Idaho: Prejudgment Interest Rules
The short answer
Yes, at a flat 12% per year, but only once the damages are "liquidated or ascertainable by mere mathematical process." Idaho doesn't have a dedicated prejudgment-interest statute — instead, courts apply the state's general legal-interest-rate statute to any claim, contract or personal injury alike, once that threshold is met. Interest runs from the date the amount first became fixed or calculable, which isn't always the date of breach or injury. If damages stay genuinely disputed or require a jury's judgment call, there's no prejudgment interest at all — only interest from the date of judgment forward. It's simple interest, never compounded.
| Governing law | Idaho Code § 28-22-104(1), Idaho's general "legal rate of interest" statute, applied to prejudgment interest by case law (Farm Dev. Corp. v. Hernandez, 93 Idaho 918 (1970); Bouten Constr. Co. v. H.F. Magnuson Co., 133 Idaho 756 (1999)) — the statute never uses the phrase "prejudgment interest" |
|---|---|
| Interest rate | 12% per year (§ 28-22-104(1)), or the rate set in the parties' own written contract if there is one; a separate, floating rate under § 28-22-104(2) applies only to interest accruing after judgment |
| When interest starts running | From the date the damages amount first becomes "fixed" or "ascertainable," which can be later than the date of the breach or injury itself if the exact figure wasn't calculable until some later event (Meldco, Inc. v. Hollytex Carpet Mills, 118 Idaho 5 (1990)) |
| Contract vs. tort claims | Unified by case law — no separate tort statute exists; the same "liquidated or ascertainable by mere mathematical process" test governs both a contract claim (Bouten) and a personal-injury tort claim (Long v. Hendricks, 117 Idaho 1051 (1990)) |
| Mandatory or discretionary | Settled as a matter of right once the claim is shown to be liquidated or ascertainable — courts don't describe this as a discretionary award, though whether a given claim clears that ascertainability threshold is itself frequently and heavily litigated |
| Simple or compound | Simple interest only; § 28-22-104 contains no compounding language, and a federal court applying Idaho law expressly declined to compound prejudgment interest annually (Ivanov v. Fitness Elite Training Ctr., No. 1:20-cv-00380 (D. Idaho 2025)) |
| Claims against the government | No express prejudgment-interest carve-out was located in the Idaho Tort Claims Act (Idaho Code §§ 6-901 to 6-929); the general ascertainability rule presumptively applies the same way it would to a private defendant, but any recovery still falls within the Act's overall $500,000-per-occurrence cap on "damages, costs and attorney's fees" (§ 6-926) and its bar on punitive damages (§ 6-918) — whether interest counts inside or on top of that $500,000 figure wasn't confirmed by any located case |
| Other exceptions | A contract's own stated interest rate displaces the 12% statutory rate; if damages remain genuinely unliquidated or require a fact-finder's judgment call at the time of judgment, no prejudgment interest accrues at all — the claimant gets only postjudgment interest under § 28-22-104(2) going forward |
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The short answer
Idaho doesn't have a statute labeled "prejudgment interest." Instead, courts borrow the state's general legal-interest-rate statute, Idaho Code § 28-22-104, and apply its 12% rate to a claim once the damages are "liquidated or ascertainable by mere mathematical process" — a phrase Idaho courts have repeated for decades. That threshold, not the label "contract" or "tort," decides who gets prejudgment interest. If it's met, interest is owed as a matter of right from the date the amount became fixed or calculable. If it's not met — most often because a jury has to weigh evidence and pick a number, as in most personal-injury cases — there's no prejudgment interest at all, only interest starting from the date of judgment.
Requirements one by one
Governing law
Idaho Code § 28-22-104 sits in the "Money of Account and Interest" chapter of the commercial transactions title. Subsection (1) sets a 12% legal rate of interest on six categories of money obligations (contract debts, money after it becomes due, money lent, money held for another's benefit and withheld, settled accounts, and old open accounts). Subsection (2) sets a separate, floating rate for interest that accrues after judgment. Neither subsection mentions "prejudgment interest" by name. Idaho courts filled that gap starting with Farm Development Corp. v. Hernandez, 93 Idaho 918 (1970), and restated most recently in Bouten Construction Co. v. H.F. Magnuson Co., 133 Idaho 756 (1999): a plaintiff can recover interest for the period before judgment under § 28-22-104(1), but only if the claim is liquidated or ascertainable.
