Missouri: Prejudgment Interest Rules

The short answer

Missouri splits prejudgment interest sharply by claim type, and the tort
side has a real procedural trap built into it. If you're owed money on a
contract or account, interest accrues automatically at 9% a year (or your
contract's own higher rate) from the date it became due -- no request, no
discretion, just a statutory entitlement once the amount is fixed or
readily calculable. If you're pursuing a tort claim -- a personal injury,
property damage, or wrongful-death case -- you get nothing before judgment
unless you first send the defendant (and their insurer, if known) a
carefully formatted written settlement demand, and the eventual judgment
ends up beating that demand. Get the demand right, and prejudgment interest
becomes mandatory; skip it, and there's no prejudgment interest at all,
however strong the case.

Requirements one by one

Governing law

Contract and ordinary debt claims are governed by RSMo § 408.020, a short,
old statute (dating to the 1930s recodification) that simply entitles a
creditor to interest once money becomes due. Judgment interest -- for both
tort and nontort claims -- is governed by RSMo § 408.040, a single section
that does two different jobs: it sets the ordinary POSTjudgment rate for
every kind of claim, and it separately creates the PREjudgment mechanism
available only in tort actions.

Interest rate

Contract and debt claims earn 9% a year under § 408.020, unless the
underlying contract itself specifies a different (and here, higher) rate,
in which case the contract rate controls. A tort claim that qualifies for
prejudgment interest earns a different, lower rate than the ordinary
postjudgment tort rate: the intended Federal Funds Rate plus 3% a year,
locked in at the rate in effect when the judgment states it, rather than
the Federal Funds Rate plus 5% that applies to the same judgment's
postjudgment interest.

When interest starts running

Contract interest starts on the date the money became due on a written
contract, or on the date of demand for an account. Tort prejudgment
interest never starts on the date of injury -- it only exists at all if the
claimant sent a qualifying written demand, and even then it starts 90 days
after that demand was received (or earlier, if the demand was flatly
rejected without a counteroffer).

Contract vs. tort claims

This is Missouri's central axis, and it's a difference in KIND, not just
degree. A contract or debt claim gets interest as a matter of course once
the amount is liquidated or can be readily calculated by an accepted
standard -- courts have denied it only where fixing the number genuinely
required judgment (for example, lost-profits damages). A tort claim gets
nothing at all unless the claimant follows a specific pre-suit ritual: a
written demand sent by certified mail, accompanied by a sworn affidavit
describing the claim and damages, and -- for wrongful death, personal
injury, or bodily injury claims -- a list of medical providers, copies of
medical bills, and signed authorizations letting the defendant obtain those
records. The demand must expressly reference § 408.040 and stay open for 90
days, and the claimant must actually file suit within 120 days of sending it
or lose prejudgment-interest eligibility altogether.

Mandatory or discretionary

Contract and debt interest is close to automatic once the underlying amount
is shown to be liquidated or ascertainable -- the statute's own language is
"creditors shall be allowed to receive interest." Tort prejudgment interest
is likewise mandatory, not left to a court's discretion, once its own
conditions are met: if the demand met every formal requirement and the
judgment exceeded it, the statute says interest "shall be awarded." The real
gatekeeping happens earlier, in whether the claimant followed the demand
procedure correctly and whether the judgment actually beat the demand.

Simple or compound

Missouri courts treat both tracks as simple interest by default (Wallemann
v. Wallemann, 817 S.W.2d 548 (Mo. Ct. App. 1991)). Two narrow exceptions
exist: a court sitting in equity may allow compounding when justice
requires it, and the parties' own contract can call for compound interest.

Claims against the government

Missouri keeps broad sovereign immunity for tort claims against the state
and political subdivisions, waived mainly for two categories: injuries from
a public employee's negligent operation of a motor vehicle, and injuries
from a dangerous condition of public property (plus any claim covered by
liability insurance the entity chooses to purchase). Damages against a
public entity are capped at a dollar amount that adjusts annually for
inflation, and punitive damages are barred entirely on any such claim.
Missouri case law confirms this cap does not limit at least postjudgment
interest on a judgment against the state (Benoit v. Mo. Highway & Transp.
Comm'n, 33 S.W.3d 663 (Mo. Ct. App. 2000)) -- no case was located addressing
whether the cap reaches prejudgment interest specifically.

