{
  "slug": "qtc-interval",
  "title": "QTc Calculator (Bazett & Fridericia)",
  "heading": "QTc Interval Calculator",
  "category": "fitness-health",
  "url": "https://www.revenuelab.fyi/toolbox/qtc-interval",
  "summary": "Correct the QT interval for heart rate using two standard formulas.",
  "description": "The QT interval on an EKG shortens as heart rate increases, so raw QT cannot be compared across different heart rates without correction. Bazett's formula (QT ÷ √RR) is the most widely taught and used in practice, but it overcorrects at high heart rates and undercorrects at low heart rates, which is why Fridericia's formula (QT ÷ RR^(1/3)) is often preferred in research and by cardiology for tachycardic or bradycardic patients. This calculator reports both so you can see where they diverge. QTc matters because prolongation increases risk of torsades de pointes, particularly from QT-prolonging medications (many antiarrhythmics, antipsychotics, antiemetics, and antibiotics), electrolyte derangements (low potassium, magnesium, or calcium), and congenital long QT syndromes. A QTc over 450 ms in men or 460 ms in women is generally considered prolonged, and values over 500 ms carry meaningfully elevated arrhythmia risk. This tool is for education and workflow support only — not medical advice. Always verify results with a clinician and use clinical judgment alongside your institution's protocols.",
  "formula": "Bazett: QTc = QT ÷ √(RR); Fridericia: QTc = QT ÷ RR^(1/3), where RR = 60 ÷ heart rate (seconds).",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=qtc-interval",
  "inputs": [
    {
      "id": "qt",
      "label": "Measured QT interval",
      "kind": "number",
      "hint": null,
      "default": 400,
      "unit": "ms",
      "min": 200,
      "max": 700
    },
    {
      "id": "hr",
      "label": "Heart rate",
      "kind": "number",
      "hint": null,
      "default": 75,
      "unit": "bpm",
      "min": 20,
      "max": 220
    },
    {
      "id": "sex",
      "label": "Sex",
      "kind": "select",
      "hint": null,
      "default": "male",
      "options": [
        {
          "value": "male",
          "label": "Male"
        },
        {
          "value": "female",
          "label": "Female"
        }
      ]
    }
  ],
  "outputs": [
    {
      "id": "bazett",
      "label": "QTc (Bazett)",
      "format": "decimal",
      "hint": null,
      "primary": true
    },
    {
      "id": "fridericia",
      "label": "QTc (Fridericia)",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "threshold",
      "label": "Prolongation threshold (ms)",
      "format": "number",
      "hint": null,
      "primary": false
    },
    {
      "id": "prolonged",
      "label": "Bazett interpretation",
      "format": "raw",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Measured QT interval: 400 ms",
      "Heart rate: 75 bpm",
      "Sex: Male"
    ],
    "outputs": [
      "QTc (Bazett): 447",
      "QTc (Fridericia): 431",
      "Prolongation threshold (ms): 450",
      "Bazett interpretation: Normal"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter measured qt interval (ms).",
      "Enter heart rate (bpm).",
      "Enter sex.",
      "Read your qtc (bazett) 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": {
        "qt": 240,
        "hr": 45,
        "sex": "male"
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "qt": 400,
        "hr": 75,
        "sex": "male"
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "qt": 640,
        "hr": 120,
        "sex": "male"
      }
    }
  ],
  "limitations": [
    "Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.",
    "Results do not account for medical conditions, medication, or pregnancy.",
    "Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it."
  ],
  "faq": [
    {
      "q": "Which formula should I trust more?",
      "a": "Fridericia performs better at heart rates outside the 60-100 range because it corrects less aggressively than Bazett. If the two formulas disagree significantly, particularly at a fast or slow heart rate, lean toward Fridericia or consult a cardiologist rather than picking whichever number looks more reassuring."
    },
    {
      "q": "What heart rate range does Bazett distort most?",
      "a": "Bazett overcorrects (reports falsely high QTc) at heart rates above about 100 bpm and undercorrects (reports falsely low QTc) at heart rates below about 60 bpm, since it assumes a fixed square-root relationship that does not hold at the extremes."
    },
    {
      "q": "What's a dangerous QTc value?",
      "a": "Above 500 ms is generally considered high risk for torsades de pointes regardless of formula, and an increase of more than 60 ms from a patient's own baseline is also concerning even if the absolute number looks moderate. Any QTc-prolonging medication should be reviewed against a fresh EKG in that range."
    },
    {
      "q": "Do electrolytes matter as much as medications?",
      "a": "Yes. Hypokalemia, hypomagnesemia, and hypocalcemia all prolong QT independently and also amplify the effect of QT-prolonging drugs. Checking and correcting electrolytes is often the fastest, safest intervention before deciding whether a medication needs to be held."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/shock-index",
    "https://www.revenuelab.fyi/toolbox/abg-interpretation",
    "https://www.revenuelab.fyi/toolbox/winters-formula"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — QTc Calculator (Bazett & Fridericia) (https://www.revenuelab.fyi/toolbox/qtc-interval)"
}