Interest rate
12% per year, flat — not tied to any market index. That's the rate under § 28-22-104(1) for a qualifying prejudgment claim. If the parties have a written contract that sets its own interest rate, that rate controls instead. Don't confuse this with § 28-22-104(2)'s postjudgment rate, which is a completely different, floating figure (5% plus a Treasury-based base rate reset every July 1st) that only starts running once judgment is entered — it has nothing to do with the prejudgment period.
When interest starts running
Interest runs from the date the damages amount first became "fixed" or "ascertainable" — not automatically from the date of breach or injury. In Meldco, Inc. v. Hollytex Carpet Mills, 118 Idaho 5 (1990), the Idaho Supreme Court explained that even though a breach-of-warranty claim legally accrues at delivery, prejudgment interest should run "from the date the damages amount first becomes 'fixed' or 'ascertainable,'" which can come later — for example, once a specific measurement or calculation pins down the number.
Contract vs. tort claims
Idaho doesn't split these into separate statutes or separate rates. The same ascertainability test applies across the board. Bouten applied it to a construction contract dispute; Long v. Hendricks, 117 Idaho 1051 (1990), applied the identical rule to a personal-injury claim for lost wages and medical expenses arising from a car accident. What actually separates claims that get interest from claims that don't isn't whether they sound in contract or tort — it's whether the dollar figure was capable of exact calculation without a judge's or jury's discretion. Most personal-injury damages (pain and suffering, for instance) fail that test and draw no prejudgment interest; a repair bill or an unpaid invoice usually passes it regardless of claim type.
Mandatory or discretionary
Once a claim clears the ascertainability bar, Idaho courts treat the interest award as settled, not a matter of discretion — the recurring language is that prejudgment interest "is available" when the standard is met, not that a court "may" award it. In practice, most of the litigation happens at the threshold question — arguing over whether a given claim actually was liquidated or ascertainable at a given point in time — rather than over whether interest should be awarded once that's established.
Simple or compound
Simple interest. Section 28-22-104 says nothing about compounding, and a federal court applying Idaho substantive law expressly held that prejudgment interest calculated under Idaho law "will not be compounded annually" (Ivanov v. Fitness Elite Training Ctr., No. 1:20-cv-00380 (D. Idaho 2025)).
Claims against the government
This survey did not find a specific prejudgment-interest bar in the Idaho Tort Claims Act, Idaho Code §§ 6-901 to 6-929 — its own table of sections has no entry addressing interest at all. That suggests the ordinary ascertainability rule applies to a state or local government defendant the same way it applies to a private one. But two other limits in the Act still matter: § 6-918 bars punitive damages against a governmental entity outright, and § 6-926 caps the "combined, aggregate liability... for damages, costs and attorney's fees" arising from a single occurrence at $500,000 (absent excess insurance). Whether an award of prejudgment interest is counted inside that $500,000 cap or added on top of it wasn't resolved by any case located for this survey — treat that as an open question rather than a settled answer.
Other exceptions
A written contract's own interest-rate clause overrides the statutory 12% rate. And because Idaho ties the entire prejudgment right to the ascertainability threshold, there's no partial or discounted award for a claim that falls short — if the amount wasn't fixed or calculable without a fact-finder's judgment call, the claimant gets no prejudgment interest at all, only interest running from the date of judgment under § 28-22-104(2)'s separate postjudgment rate.
What trips people up
The biggest surprise for most people is that Idaho has no statute that actually says "prejudgment interest" — it's built entirely from case law interpreting a general interest-rate statute that was written for ordinary debts and contracts, not litigation. Someone reading § 28-22-104 cold would have no idea it also governs personal-injury judgments.
