Texas mode uses Finance Code §302.002 (6% a year from the 30th day), and cites §302.001(b) and §§303.002, 303.003 and 303.009. It does not show this week's ceiling. Informational, not legal advice.
Simple interest, no compounding
Days late = payment date − due date. Accruing days = max(0, days late − grace days). If you pick monthly, the annual rate is that percent × 12. Interest = invoice × annual rate × accruing days ÷ 365. Total = invoice + flat fee + interest. The flat late-fee share, with no dates, stays on the flat late-fee card.
Texas, when no rate was agreed
The same $5,000, due 2026-09-01 and paid 2026-10-16, accrues from 2026-10-01 (the 30th day). That is 15 days at 6% a year: $5,000 × 0.06 × 15 ÷ 365 = $12.33. The day count is this page's model. Other states are not listed.
Questions
How is late-payment interest calculated?
Days late is the payment date minus the due date. Accruing days are days late minus grace days, and never below zero. Interest is invoice × annual rate × accruing days ÷ 365. Total due is invoice + flat fee + interest. A cap, if you type one, limits the interest. This page does not compound.
Can I add a late fee if it was not in the contract?
Put the rate in the contract or the invoice first. Texas Finance Code §302.002 allows 6% a year beginning on the 30th day after the amount is due when no interest was agreed. Whether a freelance late charge counts as interest is a legal question. Informational, not legal advice.
What is the difference between a monthly rate and an annual rate?
A monthly rate on this page is multiplied by 12 to make a simple annual rate. There is no compounding.
What is the most I can charge in Texas?
Texas Finance Code §302.002 is 6% a year from the 30th day when no rate was agreed. §302.001(b) states a 10% a year maximum except as otherwise provided by law. §§303.002, 303.003 and 303.009 let a written agreement use the weekly ceiling, floored at 18% and capped at 24% (28% for business or commercial credit). This page does not show this week's ceiling. Informational, not legal advice.
Should I charge a flat fee or interest?
Both are inputs. This page does not recommend a number.