US Federal Post-Judgment Interest (28 USC §1961): Calculator and Methodology Guide

Federal post-judgment interest is rarely controversial in principle. The statute is clear enough, the rate is published, and the method is familiar. The difficulty is that, by the time interest becomes the focus, it is usually the last unresolved detail in a long-running matter — and small mechanical errors start to matter.

The Statutory Framework

Under 28 U.S.C. § 1961, post-judgment interest on federal money judgments is tied to the weekly average one-year constant maturity Treasury yield for the calendar week preceding the date of judgment. That rate applies as of the judgment date and, unless displaced by statute or contract, runs until payment, compounded annually.

Where Things Go Wrong

Where things tend to go wrong is not the headline rule, but the follow-through. A single payment in full is usually straightforward if the correct rate and date are used. Once payments are made in stages, or credits are applied part-way through the accrual period, the calculation becomes more fragile. Days can be counted twice or missed altogether, interest can continue to run on sums that have already been reduced, and assumptions about compounding or timing can shift without anyone noticing.

This is the point at which spreadsheets begin to fail. They are quick to produce a number, but they are poor at preserving a clear, repeatable account of how that number was reached, particularly when the calculation needs to be revisited months later or shared with someone else.

How This Calculator Works

The calculator on Interest.law is designed to deal with those ordinary realities in a controlled way. It applies § 1961 mechanically, using the correct Treasury yield for the relevant judgment date, and carries that rate through consistently over the period specified. It handles payment in full, staged or interim payments, and credits by applying interest only to the outstanding balance at each stage. The method used to count days, deal with leap years, and avoid overlapping accrual periods is fixed and published.

Usable Output

The calculation itself is instant, but the output is intended to be usable. The tool produces a short, neutral paragraph that can be copied directly into correspondence or a schedule, a clear breakdown showing the running balance and accrued interest, and a permanent, read-only link that can be shared with clients, colleagues, opposing counsel, or the court. The shared view reflects the calculation as at the time it was run and does not expose client identifiers or allow the inputs to be altered.

What the Calculator Does Not Do

The calculator does not attempt to resolve disputed legal questions. It does not decide whether § 1961 applies, from which date interest should run, or whether a different rate should displace the statutory default. Those are matters of judgment. Once they are fixed, however, the arithmetic that follows should be stable, transparent, and easy to explain.

That is the purpose of the tool: to produce a federal post-judgment interest figure that does not become a point of argument in its own right. And to do so cheaply and well.

Sources

• 28 U.S.C. §1961: Interest
• US Courts: Post-Judgment Interest Rate Table
• FRED: 1-Year Treasury Constant Maturity Rate
• Federal Reserve H.15 Release