Late Payment Interest Calculator

How to Calculate Late Payment Interest

Late payment interest compensates you for the time value of money when a client pays after the agreed due date. The three most common methods are:

Daily interest — Divide the annual rate by 365, then multiply by the outstanding amount and the number of days late. This is the most precise and common method for B2B invoices.
Percentage penalty — Charge a one-time percentage of the invoice total (e.g., 2% or 5%). Simple and easy for clients to understand.
Flat fee — Add a fixed dollar amount (e.g., $25 or $50). Common in consumer billing and subscription services.

The formula for daily interest:

Late Fee = Invoice Amount × (Annual Rate ÷ 365) × Days Overdue

For example, a $5,000 invoice that is 30 days overdue at 18% annual interest: $5,000 × 0.0493% × 30 = $73.97.

Common Late Fee Structures

Type	Typical Range	Best For
Flat fee	$25 – $50	Small invoices, subscriptions
Percentage	1% – 5% of total	Simple B2B billing
Monthly interest	1% – 1.5% per month	Ongoing contracts
Daily interest	8% – 18% annual	Large invoices, long overdue

Choose the method that matches your industry norms and contract terms. Whatever you choose, make sure it is clearly stated on every invoicebefore the work begins.

Late Fee Wording for Your Invoices

Clear, professional language protects you legally and sets expectations. Here are copy-paste examples for your invoice terms:

Daily interest

“A late fee of 1.5% per month (18% per annum) will be applied to all invoices not paid within the stated payment terms. Interest accrues daily from the due date until payment is received in full.”

Percentage penalty

“Invoices not paid within 30 days of the due date will incur a late payment penalty of 5% of the outstanding balance.”

Flat fee

“A late payment fee of $50.00 will be added to any invoice not paid by the due date. Additional fees may apply for each subsequent 30-day period the invoice remains unpaid.”

Graduated penalty

“Invoices overdue by 1–30 days: 2% penalty. 31–60 days: 5% penalty. Over 60 days: 5% penalty plus 1.5% monthly interest on the remaining balance.”

State-by-State Late Fee Limits

In the United States, late fee limits vary by state and transaction type. While B2B transactions generally have more flexibility than consumer contracts, here are some key guidelines:

California — Maximum 10% annual interest on B2B debts unless a different rate is specified in writing. Consumer late fees must be “reasonable.”
New York — Usury limit of 16% annual interest. Criminal usury above 25%.
Texas — Maximum 18% annual for written contracts. 6% if no rate is agreed upon in writing.
Florida — 18% annual for amounts under $500,000. 25% for amounts above.
Illinois — 5% annual default rate. Contracted rates can be higher for B2B.

Important disclaimer

This is general information, not legal advice. Late fee regulations change frequently and vary by jurisdiction, industry, and contract type. Always consult a licensed attorney in your state before setting late fee policies.

How Billbot Handles Late Fees Automatically

Manually calculating and chasing late fees is tedious. Billbot automates the entire process:

Set your terms once — Configure your default late fee structure (flat, percentage, or daily interest) in your account settings.
Automatic tracking — Billbot monitors every invoice and knows exactly when a payment becomes overdue.
Smart reminders — Polite, professional reminder emails go out automatically, so you never have to send an awkward “just checking in” message.
API-first — Your AI agents and automations can query overdue invoices and late fees via the REST API.

Stop chasing payments manually

Let Billbot handle late fee calculations, reminders, and payment tracking. Start free — upgrade when you’re ready.