Other exceptions

Missouri's Supreme Court has held that § 408.040 lets prejudgment interest
apply to an entire judgment, including a punitive-damages award, not just
the compensatory portion (Werremeyer v. K.C. Auto Salvage Co., 134 S.W.3d
633 (Mo. banc 2004)) -- a real outlier, since most states exclude punitive
damages from prejudgment interest altogether. That said, a public-entity
defendant can't be hit with this rule in practice, since punitive damages
against the government are barred outright. Medical malpractice claims are
carved entirely out of § 408.040's tort mechanism and governed instead by a
separate chapter (RSMo ch. 538), which this survey doesn't cover in detail.

What trips people up

The single biggest trap is treating the tort demand requirement as
optional or a formality. Missouri courts apply the formal requirements
strictly -- the demand has to be sent by certified mail, come with a
detailed sworn affidavit and (for personal injury and wrongful death
claims) medical records and authorizations, name the statute, and stay open
for 90 days. Miss any element, and there is no prejudgment interest to
award later, no matter how clearly the eventual verdict beat what would
have been a reasonable settlement number.

The 120-day filing deadline after sending the demand is easy to overlook,
especially if settlement talks continue past that point. Waiting too long
to actually file the lawsuit forfeits prejudgment interest even if the
demand itself was perfect.

Don't confuse the tort PREjudgment rate (Federal Funds Rate plus 3%) with
the tort POSTjudgment rate on the very same case (Federal Funds Rate plus
5%) -- they're deliberately different, and using the wrong one under- or
over-states the interest owed.

Common questions

How do I get prejudgment interest on a Missouri personal injury claim?
Send the defendant (and their insurer, if known) a written settlement
demand by certified mail that meets § 408.040's formal requirements --
sworn affidavit, medical records, an explicit reference to the statute, and
a 90-day open period -- then file suit within 120 days. If your eventual
judgment exceeds the demand, prejudgment interest is mandatory from 90 days
after the demand.

What's the interest rate on an unpaid Missouri invoice or contract?
9% a year under § 408.020, unless the contract itself sets a higher rate.

Does Missouri prejudgment interest apply to punitive damages?
Yes, unlike most states -- the Missouri Supreme Court has held § 408.040
lets prejudgment interest reach the entire judgment, including any punitive
award, against a private defendant.

Can I get prejudgment interest against the State of Missouri or a city?
Only if sovereign immunity is waived for your claim in the first place
(mainly motor-vehicle negligence or a dangerous property condition), and
damages are capped regardless; punitive damages are unavailable entirely
against a public entity.

Statutes and sources

• RSMo § 408.020 -- "Creditors shall be allowed to receive interest at the
  rate of nine percent per annum, when no other rate is agreed upon, for
  all moneys after they become due and payable, on written contracts, and
  on accounts after they become due and demand of payment is made; for
  money recovered for the use of another, and retained without the owner's
  knowledge of the receipt, and for all other money due or to become due
  for the forbearance of payment whereof an express promise to pay interest
  has been made." Accessed 2026-07-05:
  https://revisor.mo.gov/main/OneSection.aspx?section=408.020
• RSMo § 408.040.2-.4 -- the full nontort/tort judgment-interest and
  demand-triggered prejudgment mechanism, quoted in full above. Accessed
  2026-07-05: https://revisor.mo.gov/main/PageSelect.aspx?section=408.040&bid=51898
• RSMo § 537.610.2-.3 -- "The liability of the state and its public
  entities on claims within the scope of sections 537.600 to 537.650, shall
  not exceed two million dollars for all claims arising out of a single
  accident or occurrence and shall not exceed three hundred thousand
  dollars for any one person... No award for damages on any claim against a
  public entity within the scope of sections 537.600 to 537.650, shall
  include punitive or exemplary damages." Accessed 2026-07-05:
  https://revisor.mo.gov/main/PageSelect.aspx?section=537.610&bid=28541
• Wallemann v. Wallemann, 817 S.W.2d 548 (Mo. Ct. App. 1991) -- simple,
  not compound, interest is the default rule on a Missouri judgment.
  Citation verified via legalresearch (courtlistener id 5253211).
• Werremeyer v. KC Auto Salvage Co., Inc., 134 S.W.3d 633 (Mo. banc 2004)
  -- held § 408.040 allows prejudgment interest on the whole judgment,
  including punitive damages. Citation verified via legalresearch
  (courtlistener id 1405398).
• Benoit v. Missouri Highway & Transportation Commission, 33 S.W.3d 663
  (Mo. Ct. App. 2000) -- held the § 537.610 damages cap does not limit
  postjudgment interest on a damage award against the state. Citation
  verified via legalresearch (courtlistener id 1673718).