My client—a contractor from Austin—was owed $8,200 for a kitchen remodel he completed in June 2025. The homeowner kept saying "check's in the mail" for 3 months.

We sent a demand letter citing Texas Property Code § 53.056 and calculated 18% annual interest on the unpaid balance. Within 48 hours, the homeowner paid in full. Why? Because we showed him exactly how much this was going to cost him.

Let me show you how to calculate late fees and interest for your unpaid invoices—and how to make sure it's actually legal.

The Two Types of Late Payment Penalties

📅 Late Fees (Flat Amount)

One-time charge for missing payment deadline.

Example:

$500 invoice + $50 late fee = $550 total

When to Use:

Fixed administrative cost. Simple. Easier to enforce.

💰 Interest (Percentage Over Time)

Percentage that accrues daily/monthly until paid.

Example:

$500 invoice at 1.5% per month for 3 months = $522.68

When to Use:

Long overdue balances. Compensates for time value of money.

⚠️ Critical Rule:

You can charge BOTH late fees AND interest—but only if your contract (or state law) allows it. If not in writing, some states won't let you collect.

How to Calculate Late Fees (Step-by-Step)

Formula for Flat Late Fees:

Late Fee = Invoice Amount × Late Fee %

Late Fee = Fixed Dollar Amount

Example 1: Percentage-Based

Invoice: $2,500

Late fee: 5% of invoice

Calculation: $2,500 × 0.05 = $125

Total owed: $2,625

Example 2: Fixed Amount

Invoice: $850

Late fee: $75 flat fee

Calculation: $850 + $75 = $925

Total owed: $925

How to Calculate Interest (Simple vs Compound)

Simple Interest (Most Common)

Interest calculated only on the original amount owed.

Interest = Principal × Rate × Time

Where:

Principal = Original invoice amount
Rate = Annual interest rate (as decimal)
Time = Days overdue ÷ 365

Example: 18% Annual Interest

Invoice: $5,000

Annual rate: 18% (0.18)

Days overdue: 90 days

Calculation: $5,000 × 0.18 × (90/365) = $221.92

Total owed: $5,221.92

Monthly Interest (Easier Math)

Interest calculated per month instead of annually.

Interest = Principal × Monthly Rate × Months

Example: 1.5% Per Month

Invoice: $3,200

Monthly rate: 1.5% (0.015)

Months overdue: 4 months

Calculation: $3,200 × 0.015 × 4 = $192

Total owed: $3,392

State-by-State Legal Interest Rates (2026)

If your contract doesn't specify an interest rate, most states have a "default judgment rate" you can use:

State	Default Interest Rate	Max Contractual Rate	Notes
California	10% per year	No limit	Judgment rate: 10%
Texas	5% per year	18% per year	Usury limit: 18%
Florida	12% per year	18% per year	Higher for businesses
New York	9% per year	16% per year	Criminal usury: 25%
Illinois	9% per year	9% per year	Strict usury laws
Pennsylvania	6% per year	6% per year	One of lowest rates
Ohio	5% per year	8% per year	Written contracts: 8%
Georgia	7% per year	16% per year	Higher for credit sales
Washington	12% per year	12% per year	Higher for businesses
Arizona	10% per year	10% per year	Consistent rates

⚠️ Usury Laws Warning:

Charging interest above your state's maximum rate is illegal and can void your entire claim. Always check your state's usury limit before charging interest.

Interactive Calculator Examples

Scenario 1: Freelance Designer

Invoice amount: $2,400

Payment due: October 1, 2025

Today's date: January 24, 2026

Days overdue: 115 days

Contract terms: 1.5% per month

Calculation:

Months overdue: 115 ÷ 30 = 3.83 months

Interest: $2,400 × 0.015 × 3.83 = $137.88

Total owed: $2,537.88

Scenario 2: Contractor (Texas)

Invoice amount: $8,200

Payment due: June 15, 2025

Today's date: January 24, 2026

Days overdue: 223 days

Texas law: 18% per year (max)

Calculation:

Time: 223 ÷ 365 = 0.611 years

Interest: $8,200 × 0.18 × 0.611 = $901.04

Total owed: $9,101.04

How to Add Late Fees to Your Contracts

✅ Enforceable Late Fee Clause (Template)

"Payment is due within [30] days of invoice date. Invoices not paid by the due date will incur a late fee of [5%] of the outstanding balance or [$50], whichever is greater. Additionally, interest will accrue on unpaid balances at a rate of [1.5%] per month ([18%] annually) until paid in full. Client agrees to pay all reasonable collection costs, including attorney fees, if legal action is required."

