What inputs does the Invoice Late Fee & Interest Calculator need?

It takes 6 inputs: invoiceAmount, dueDate, paymentDate, gracePeriodDays (default 0), flatLateFee (default 0), annualInterestRate (default 0). Outputs returned: daysLate, interestAmount, totalOwed.

Can I verify the Invoice Late Fee & Interest Calculator with a worked example?

Yes. With invoice = $5,000, daysLate = 30, flatFee = $25, annualRate = 12%. the tool returns interest ≈ $49.32, total_owed ≈ $5,074.32.

Where does the Invoice Late Fee & Interest Calculator get its benchmark data?

Reference data is sourced from: US Uniform Commercial Code — Article 2 (Sales), via Cornell Legal Information Institute (as of 2024).

What can the Invoice Late Fee & Interest Calculator not tell me?

Known limitations: State usury caps on permissible interest rates vary (5–24%+). Excess is unenforceable. EU customers: EU Directive 2011/7 on combating late payment governs statutory interest (ECB reference rate + 8 pp). Use that as a benchmark for B2B EU invoices.

Methodology: Invoice Late Fee & Interest Calculator

1. Scope

Calculates late-payment penalties from invoice amount, grace period, a fixed fee, and an annual interest rate. US-centric defaults. Not legal advice — enforceable penalty rates are governed by state usury law and contract terms.

2. Inputs and outputs

Inputs

invoiceAmount number (currency)
dueDate date
paymentDate date
gracePeriodDays number default: 0
flatLateFee number (currency) default: 0
annualInterestRate percent default: 0

Outputs

daysLate

max(0, paymentDate − dueDate − gracePeriod).
interestAmount

invoice × (annualRate / 365) × daysLate.
totalOwed

invoice + flatFee + interest.

Engine source: src/lib/invoice-late-fee-interest-calculator/engine.ts

3. Formula / scoring logic

days_late   = max(0, date(paid) - date(due) - grace_days)
interest    = invoice * (annual_rate / 365) * days_late
total_owed  = invoice + flat_fee + interest

4. Assumptions

Simple-interest accrual daily. Compounding and payment-allocation rules (fee first or interest first) are contract-specific.
Banker's 365-day year, not 360 or 30/360.
No withholding or VAT logic — entered at gross.

5. Data sources

US Uniform Commercial Code — Article 2 (Sales), via Cornell Legal Information Institute as of 2024

6. Known limitations

State usury caps on permissible interest rates vary (5–24%+). Excess is unenforceable.
EU customers: EU Directive 2011/7 on combating late payment governs statutory interest (ECB reference rate + 8 pp). Use that as a benchmark for B2B EU invoices.

7. Reproducibility

Input
invoice = $5,000, daysLate = 30, flatFee = $25, annualRate = 12%.

Expected output
interest ≈ $49.32, total_owed ≈ $5,074.32.

8. Change log

2026-04-24 methodology page first published.

Worked example

Run live against the same engine this site ships (/engines/invoice-late-fee-interest-calculator.js). The inputs and outputs below are recomputed on every build and independently re-verified in CI — they are never hand-authored.

Input

tool: invoice_late_fee_interest
invoice_amount: 5400
days_late: 24
annual_interest_percent: 12
fixed_late_fee: 40
grace_days: 7

Output

primaryLabel: Total amount due
primaryValue: 5470.18
primaryFormat: currency
summary: Includes principal invoice amount, fixed late fee, and prorated daily interest.
metrics[0].label: Chargeable days
metrics[0].value: 17
metrics[0].format: days
metrics[1].label: Late fee
metrics[1].value: 40
metrics[1].format: currency
metrics[2].label: Interest
metrics[2].value: 30.18
metrics[2].format: currency
metrics[3].label: Effective penalty
metrics[3].value: 1.3
metrics[3].format: percent
assumptionsEcho.invoice_amount: 5400
assumptionsEcho.days_late: 24
assumptionsEcho.annual_interest_percent: 12
assumptionsEcho.fixed_late_fee: 40
assumptionsEcho.grace_days: 7

Frequently asked questions

What does the Invoice Late Fee & Interest Calculator calculate?: Calculates late-payment penalties from invoice amount, grace period, a fixed fee, and an annual interest rate. US-centric defaults. Not legal advice — enforceable penalty rates are governed by state usury law and contract terms.
What inputs does the Invoice Late Fee & Interest Calculator need?: It takes 6 inputs: invoiceAmount, dueDate, paymentDate, gracePeriodDays (default 0), flatLateFee (default 0), annualInterestRate (default 0). Outputs returned: daysLate, interestAmount, totalOwed.
What formula does the Invoice Late Fee & Interest Calculator use?: The exact computation is: days_late = max(0, date(paid) - date(due) - grace_days); interest = invoice * (annual_rate / 365) * days_late; total_owed = invoice + flat_fee + interest
Can I verify the Invoice Late Fee & Interest Calculator with a worked example?: Yes. With invoice = $5,000, daysLate = 30, flatFee = $25, annualRate = 12%. the tool returns interest ≈ $49.32, total_owed ≈ $5,074.32.
Where does the Invoice Late Fee & Interest Calculator get its benchmark data?: Reference data is sourced from: US Uniform Commercial Code — Article 2 (Sales), via Cornell Legal Information Institute (as of 2024).
What can the Invoice Late Fee & Interest Calculator not tell me?: Known limitations: State usury caps on permissible interest rates vary (5–24%+). Excess is unenforceable. EU customers: EU Directive 2011/7 on combating late payment governs statutory interest (ECB reference rate + 8 pp). Use that as a benchmark for B2B EU invoices.