Interest on unpaid invoices: 4 Missouri court limits

Key takeaways

• Check whether records establish the amount owed.
• Separate lost profits from completed-work invoices.
• Ask a lawyer to review the interest request before filing.
• Keep an understandable interest calculation with your records.

Adding a line to the invoice does not answer that. A judge looks at what kind of debt you are owed, how clearly the amount is known, and whether the claim was stated properly in a lawsuit.

This post walks through four limits that appear in Missouri court decisions. They come from the case annotations on the Missouri Revisor of Statutes page for Section 408.020, which summarize how courts applied that section to specific disputes. A summary is not the full statute, and it cannot tell you how a judge will rule on your facts. Use this as a starting point, and talk to a Missouri lawyer before you sue.

Small-business owners and bookkeepers face the same choice again and again: send a firm reminder, add interest, call a collection service, or file suit. Interest is one lever among several. It is only useful if you can support it, so the sections below explain what to check before you put a number on the page.

Why certainty matters

Courts want to know what the other side actually owes before they add interest on top of it. A debt that is clear on paper, agreed in writing, and tied to a known date is easy to work with. A debt that depends on who remembers what, or on which version of a conversation a judge believes, is harder to put a number on. Interest is a cost layered onto the principal, so the principal has to be solid first.

That is the thread running through the summaries. The sections below cover four limits: the amount must be fixed, lost profits are treated differently, interest must appear in the lawsuit, and jury instructions, verdict presentation and corrections matter.

Limit 1: The amount must be fixed or readily ascertainable

The strongest theme in the annotations is that the amount of the debt has to be clear. The summaries describe a requirement that the amount be "liquidated or readily ascertainable by reference to recognized legal standards" as the section requires. The annotations on the statute page describe one case where the amount was disputed, and one party claimed the other was estopped from claiming the true debt. The court found the amount was not liquidated or readily ascertainable, so the requirement was not met.

Another summarized case shows the limit is not absolute. A subcontractor sued for the balance due under a subcontract to cover a subgrade with topsoil. The parties honestly disagreed about how much fill had been provided. The court held that this disagreement did not make the sum unascertainable, and it did not err in allowing interest from the date the work was accepted. The same statute page carries that summary.

Set those two cases side by side and the question for your own invoice becomes clear: can the amount be worked out from records and recognized standards, or does the customer's position turn it into a guess? An honest argument about quantities did not defeat the subcontractor's claim. In the first case, the disputed amount and estoppel argument meant the amount failed the ascertainability requirement for interest.

Consider two invoices. In the first, you quoted a fixed price for installing a server, the customer signed an acceptance form on a known date, and the invoice matches the quote. The amount is easy to show. In the second, you billed hourly on an open-ended project, the customer says half those hours were never authorized, and nobody wrote down what was agreed. The second invoice is much harder to call fixed, and any interest claim on it is weaker.

Bookkeepers can prepare for this now. Keep the signed contract or purchase order, the record of delivery or completion, the acceptance date, the invoice, and every payment and credit. The subcontract case ran interest from the date of acceptance, which is why that date matters. Write it on the invoice or in the file so it is never a memory test later. For a related billing review, use the invoice approval checklist.

Limit 2: Lost profits are a different kind of claim

The annotations also say that prejudgment interest is not available for breach of a contract where damages are based on lost profits. The same statute page summarizes that holding.

This matters when a customer cancels a contract and you want to recover what you would have earned had the deal gone through. That is a lost-profits claim. An unpaid invoice for work you already completed is different. It is money you earned and billed. Treating the two as one claim invites a fight over which part the interest applies to, and the summary says interest is not available for the lost-profits kind of claim.

In practice, separate the claims in your records and your demand letter. List the invoice balance on its own line, with its own dates. If you also claim lost profits, state them separately, and do not add interest to them unless a lawyer has told you it is available. A clean split also helps you explain the amount if the customer or a judge asks how you reached it.

Illustrative example: Separate the invoice and lost profits

A customer orders a custom build, you complete half and invoice 80 for it, and the customer then cancels the rest. The invoice for the completed half is a sum already earned. Put them in separate columns of your demand, and make clear which number is the invoice and which is the lost profit. These amounts illustrate the distinction, not an interest calculation.

Limit 3: Interest must be pleaded in the lawsuit

The 1985 annotation says that the issue of prejudgment interest and all the facts necessary for an award must appear in the petitions. The statute page includes that summary. The summary means interest cannot simply be saved for trial. The claim for it and the facts behind it have to be in the petition.

A demand letter that mentions interest is not the same as a petition that asks for it. Do not assume a judge will add interest at the end of the case if the petition never raised it.

