Interest reference snapshot for United States (Federal)

Rule or statute summary

Run this scenario in DocketMath using the Interest calculator.

This reference snapshot highlights key interest rules at the federal level in the United States—specifically how interest is commonly computed and charged, where it shows up (e.g., court judgments, tax, and certain federal payment/contract contexts), and what you can plug into DocketMath’s interest calculator to produce quick estimates.

Because “interest” can mean different things depending on context, the rules below are organized by the most frequent federal categories:

• Federal judgments and post-judgment interest (interest on a court-awarded amount after judgment)
• Federal tax underpayment/overpayment interest (interest calculated under the Internal Revenue Code by the IRS)
• Federal contractor/vendor late-payment interest (where applicable under federal payment statutes, often tied to the Prompt Payment Act)
• Interest on claims against the U.S. and the “no interest” baseline (interest may be barred unless Congress waives immunity or a specific statute authorizes it)

Quick “what to calculate” map

Use this table as a practical guide to decide which interest concept you’re working with:

Citations

Use these sources to confirm the authoritative text before finalizing the calculation.

• 28 U.S.C. § 1961

If an assumption is uncertain, document it alongside the calculation so the result can be re-run later.

Capture the source for each input so another team member can verify the same result quickly.

1) Post-judgment interest (federal court judgments)

Federal post-judgment interest is governed by:

• 28 U.S.C. § 1961 (post-judgment interest; federal rate derived using a Treasury securities methodology)

Key calculation points typically include:

• Start date: generally the date of entry of judgment.
• Rate: not usually a single fixed statutory percentage; it is tied to the § 1961 Treasury-based mechanism.
• Accrual/compounding: governed by the federal rule under § 1961.

2) Federal tax interest (IRS underpayment/overpayment)

Federal tax interest provisions are primarily in the Internal Revenue Code (IRC):

• 26 U.S.C. § 6601 (interest on underpayments of tax)
• 26 U.S.C. § 6611 (interest on overpayments of tax)
• 26 U.S.C. § 6621 (rate of interest determined under IRS rate-setting rules)

Practical calculation implications:

• Rates change: IRS interest rates are updated periodically. If your period spans multiple rate changes, a single annual rate can be misleading.
• Date selection matters: tax interest windows can depend on events such as assessment or payment timing (per the applicable IRC framework).

3) Late payment interest in federal procurement/payment contexts

Late-payment interest rules can arise in federal procurement/payment contexts, often associated with the:

• 31 U.S.C. §§ 3901–3906 (Prompt Payment Act late payment interest framework)

What to watch for in your estimate:

• Trigger timing: interest accrues only when statutory payment conditions are met.
• Rate/accrual method: defined by the Prompt Payment Act framework (so your estimate should mirror the statutory method as closely as possible).

4) Baseline rule: interest against the U.S. may be limited by waiver

As a general principle, the United States is immune from interest claims unless Congress has clearly waived that immunity or provided a specific statutory basis:

• This concept is commonly discussed under sovereign immunity principles; interest entitlements often require an express statute (for example, § 1961 for post-judgment interest, or specific IRC provisions for tax interest).

Use the calculator

DocketMath’s interest calculator helps you estimate accrued interest using common inputs. Since many federal interest regimes depend on rate changes and specific dates, the calculator is most useful for:

• Quick estimates using a single APR over a limited time window, and/or
• Time-sliced estimates where you segment the period and apply the correct rate for each segment.

Inputs to gather (federal context)

Before using DocketMath, collect:

• Principal amount (e.g., judgment amount or tax amount at issue)
• Annual interest rate (APR) (as a %). For regimes tied to a methodology (like § 1961), use the applicable computed rate you’ve identified for the period.
• Start date and end date (or the number of days)
• Compounding/accrual convention (if DocketMath asks). If not, use the tool’s default and adjust by segmentation if needed.

What outputs to expect

After running the calculation, review:

• Total interest accrued over the selected period
• Sensitivity to date changes (interest totals often change meaningfully with even modest date differences)
• Effect of compounding (if applied)

Example workflow (segmentation where rates change)

Tax interest under 26 U.S.C. §§ 6601, 6611, 6621 often requires rate changes over time. A practical approach:

1. Break the overall window into sub-periods where a single IRS rate applies.
2. Run DocketMath for each sub-period using the relevant start/end dates.
3. Sum the sub-results for an overall estimate.

Segmentation checklist:

Run it from the primary CTA

Use DocketMath’s interest tool directly:

• /tools/interest

For a federal judgment estimate, your typical setup is:

• Principal = judgment amount
• Start date = judgment entry date
• End date = payoff/estimate date
• Rate = the applicable § 1961 federal post-judgment interest rate for the timeframe

For tax estimates, do the same but segment by IRS rate changes if needed.

Reminder: DocketMath helps compute the arithmetic. The legally correct rate/date selection comes from the governing statute and the specific administrative/court record.

Common “gotchas” to check before relying on the number

Related reading

• Interest rule lens: Maine — The rule in plain language and why it matters
• Common interest mistakes in Rhode Island — Common errors and how to avoid them
• Worked example: interest in Maine — Worked example with real statute citations