The ascertainability requirement is also easy to underestimate. It's not enough that a plaintiff eventually proves a specific dollar amount at trial — Idaho courts ask whether the amount was capable of exact, non-discretionary calculation before that proof was needed. A construction dispute where the final cost overrun depends on a court weighing conflicting evidence about which changes were reasonable, for example, isn't ascertainable until the court actually rules — so interest starts only from that ruling, not from the date the work was done.
Common questions
Does Idaho pay prejudgment interest on a personal-injury verdict?
Only if the specific damages were ascertainable by mathematical calculation, which is unusual for pain-and-suffering or general tort damages. A liquidated component, like an exact, undisputed medical bill, could qualify even within a broader personal-injury case.
What's Idaho's prejudgment interest rate?
12% per year under Idaho Code § 28-22-104(1), unless the parties' own written contract sets a different rate.
Does Idaho's prejudgment interest compound?
No. It's simple interest; nothing in the statute authorizes compounding, and courts applying Idaho law have said so directly.
Can I get prejudgment interest on a claim against an Idaho city or county?
The Idaho Tort Claims Act doesn't contain an express bar on prejudgment interest, so the general ascertainability rule likely applies the same as it would against a private defendant — but any recovery is still capped, along with other damages, at $500,000 per occurrence unless the entity carries more insurance.
Statutes and sources
- Idaho Code § 28-22-104(1) — "When there is no express contract in writing fixing a different rate of interest, interest is allowed at the rate of twelve cents (12¢) on the hundred by the year on: 1. Money due by express contract. 2. Money after the same becomes due. 3. Money lent. 4. Money received to the use of another and retained beyond a reasonable time without the owner's consent, express or implied. 5. Money due on the settlement of mutual accounts from the date the balance is ascertained. 6. Money due upon open accounts after three (3) months from the date of the last item." Accessed 2026-07-05: https://legislature.idaho.gov/statutesrules/idstat/title28/t28ch22/sect28-22-104/
- Idaho Code § 28-22-104(2) — "The legal rate of interest on money due on the judgment of any competent court or tribunal shall be the rate of five percent (5%) plus the base rate in effect at the time of entry of the judgment." Accessed 2026-07-05: https://legislature.idaho.gov/statutesrules/idstat/title28/t28ch22/sect28-22-104/
- Idaho Code § 6-926 — "The combined, aggregate liability of a governmental entity and its employees for damages, costs and attorney's fees under this chapter... shall not exceed and is limited to five hundred thousand dollars ($500,000), unless the governmental entity has purchased applicable, valid, collectible liability insurance coverage in excess of said limit..." Accessed 2026-07-05: https://legislature.idaho.gov/statutesrules/idstat/title6/t6ch9/
- Bouten Construction Co. v. H.F. Magnuson Co., 133 Idaho 756, 992 P.2d 751 (1999) — "It is settled law in Idaho that pre-judgment interest is available only when damages are liquidated or are ascertainable by mere mathematical process." Accessed 2026-07-05: https://caselaw.findlaw.com/court/id-supreme-court/1489343.html
- Long v. Hendricks, 117 Idaho 1051, 793 P.2d 1223 (1990) — a personal-injury (auto accident) case applying the same ascertainability rule used in contract cases, cited and discussed within the Bouten opinion. Accessed 2026-07-05: https://caselaw.findlaw.com/court/id-supreme-court/1489343.html
- Ivanov v. Fitness Elite Training Ctr., Inc., No. 1:20-cv-00380 (D. Idaho) — a federal decision applying Idaho's 12% statutory prejudgment interest rate under § 28-22-104 and declining to compound it annually. Accessed 2026-07-05: https://law.justia.com/cases/federal/district-courts/idaho/iddce/1:2020cv00380/46301/178/
- Idaho State Treasurer's Office, Legal Rate of Interest table — publishes the current § 28-22-104(2) postjudgment rate (distinct from the prejudgment rate discussed on this page). Accessed 2026-07-05: https://sto.idaho.gov/Banking/Legal-Rate-of-Interest
Source links
Every statute quoted above, linked, with the date we checked it.