{
  "slug": "late-payment-interest",
  "title": "Late Payment Interest Calculator",
  "heading": "Late Payment Interest Calculator",
  "category": "financial",
  "url": "https://www.revenuelab.fyi/toolbox/late-payment-interest",
  "summary": "Interest and fees owed on an overdue invoice.",
  "description": "Most contracts allow interest on late invoices. Enter the invoice, days overdue and your contractual rate to see exactly what to add.",
  "formula": "Interest = amount × annual rate × days ÷ 365.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=late-payment-interest",
  "inputs": [
    {
      "id": "amount",
      "label": "Invoice amount",
      "kind": "number",
      "hint": null,
      "default": 4500,
      "unit": "$",
      "min": null,
      "max": null
    },
    {
      "id": "days",
      "label": "Days overdue",
      "kind": "number",
      "hint": null,
      "default": 45,
      "unit": null,
      "min": 0,
      "max": null
    },
    {
      "id": "rate",
      "label": "Annual interest rate",
      "kind": "number",
      "hint": null,
      "default": 12,
      "unit": "%",
      "min": 0,
      "max": null
    },
    {
      "id": "fee",
      "label": "Flat late fee",
      "kind": "number",
      "hint": null,
      "default": 40,
      "unit": "$",
      "min": null,
      "max": null
    }
  ],
  "outputs": [
    {
      "id": "total",
      "label": "Total now due",
      "format": "currency",
      "hint": null,
      "primary": true
    },
    {
      "id": "interest",
      "label": "Interest accrued",
      "format": "currency",
      "hint": null,
      "primary": false
    },
    {
      "id": "daily",
      "label": "Interest per day",
      "format": "currency",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Invoice amount: 4500 $",
      "Days overdue: 45",
      "Annual interest rate: 12 %",
      "Flat late fee: 40 $"
    ],
    "outputs": [
      "Total now due: $4,606.58",
      "Interest accrued: $66.58",
      "Interest per day: $1.48"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter invoice amount ($).",
      "Enter days overdue.",
      "Enter annual interest rate (%).",
      "Enter flat late fee ($).",
      "Read your total now due on the right — it updates as you type.",
      "Hit Share to keep the scenario or send it to someone."
    ]
  },
  "scenarios": [
    {
      "name": "Conservative",
      "description": "Lower-end numbers — what if things land soft?",
      "values": {
        "amount": 2700,
        "days": 27,
        "rate": 12,
        "fee": 25
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "amount": 4500,
        "days": 45,
        "rate": 12,
        "fee": 40
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "amount": 7200,
        "days": 72,
        "rate": 12,
        "fee": 65
      }
    }
  ],
  "limitations": [
    "Results are estimates before tax, fees, and inflation unless an input explicitly covers them.",
    "Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.",
    "This is educational maths, not financial advice. Check anything contractual with the lender or your accountant."
  ],
  "faq": [
    {
      "q": "How does the late payment interest calculator work?",
      "a": "Most contracts allow interest on late invoices. Enter the invoice, days overdue and your contractual rate to see exactly what to add. The underlying maths is: Interest = amount × annual rate × days ÷ 365."
    },
    {
      "q": "What do I need to enter?",
      "a": "4 values: invoice amount, days overdue, annual interest rate, and flat late fee. Each field starts with a sensible default, so you can change one number at a time and watch the result move."
    },
    {
      "q": "What does the total now due result mean?",
      "a": "It is returned as a money amount in US dollars and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds."
    },
    {
      "q": "Is this calculator free, and do I need an account?",
      "a": "Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser."
    },
    {
      "q": "How accurate is the result?",
      "a": "It applies the standard formula exactly, so the arithmetic is precise. Results are estimates before tax, fees, and inflation unless an input explicitly covers them."
    },
    {
      "q": "Who is this tool for?",
      "a": "It's built for anyone comparing money scenarios before committing — budgeting a payment, sanity-checking a quote, or seeing what a change in rate or term actually costs."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/invoice-factoring-cost",
    "https://www.revenuelab.fyi/toolbox/cash-conversion-days"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Late Payment Interest Calculator (https://www.revenuelab.fyi/toolbox/late-payment-interest)"
}