Yes, you can charge interest on a late invoice as a sole proprietor, but only under specific conditions. The rules vary by state, and there’s one timing rule that makes most late fees unenforceable: the fee must be spelled out in your original invoice or terms of service. Adding a fee after the fact won’t hold up.
The core rule
A late fee or interest charge is enforceable only if your customer agreed to it before they incurred the debt. That means:
- It must appear in the original invoice, contract, or clearly stated terms of service.
- The customer must have seen it (or had reasonable access to it) before engaging your services.
- It must be a reasonable amount (typically 1.5% per month, or 18% annually) or lower, depending on your state.
If your invoice just said “net 30” with no mention of late fees, you cannot retroactively charge interest on a past-due balance. Adding it later is not enforceable in most US states and actually gives the debtor a basis to dispute the whole invoice.
How much can you charge?
The legal limit varies by state, but most fall into these ranges:
Contractual late fees (charged only if agreed to in advance):
- Typically capped at 1.5% per month (18% annually)
- Some states allow up to 2% per month
- A few states cap late fees at a flat percentage of the invoice amount (e.g., $25 or 5%, whichever is less)
Interest under state law (when no contract specifies a rate):
- Ranges from 4% to 12% annually depending on state
- This is the default fallback if you have no written agreement
- Varies for different debt types (judgment debt, prejudgment interest, etc.)
Check your state’s rules before committing to a late-fee structure.
The most common mistake is adding late fees retroactively. Don’t. It won’t stick, and it gives the debtor a dispute argument.
What counts as late?
Most late-fee clauses trigger on invoices past the agreed due date. If you invoice on the first with “net 30,” the fee kicks in on day 31. Some businesses charge a grace period (net 30 + 5 days, then fee applies), which is reasonable and harder to dispute.
Document your terms clearly on every invoice. A sample structure:
Due date: [date] Late fee: 1.5% per month (18% annually) applies to amounts unpaid 15 days past due date.
That clarity matters if the account ever needs escalation.
Interest vs. late fees: the difference
In most states, these are the same thing. A late fee is a charge for using someone else’s money past the due date. Some states distinguish:
- Late fee: A flat amount or percentage that applies once, at a specific point (e.g., “$50 if unpaid after day 30”)
- Interest: A monthly or daily rate that accrues continuously (e.g., “1.5% per month”)
For sole proprietors, the practical difference is minor. Pick whichever is clearer to your customer and easier for you to track. State law may have opinions on which one is more enforceable in disputes.
Special case: retainers and subscriptions
If you bill on a retainer or subscription (monthly recurring charges), some states apply different rules. A subscription customer who stops paying is sometimes treated differently from a one-time invoice dispute. Check your state’s consumer protection or commercial code for specifics if you have recurring customers.
Can you charge interest if you file a lawsuit?
Yes. If you sue for an unpaid invoice and win, most states allow you to claim prejudgment interest from the original due date, even if your contract didn’t specify a rate. This is in addition to any contractual late fee. Prejudgment interest rates vary by state (typically 5% to 9% annually).
Rule: write it into your terms first, add it later at your own cost
Late fees added after the fact are not enforceable in most states and create a dispute surface. Include late-fee language on every invoice, starting now. It's the only enforceable way.
When interest won’t help (and what to do instead)
Here’s the truth: chasing a past-due invoice using interest is slow math. By the time interest accrues enough to matter (weeks or months), the invoice is already old, the debtor is further from paying voluntarily, and you’re still in the inbox.
If you’re looking at a late invoice right now:
Days 1 to 30: Interest isn’t the lever. A clear, specific reminder is. “Invoice #2401 ($3,500) is now 15 days past due. Can you confirm payment is scheduled?”
Days 30 to 60: A discount is far more effective than interest. Offering 20% off to settle today nets you money now instead of 3-6 months later. The interest math usually loses to the settlement math.
Days 60+: The inbox phase is over. The interest clause isn’t going to move them. You either escalate to formal recovery (demand letter, third-party recovery service, or legal action) or you write it off.
Source: Captira analysis of AR recovery patterns across 20+ years of customer software work.
What you should do instead
Include late-fee language in your standard invoice template now and on every invoice going forward. Use 1.5% per month or a flat late fee (e.g., “$25 after 30 days late”), whichever works for your business. This protects you for future invoices.
For invoices already past due without a late-fee clause, don’t try to add one. Instead:
- Send a clear reminder with a specific date to pay (day 1 to day 30).
- Offer a settlement if they’re stuck (day 30 to day 60). A 20% discount nets you more than waiting six months for full payment.
- Send a final notice explaining that if unpaid by a specific date, you’ll escalate to formal recovery (day 60).
- Escalate or write off (day 60+). The interest accrual won’t move them.
If you have a customer currently more than 60 days late, the time for interest arguments is past. Get a free analysis of your aging report and we’ll walk you through which accounts are worth pursuing, which ones to settle, and which to write off.
Frequently asked
Can I charge interest if there’s no written agreement?
Some states allow you to claim the state’s default judgment interest rate (usually 5-9% annually) even without a written agreement, but only if you sue and win. Without a judgment, the agreement controls. Always write it down.
What if my invoice says “late fees apply per state law” without specifying a rate?
Risky. Courts read this as you claiming the state’s legal interest rate (the fallback). You’re better off specifying a clear rate (1.5% per month) on the invoice itself. Be explicit.
Can I charge interest on a retainer balance if the client doesn’t spend it all?
No. Retainer balances are held in trust and have different rules. Interest on retainers is not standard and may violate your state’s ethics rules if you’re a professional (lawyer, accountant, etc.). Ask your accountant or state bar if this applies to you.
If I charge late fees and they dispute the invoice, can I still collect?
Yes, but only the undisputed portion. If a customer disputes the invoice amount while owing the late fee, most courts require you to resolve the underlying dispute first. The late fee follows, not the other way around. Get the facts straight before escalating.
What if I’m in a state with no specified interest rate?
Unlikely. Every state has some default interest or late-fee law. Look up your state’s commercial code or contact your accountant. If you’re in an unusual situation, consult a business lawyer before billing late fees.