Post Judgment Interest Calculator: How It Works

A post-judgment interest calculator works out the interest that accrues on a court judgment from the date it is entered until the date it is paid. You enter the judgment amount, the applicable annual interest rate, and the start and end dates, and it returns the total interest owed and the total amount due. The calculation uses simple interest, which is the standard method for post-judgment interest in most U.S. jurisdictions.

Formula

The formula for post-judgment interest is simple interest: Interest = Principal × Rate × Time. In symbols:

I = P × (r / 100) × (d / 365)

Where:

• P = principal judgment amount (in dollars)
• r = annual interest rate (as a percentage, e.g., 5 for 5%)
• d = number of days between the judgment date and the payment date
• 365 = number of days in a year (some jurisdictions use 360; see below)

The total amount due is simply P + I.

Worked Example

Using the defaults from the calculator: a judgment of $50,000, an annual interest rate of 5%, a judgment date of January 15, 2024, and a payment date of January 15, 2025.

1. Calculate the number of days: January 15, 2025 minus January 15, 2024 = 365 days (2024 is a leap year, but the difference is exactly one year).
2. Convert the rate to a decimal: 5% / 100 = 0.05.
3. Apply the formula: I = 50,000 × 0.05 × (365/365) = 50,000 × 0.05 × 1 = $2,500.
4. Total due = 50,000 + 2,500 = $52,500.

The calculator shows the same result: interest of $2,500.00 and a total of $52,500.00.

When to Use a Post-Judgment Interest Calculator

You use this calculator any time you need to know how much a court judgment will cost or yield over time. Common scenarios include:

• Plaintiffs who won a money judgment and want to know what the defendant owes if payment is delayed.
• Defendants who want to budget for a judgment they plan to pay later, to avoid surprises.
• Lawyers and paralegals preparing settlement statements or payoff letters.
• Judges and court clerks who need to compute interest when drafting final orders.
• Anyone negotiating a lump-sum settlement after a judgment—knowing the accrued interest is essential.

The calculator is also useful for comparing the cost of paying early versus waiting, or for verifying a number provided by an opposing party.

How to Choose the Correct Interest Rate

The annual interest rate is the most critical input, and it varies widely by jurisdiction. In U.S. federal courts, post-judgment interest is set by 28 U.S.C. § 1961, which uses the weekly average one-year constant maturity Treasury yield. That rate changes weekly, so you must look up the rate for the week the judgment was entered—not the current rate.

State courts have their own rules. Some states set a fixed statutory rate (e.g., 7% per year), others use the prime rate, and a few allow the judgment itself to specify a contractual rate. Always check the statute or court rule applicable to your case. Many state court websites publish the current post-judgment interest rate.

If the judgment includes pre-judgment interest, that is a separate calculation with its own rules. This calculator only handles post-judgment interest.

Common Mistakes and Pitfalls

Several errors are common when calculating post-judgment interest manually:

• Using the wrong rate. The rate may change during the life of the judgment. Some jurisdictions require you to recompute the rate annually or use the rate in effect at the time of the judgment for the entire period. Others apply a new rate each year. Verify the rule for your jurisdiction.
• Miscounting days. The start date is the date the judgment is entered (not the date of the verdict), and the end date is the day before payment is made, or the day payment is received, depending on local rules.
• Using 360 vs. 365 days. Some states use a 360-day year (banker’s year) for interest calculations. If your jurisdiction uses 360, the daily rate is r/360, not r/365. The calculator uses 365, which is the federal standard and the most common.
• Ignoring compounding. Post-judgment interest is almost always simple interest, not compounded. If a judgment says “interest at the rate of 5% per annum,â€� it means simple interest, not monthly compounding.
• Forgetting partial payments. If the defendant makes a partial payment, interest stops accruing on that portion. The calculator assumes no payments until the end date. For partial payments, you must adjust the principal accordingly and recalculate.

How to Read the Results

The calculator returns two numbers: the interest and the total due. The interest is the amount that has accrued on top of the original judgment. The total due is what the defendant must pay to satisfy the judgment in full.

If you are the plaintiff, you would add any court costs or attorney fees that are part of the judgment to the total. If you are the defendant, you should confirm that the judgment does not include additional amounts like sanctions or statutory penalties that are not part of the principal.

Also note that interest continues to accrue daily. The result is accurate only for the exact end date you enter. If you need a payoff amount for a specific day, use that day as the end date. If you are preparing a final satisfaction of judgment, you may need to calculate interest up to the day before the payment is actually tendered.

Limitations of the Calculator

This calculator provides a general estimate based on the most common rules. It does not account for:

• Variable rates that change over time (e.g., federal rate changes weekly).
• Partial payments or credits applied to the judgment.
• Compound interest if a specific statute or contract requires it.
• Different day-count conventions (360-day year).
• State-specific rules on when interest begins (some states start from the date of the verdict, not the judgment).

For an exact figure, consult the court’s clerk or a qualified attorney. The calculator is a tool for estimation and verification, not a substitute for legal advice.

Frequently Asked Questions

What is the difference between pre-judgment and post-judgment interest?

Pre-judgment interest accrues on damages from the time the injury or loss occurred until the judgment is entered. Post-judgment interest runs from the judgment date until payment. They are calculated separately and may use different rates. This calculator only handles post-judgment interest.

Does post-judgment interest compound?

No, in virtually all U.S. jurisdictions, post-judgment interest is simple interest. It is calculated on the original principal amount only, not on previously accrued interest. If you see a term like “interest on interest,� that would be compounding, but it is rare and usually requires explicit statutory authority.

What if the judgment is paid in installments?

If the defendant pays in installments, interest accrues on the unpaid balance. Each payment reduces the principal, and interest stops on that portion. You would need to recalculate the interest for each period between payments. This calculator assumes a single lump-sum payment at the end date.

How do I find the federal post-judgment interest rate?

The federal rate is the weekly average one-year constant maturity Treasury yield, published by the U.S. Treasury. You can find it on the Treasury’s website or through the Federal Reserve’s H.15 release. The rate for the week the judgment was entered is the one to use, and it remains fixed for that judgment unless a statute says otherwise.

Can I use this calculator for a judgment from a small claims court?

Yes, but you must verify the interest rate and rules for your specific small claims court. Many small claims courts use the same statutory rate as the general trial court. Check the court’s website or ask the clerk. The formula is the same; only the rate and day-count may differ.

What is the “banker’s year� of 360 days?

Some states and many financial contracts use a 360-day year to simplify interest calculations. The daily interest rate is the annual rate divided by 360, which results in slightly higher daily interest than a 365-day year. If your jurisdiction uses 360, you can adjust the calculation by dividing the rate by 360 instead of 365. This calculator uses 365, the federal standard.

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in p | Principal judgment amount | $ | 50000 | 0 | 10000000
in r | Annual interest rate | % | 5 | 0 | 30
in d1 | Start date (judgment date) | - | 2024-01-15 | 1900-01-01 | 2100-12-31
in d2 | End date (payment date) | - | 2025-01-15 | 1900-01-01 | 2100-12-31
out i | Interest | $ | p * r/100 * (d2-d1)/365 | 2
out t | Total due | $ | p + p * r/100 * (d2-d1)/365 | 2