California Invoice Late Fees: Review the Basis Before Adding a Charge

Begin with the accepted agreement

Collect the signed proposal or contract, accepted payment terms, amendments, invoice and evidence of delivery. Identify the due date and any grace period. A term printed for the first time on a reminder does not by itself show that the client accepted it.

Make a short record of the legal issue you need resolved: Is there an agreed late charge? Is it interest? Is it a liquidated-damages provision? Are you considering interest as damages after breach? These are different routes, and the same percentage cannot be assumed valid under each.

Understand the two commonly cited statutes

Civil Code section 3289 preserves a legal contract interest rate after breach and provides 10% annual interest after breach for qualifying post-January 1, 1986 contracts without a stated legal interest rate. It excludes a note secured by a real-property deed of trust from that latter definition. It does not set a general 10% consumer late-fee cap. Civil Code §3289

Section 1671 addresses liquidated damages and treats specified personal, family or household transactions differently from the general rule. It supplies no automatic 1.5% monthly approval. Civil Code §1671

Eligibility for interest and when it begins may also involve the certainty and timing of damages under section 3287. Do not assume every disputed invoice produces the same interest start date. Civil Code §3287

For an actual demand, have a California lawyer assess the transaction and contract, including any other applicable interest restrictions. A clause saying “maximum allowed by law” does not replace that analysis or identify the number to enter.

Draw the timeline before doing the math

In a worked calendar example, the parties have expressly set a September 15 due date and a five-calendar-day grace period covering September 16 through 20. Charges start September 21. Through September 30, that gives 10 chargeable days, counting September 21 through 30 inclusively.

Those dates are illustrative terms, not California's default rule. The point is to distinguish the invoice date, due date, grace-period end and accrual start. A bill due 30 days after issuance is not 30 days overdue on its due date.

Worked examples with explicit assumptions

Assume an eligible $5,000 principal and a separately verified 6% annual simple-interest provision, calculated on actual/365 for the 10 chargeable days above:

$5,000 × 0.06 × 10 ÷ 365 = $8.22, rounded to cents.

The statement would show $5,000 principal and $8.22 interest, totaling $5,008.22, before other credits. The assumed 6% is not a recommended rate or legal conclusion.

In a separate statutory-interest illustration, assume a reviewer has established that section 3289's 10% rule applies to an $8,000 obligation for 60 days after breach. On an actual/365 basis:

$8,000 × 0.10 × 60 ÷ 365 = $131.51.

That number is conditional on entitlement, principal, dates and convention. It is not authority to add $131.51 merely because an invoice has remained unpaid for 60 days.

Make the balance understandable

Show the principal, contract or other established basis, accrual dates, rate unit, formula and payments credited. Keep interest separate from the principal under a simple-interest calculation. If principal changes, split the period and explain how the payment was allocated.

When a client disputes a line, record the dispute and supporting documents. Do not assume either that every dispute stops interest automatically or that every asserted fee continues unaffected. Resolve the applicable treatment before escalating a contested charge.

Use the late fee calculator to check a supported calculation. For date and partial-payment mechanics, see the invoice late-fee calculation guide. The tool cannot decide California legal entitlement, contractual acceptance or a court's treatment of the charge.