Prejudgment Interest Rules in Idaho
At a glance
| Governing law | Idaho Code § 28-22-104(1) sets the legal interest rate for listed money obligations; Idaho decisions apply it to liquidated or mathematically ascertainable prejudgment claims (Bouten Construction Co. v. H.F. Magnuson Co., 133 Idaho 756 (1999)). |
|---|---|
| Interest rate | 12% a year under § 28-22-104(1), unless an express written contract sets a different rate. Subsection (2) instead sets a floating rate for money due on a judgment. |
| When interest starts running | For a qualifying amount, interest runs when the money becomes due or, for a settled mutual account, when its balance is ascertained (§ 28-22-104(1)); the damages must be liquidated or calculable by mere mathematical process (Bouten). |
| Contract vs. tort claims | Section 28-22-104(1) lists types of money due rather than separate contract and tort rates. The claimant must meet the liquidated-or-mathematically-ascertainable threshold recognized in Bouten. |
| Mandatory or discretionary | Statutory interest is allowed on the listed money obligations once the amount qualifies; whether damages were liquidated or ascertainable is the decisive threshold (§ 28-22-104(1); Bouten). |
| Simple or compound | Prejudgment interest under § 28-22-104 is simple, as held in Holladay v. Lindsay, 143 Idaho 767 (Ct. App. 2006). That opinion considered a separate equitable claim for compounded gains. |
| Claims against the government | The Idaho Tort Claims Act (§ 6-901) caps combined governmental liability for damages, costs, and attorney fees at $500,000 per occurrence unless excess insurance applies (§ 6-926(1)); it also bars punitive damages (§ 6-918). These sections do not specify prejudgment interest’s treatment within the cap. |
| Other exceptions | A written contract setting a different interest rate displaces § 28-22-104(1)’s 12% default; an amount that is not liquidated or mathematically ascertainable fails the Bouten prejudgment-interest threshold. |
Requirements one by one
Governing law
Idaho Code § 28-22-104(1) allows interest on specified money obligations. Bouten Construction Co. v. H.F. Magnuson Co. states that prejudgment interest is available only when damages are liquidated or ascertainable by mere mathematical process.
Interest rate
The statutory rate is twelve cents on the hundred by the year, or 12%, unless an express written contract fixes a different rate (§ 28-22-104(1)). Subsection (2) supplies a different, floating rate for money due on a judgment.
When interest starts running
Section 28-22-104(1) speaks of money after it becomes due and, for mutual accounts, from the date the balance is ascertained. A disputed amount that requires a fact-finder to set damages does not meet Bouten's liquidated-or-calculable test.
Contract vs. tort claims
The statute lists categories of money due; it does not assign one rate to contracts and another to torts. The legal-interest question depends on the obligation and whether the amount is liquidated or calculable (§ 28-22-104(1); Bouten).
Simple or compound
In Holladay v. Lindsay, the Idaho Court of Appeals affirmed simple prejudgment interest under § 28-22-104. The court separately discussed whether proven compounded returns might be recoverable as unjust-enrichment damages; that is a different claim from compounding the statute's 12% legal rate.
Claims against the government
The Idaho Tort Claims Act is named in § 6-901. Section 6-926(1) limits combined governmental liability under that act to $500,000 per occurrence unless excess liability insurance controls. Section 6-918 bars punitive damages. The listed cap components are damages, costs, and attorney fees; § 6-926 does not expressly classify prejudgment interest for that limit.
What trips people up
The 12% prejudgment rate under § 28-22-104(1) is different from subsection (2)'s postjudgment rate. In Holladay, a possible equitable recovery of compounded gains did not convert the statutory interest itself into compound interest.
Common questions
Can a written contract set a different rate? Yes. Section 28-22-104(1) applies when there is no express written contract fixing another interest rate.
Does an unliquidated claim automatically earn interest? No. Bouten requires the amount to be liquidated or ascertainable by mere mathematical process.
Statutes and sources
- Idaho Code § 28-22-104(1) — https://legislature.idaho.gov/statutesrules/idstat/title28/t28ch22/sect28-22-104/ (accessed 2026-10-06).
- Idaho Code § 28-22-104(2) — https://legislature.idaho.gov/statutesrules/idstat/title28/t28ch22/sect28-22-104/ (accessed 2026-10-06).
- Idaho Code § 6-926 — https://legislature.idaho.gov/statutesrules/idstat/title6/t6ch9/sect6-926/ (accessed 2026-10-06).
- Bouten Construction Co. v. H.F. Magnuson Co., 133 Idaho 756, 992 P.2d 751 (1999) — https://caselaw.findlaw.com/court/id-supreme-court/1489343.html (accessed 2026-10-06).
- Holladay v. Lindsay, 143 Idaho 767, 152 P.3d 638 (Ct. App. 2006) — https://caselaw.findlaw.com/court/id-court-of-appeals/1175702.html (accessed 2026-10-06).
- Idaho Code § 6-901 — https://legislature.idaho.gov/statutesrules/idstat/title6/t6ch9/sect6-901/ (accessed 2026-10-06).
- Idaho Code § 6-918 — https://legislature.idaho.gov/statutesrules/idstat/title6/t6ch9/sect6-918/ (accessed 2026-10-06).
Source links
Every statute quoted above, linked, with the date we checked it.
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