{
    "name": "Bolt Torque Calculator",
    "url": "https://calculators.geekmidst.com/calculators/bolted-joint-torque-calculator",
    "markdown": "https://calculators.geekmidst.com/calculators/bolted-joint-torque-calculator.md",
    "category": "Science & engineering",
    "description": "Convert between bolt tightening torque and clamp preload with T = K × F × d, and check the bolt stress as a percentage of its proof load.",
    "summary": "The short-form torque equation T = K × F × d links tightening torque to bolt preload. The nut factor K lumps together thread and under-head friction, so it dominates the accuracy.",
    "also_called": [
        "Bolt preload calculator",
        "Torque–tension calculator"
    ],
    "applies_to": "Worldwide",
    "last_updated": "2026-10-29",
    "computation": "Results are computed in the visitor’s browser on the HTML page. This API describes the calculator; it does not compute results.",
    "inputs": [
        {
            "name": "solve_for",
            "label": "Find",
            "type": "segmented",
            "default": "torque",
            "options": {
                "torque": "Torque",
                "preload": "Preload"
            }
        },
        {
            "name": "d",
            "label": "Nominal bolt diameter d",
            "type": "number",
            "default": "10",
            "unit": "mm"
        },
        {
            "name": "k",
            "label": "Nut factor K",
            "type": "number",
            "default": "0.2"
        },
        {
            "name": "preload",
            "label": "Target preload F",
            "type": "number",
            "default": "20",
            "unit": "kN",
            "shown_when": {
                "solve_for": "torque"
            }
        },
        {
            "name": "torque",
            "label": "Tightening torque T",
            "type": "number",
            "default": "40",
            "unit": "N·m",
            "shown_when": {
                "solve_for": "preload"
            }
        },
        {
            "name": "as",
            "label": "Tensile stress area A_s",
            "type": "number",
            "default": "58",
            "unit": "mm²"
        },
        {
            "name": "proof",
            "label": "Proof strength",
            "type": "number",
            "default": "580",
            "unit": "MPa"
        }
    ],
    "outputs": [
        {
            "key": "torque",
            "label": "Tightening torque",
            "format": "number"
        },
        {
            "key": "preload",
            "label": "Preload (clamp force)",
            "format": "number"
        },
        {
            "key": "stress",
            "label": "Bolt tensile stress",
            "format": "number"
        },
        {
            "key": "pct_proof",
            "label": "Share of proof load",
            "format": "percent"
        },
        {
            "key": "proof_load",
            "label": "Proof load A_s × proof strength",
            "format": "number"
        }
    ],
    "formula": {
        "text": "T = K × F × d;  F = T ÷ (K × d);  σ = F ÷ A_s;  proof load = A_s × proof strength",
        "variables": {
            "T": "tightening torque",
            "K": "nut factor (dimensionless)",
            "F": "bolt preload",
            "d": "nominal diameter",
            "A_s": "tensile stress area"
        }
    },
    "worked_example": {
        "inputs": {
            "solve_for": "torque",
            "d": "10",
            "k": "0.2",
            "preload": "20",
            "torque": "40",
            "as": "58",
            "proof": "580"
        },
        "steps": [
            "T = 0.2 × 20,000 N × 0.010 m = 40 N·m.",
            "σ = 20,000 N ÷ 58 mm² = 344.8 MPa.",
            "Proof load = 58 × 580 = 33,640 N; 20 kN is 59.5% of it."
        ],
        "result": "Clamping an M10 bolt to 20 kN with K = 0.2 needs about 40 N·m, loading it to about 60% of a 580 MPa proof strength."
    },
    "interpretation": "Preloads of roughly 70–90% of proof load are common for reusable and permanent joints respectively, but follow your standard or the fastener maker. Because K varies widely with lubrication and finish, torque control alone typically gives preload scatter of ±25% or more.",
    "assumptions": [
        "Short-form torque–preload relation with a single nut factor; standard threads; preload carried by the tensile stress area."
    ],
    "limitations": [
        "Does not model joint stiffness, external load sharing, embedding, relaxation or prevailing-torque nuts.",
        "For critical joints, use the fastener manufacturer’s data or measure preload directly."
    ],
    "rules": null,
    "faqs": [
        {
            "question": "Why does lubrication matter so much?",
            "answer": "Most of the torque (often around 90%) is used overcoming friction. Lowering friction lowers K, so the same torque produces much more preload and can overload the bolt."
        },
        {
            "question": "Which diameter is d?",
            "answer": "The nominal (major) thread diameter, in metres in the formula."
        }
    ],
    "sources": [],
    "related": [
        {
            "name": "Torsion Stress Calculator",
            "url": "https://calculators.geekmidst.com/calculators/torsion-stress-calculator"
        },
        {
            "name": "Factor of Safety Calculator",
            "url": "https://calculators.geekmidst.com/calculators/safety-factor-calculator"
        }
    ],
    "disclaimer": "Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials.",
    "publisher": "GM¹X CalcVerse"
}