What makes this enforceable:

Specific payment deadline (30 days)
Clear late fee amount (5% or $50)
Defined interest rate (1.5% per month = 18% annual)
Collection costs clause (attorney fees, court costs)
Client acknowledgment ("Client agrees...")

Common Mistakes That Kill Late Fee Claims

❌ Mistake 1: No written agreement

Saying "I charge late fees" verbally doesn't count. It must be in writing before work begins.

❌ Mistake 2: Charging above state usury limits

24% interest might sound good, but if your state caps it at 18%, you just voided your claim.

❌ Mistake 3: Adding late fees retroactively

You can't decide to charge 10% late fee on an old invoice that didn't mention fees.

❌ Mistake 4: Unclear calculation method

"Interest may apply" is not enforceable. Must specify rate and how it's calculated.

❌ Mistake 5: Outrageous penalties

$500 late fee on a $200 invoice? Judge will call it a "penalty" and throw it out.

What's "Reasonable"? Court Standards

Courts generally accept late fees and interest if they're "reasonable compensation" for administrative costs and time value of money:

✅ Generally Reasonable:

✓ Late fees: 5-10% of invoice
✓ Interest: 12-18% annually (1-1.5% monthly)
✓ Minimum fee: $25-50 for small invoices
✓ Collection costs: Actual attorney fees

❌ Likely Unreasonable:

✗ Late fees: Over 15% of invoice
✗ Interest: Over 24% annually (2% monthly)
✗ Flat fee exceeding invoice amount
✗ Compounding interest daily

Sample Demand Letter With Late Fees

[Your Name]
[Your Address]
[City, State ZIP]

[Date]

[Client Name]
[Client Address]
[City, State ZIP]

RE: Outstanding Invoice #2025-0847 + Late Fees & Interest

Dear [Client Name],

This letter concerns Invoice #2025-0847 dated October 1, 2025, for $2,400.00, 
which remains unpaid as of January 24, 2026—115 days past due.

ORIGINAL INVOICE AMOUNT:              $2,400.00
LATE FEE (5% per contract):             $120.00
INTEREST (1.5%/month × 3.83 months):    $137.88
----------------------------------------
TOTAL AMOUNT DUE:                     $2,657.88

Per our signed contract dated September 15, 2025 (Section 4: Payment Terms), 
invoices not paid within 30 days incur a 5% late fee plus 1.5% monthly 
interest (18% annually). This clause was agreed to before work commenced.

Payment must be received within 10 days of this letter (by February 3, 2026) 
to avoid further interest accrual and potential legal action. Interest continues 
to accrue at $36/month ($1.20/day) until paid in full.

Payment Options:
• Check: [Your Business Name], mailed to address above
• Zelle: [phone/email]
• Wire transfer: [bank details]

If payment is not received by February 3, 2026, I will file a claim in small 
claims court for the full amount plus court costs and attorney fees as 
permitted under Texas law.

Sincerely,
[Your Signature]
[Your Printed Name]

Quick Reference Cheat Sheet

Late Fee Formula:

Invoice × Late Fee %

Example: $1,000 × 0.05 = $50

Interest Formula:

Principal × Rate × (Days ÷ 365)

Example: $1,000 × 0.18 × (60÷365) = $29.59

Safe Ranges:

✓ Late fee: 5-10% of invoice
✓ Annual interest: 12-18%
✓ Monthly interest: 1-1.5%
✓ Check state usury limits first
✓ Must be in contract before work starts
✓ Specify exact rates, not "may apply"

Auto-Calculate Late Fees in Your Demand Letter

Our tool automatically calculates late fees and interest based on your state's laws and contract terms. Just enter the invoice amount, due date, and your state—we handle the math and legal citations.

Create Your Free Demand Letter →

How to Calculate Late Fees and Interest on Unpaid Invoices (State-by-State)