and the lender’s disclosure gives the exact figures.

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in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2