Using a torque wrench correctly
A torque spec only means what it says under the conditions it assumed. If a chart's dry figure was calculated assuming clean, dry, as-received threads and you run a lubricated or reused bolt to that same number, you'll overtighten it, because lubrication let more of that torque become clamping force instead of getting eaten by friction. Always match the torque figure to the actual condition of the joint you're tightening: clean and dry, lubricated, plated, or reused, not whichever number happens to be easiest to find.
Why lubricated torque is lower for the same tension
This surprises people the first time they see it: a lubricated bolt is torqued to a lower number than the same bolt dry, even though the goal, the clamping force, is identical. Torque has two jobs when you tighten a fastener: some of it stretches the bolt to create clamping force, and the rest is wasted overcoming friction under the bolt head and in the threads. Lubrication cuts that friction, so a smaller applied torque gets a larger share of itself converted into clamping force. Reach the same target preload with less wasted torque, and the number on the wrench is lower, not because the bolt needs less strength, but because less of the torque is being thrown away.
Reading grade and class markings
SAE grade is stamped on the bolt head as radial lines: no lines is Grade 2 (or unmarked/lower-grade), three lines is Grade 5, five lines is Grade 7, and six lines is Grade 8. More lines means a higher-strength alloy and heat treatment, not a bigger bolt. Metric property class is stamped as a number like 8.8, 10.9, or 12.9. The first number, times 100, is roughly the tensile strength in MPa (10.9 is about 1040 MPa tensile); the second number, divided by 10, is roughly the ratio of yield strength to tensile strength (10.9 yields at about 90% of its tensile strength). Both marking systems exist so you can identify a bolt's strength class without pulling a spec sheet, as long as you know how to read the stamp. If you're tapping the hole this bolt threads into rather than using a nut, the tap drill chart covers the matching drill sizes and thread engagement for the same Unified thread sizes.
Reusable versus permanent preload
The preload fraction (C in the formula above) is a safety margin against the bolt's proof strength, and it's lower for connections you expect to reuse. A reusable joint, one that will be taken apart and retightened, typically targets about 75% of proof strength as clamping force, leaving margin for the small amount of preload lost each time a fastener is removed and reinstalled. A permanent or rarely-disturbed joint can push closer to 90% of proof strength, extracting more clamping force from the same bolt since it won't be cycled repeatedly. Neither number is a hard rule; both are conventions this calculator uses as presets, adjustable if your application calls for something else.
This method's real limits
Standard torque-to-preload, the method this whole page calculates, assumes friction-based tightening: you're estimating clamping force from torque and an assumed nut factor. It does not apply to torque-to-yield or torque-angle fasteners, common on engine head bolts and similar critical joints, which are tightened by a completely different method (torque to a low value, then rotate a further specified angle) precisely because friction-based torque alone isn't precise enough for that application. It also isn't a substitute for a manufacturer's torque spec when one exists, or for an engineer's specification on a structural or safety-critical joint. Use this calculator and these tables for standard fastener tightening where a friction-based estimate is the right tool, and defer to the actual spec everywhere one exists. If you're not sure which category a joint falls into, that uncertainty is itself a reason to look up the manufacturer's documentation before tightening rather than assume a generic chart applies.