Bolt Torque Calculator

Calculate recommended tightening torque for bolts based on size, grade, and lubrication condition.

Calculator

How to Use

  1. 1
    Select Bolt Size, Grade, and Thread Type

    Choose the bolt diameter, thread pitch (metric M designation or imperial UNC/UNF), and bolt grade (ISO 8.8, 10.9, 12.9 or SAE Grade 5, 8) to retrieve proof and yield strength.

  2. 2
    Specify the Lubrication Condition

    Select the friction coefficient corresponding to the surface condition: dry, lightly oiled, zinc-plated, or specified lubricant. Friction has the dominant effect on clamp load for a given torque.

  3. 3
    Read the Tightening Torque and Clamp Load

    The calculator outputs the tightening torque (in N·m and lbf²in) to achieve the target preload (typically 70% of proof strength), the resulting bolt tension (clamp load), and the torque for the full proof load.

About

Proper bolt tightening is one of the most common and most frequently underestimated tasks in mechanical assembly. Insufficient preload allows joint members to separate or fret under dynamic loading, leading to fatigue failure of the bolt or loss of seal integrity. Excessive torque yields the bolt, permanently reducing clamp load and potentially fracturing the fastener in service.

The AlloyFYI Bolt Torque Calculator implements the standard torque-tension equation using friction coefficients consistent with ISO 16047 fastener testing and the VDI 2230 bolted joint design guideline — the most comprehensive international standard for high-duty bolted connections. The tool covers ISO metric (4.6 through 12.9), SAE (Grade 2 through 8), ASTM A325 and A490 structural bolts, and stainless steel bolt grades (A2-70, A4-80), with density-aware weight calculations when multiple fasteners must be considered in assembly weight budgets.

FAQ

Why does lubrication affect the torque-clamp load relationship so strongly?
Approximately 90% of the applied tightening torque is consumed overcoming friction — roughly 50% under the bolt head or nut bearing face and 40% in the thread engagement. Only the remaining 10% converts to bolt elongation and clamp load. The K-factor (nut factor or friction coefficient) directly relates torque to clamp load: T = K×D×F, where T is torque, D is nominal bolt diameter, and F is clamp load. K ranges from approximately 0.10 for well-lubricated galvanized bolts to 0.20 for dry cadmium-plated, to 0.35 or more for heavily galvanized or dry rusty bolts. A 2× variation in K produces a 2× variation in clamp load for the same torque.
What is proof load and how does it differ from yield strength?
Proof load is the maximum tensile load that a bolt or fastener must sustain without acquiring any measurable permanent set. It is typically 85–92% of yield load and is specified in ASTM F606 and ISO 898-1 for metric bolts. Proof strength (proof load/stress area) is the design reference for tightening calculations because it ensures the bolt remains elastic after tightening, preserving the clamp load relationship under future thermal cycling and vibration. Tightening to 70% of proof strength provides margin against the torque-to-preload uncertainty (±30%) introduced by friction variability.
What is stress area and how is it different from the minor diameter area?
The tensile stress area is a calculated equivalent area used to convert tensile load to nominal stress for threaded fasteners. It accounts for the helical thread profile and is defined as A_s = π/4 × [(d_2 + d_3)/2]², where d_2 is the pitch diameter and d_3 is the minor diameter of the external thread. Stress area is approximately 75% of the nominal shank area for metric bolts and is slightly larger than the minor diameter area, reflecting that thread roots share load with the core. ASME and ISO standards specify the stress area to use for calculating bolt strength and tightening torque.
How do I prevent bolt self-loosening under vibration?
Bolt self-loosening occurs when vibratory transverse loads cause micro-slip between the bearing faces of the bolt head, nut, and joint members, gradually unwinding the thread engagement. The primary countermeasure is adequate clamp load — maintaining joint slip below the friction threshold prevents self-loosening. Secondary measures include prevailing torque nuts (nylon insert or all-metal prevailing torque designs per ASTM F1136), thread-locking compounds (anaerobic adhesives per MIL-S-22473), tang-type locking washers for non-critical applications, and castellated nuts with cotter pins for safety-critical linkages.
What torque values apply to metric bolts of different grades?
Representative approximate tightening torques (lightly oiled, 70% proof load, K=0.15) for common bolt sizes: M10 8.8 grade ≈ 47 N·m; M10 10.9 grade ≈ 68 N·m; M12 8.8 ≈ 82 N·m; M12 10.9 ≈ 118 N·m; M16 8.8 ≈ 200 N·m; M16 10.9 ≈ 285 N·m. These values scale with the cube of the bolt diameter (larger bolts carry much more torque). Always verify against the specific bolt manufacturer's data sheet for the exact grade and lubrication condition, as K-factor variations dominate the uncertainty.
All tools