When you or your lawyer drafts a petition, it should include:

• The principal amount you are owed.
• The date the amount became due, and the date of acceptance or completion if that is your starting point.
• The facts showing the amount is fixed, such as the contract, the order, and the acceptance record.
• A clear request for interest as part of the relief you seek.
• Any other facts that support the interest claim.

Ask a lawyer to review the draft before you file. A missing fact can cost you the interest claim even when the underlying debt is solid, and fixing that after filing is harder than getting it right the first time.

Limit 4: Jury instructions, verdicts and corrections matter

Two points from the annotations matter here. First, a 1966 summary says the court did not err in directing the jury to award interest if it found for the plaintiff, instead of only permitting it to do so. The statute page carries that summary. So the way a court frames the question to the jury can matter.

Second, the page notes that the verdict need not separately state the amount of interest allowed, and that an error in the amount allowed may be cured by remittitur.

For you, the takeaway is that the number you bring to court must be easy to check. If a verdict folds interest into a total, keep your own records of the calculation you submitted. Keep a calculation sheet that shows the principal, the start date, the end date, the method you used, and the result. If a judge or the other side questions a figure, you can show exactly how it was built.

Build the sheet early, not after the dispute gets hot. A calculation prepared at the time of the invoice is easy to defend. One reconstructed months later from scattered emails is not.

Mistakes bookkeepers make

Watch for these recordkeeping and claim problems:

• Failing to document the applicable basis for interest. Keep the records supporting your request.
• Treating every disputed invoice as unascertainable. An email questioning the amount records a dispute, but disagreement alone does not establish that interest is unavailable.
• Mixing lost profits in with the invoice balance, which blurs the basis for any interest claim.
• Leaving the interest request out of the petition when the case goes to court.
• Keeping the calculation only in a spreadsheet formula no one else can read, or only in your head.
• Sending a reminder that claims interest without checking its basis.

Better record keeping can address missing records and unclear calculations; ask a lawyer about the legal basis for interest or an omitted petition request. The cost of skipping them is usually a weaker claim in exactly the moment you need it to be strong.

Talking to the customer before you sue

Many disputes that look like interest problems are really communication problems. Before you escalate, call or write to the customer, confirm the amount, and ask whether anything is disputed. If the customer says the work was not accepted, or that the price was never agreed, you learn that early and can respond with records.

Put the conversation in writing. A short email that restates the principal, the due date, and the basis for any interest is more useful than a phone call nobody can prove. If the customer replies with a dispute, reply again with the records that answer it. That thread becomes part of your file, and it shows a court that you tried to resolve the matter before filing.

Be careful about tone and threats. A demand that states interest you cannot support can make you look less credible, even if the underlying debt is clear. Keep the request factual. If you are considering another way to resolve the dispute, read about small claims mediation for an unpaid invoice.

A practical sequence before you demand payment

Work through these steps in order:

1. Confirm the work was done, and note the completion or acceptance date.
2. Check the amount against the contract, the order, and your delivery or completion records. Write down anything the customer disputes.
3. Decide whether the claim is an ordinary invoice or a lost-profits claim. Keep the two separate.
4. If the amount is clear, state it in your demand with the principal, the due date, and how you calculated any interest.
5. If the customer disputes the amount, respond in writing, keep the thread, and assess whether records still establish the amount.
6. Before you sue, have the petition ask for interest and state the facts that support it.
7. Keep the calculation sheet and the full file together, so anyone reviewing the matter can follow it.

Following this sequence does not guarantee interest. It does mean that if you do claim interest, you have a record that supports it.

What this post does not cover

This post summarizes Missouri court decisions as they appear in the annotations to one statute section. It is not legal advice. Use it to organize your records and questions, rather than to calculate an interest rate. This post also does not address the rules in other states, which can differ from Missouri's.

Read the current statute and the cases it cites, since the annotations are summaries and may not reflect every later decision. Then have a Missouri lawyer look at the facts of your own dispute before you demand interest in writing or file suit.

Your next step today

Choose one unpaid invoice and put its contract, acceptance record, payment history and interest calculation together. Mark anything missing for follow-up before making a demand.

What should you check before claiming interest?

Does a disputed amount always prevent interest?

No: the subcontract case allowed interest despite an honest disagreement about the quantity of fill. Check whether records still establish the amount owed.

Can a verdict combine interest with the total?

Yes: a verdict need not state interest separately. Keep your calculation understandable so you can explain the amount you requested.

What should you do about an error in the interest award?

Ask a lawyer about correction by remittitur. Keep the calculation and supporting records available for that review.