Torque is where unit mistakes become physical. When a pressure conversion is wrong, a spreadsheet looks odd. When torque is wrong, a bolt can loosen, yield, snap, or quietly fail later. In mechanical and structural work, torque specifications often sit in the middle of a chain of responsibility: the designer specifies a value, the field applies it, and everyone assumes it was done correctly.
The two most common torque units are newton-meters (N m) and pound-feet (lb-ft). Both are standard. Both are easy to convert. The problem is that people occasionally treat them as interchangeable because the numbers are close enough to feel familiar. "Close enough" is not a design method.
First, a quick unit reminder
Torque is force times perpendicular distance. In SI, it is newtons times meters. In imperial practice, it is pound-force times feet. Converting between them is a scale conversion, but you need to be clear about which pound you mean. In most torque contexts, "lb" means pound-force.
A useful reference:
- 1 lb-ft is about 1.35582 N m
- 1 N m is about 0.73756 lb-ft
A torque of 100 lb-ft is about 135.6 N m. A torque of 100 N m is about 73.8 lb-ft. Those are not close.
Mistake 1: reading a spec in one unit and setting a wrench in another
This is the most common field mistake. The designer specifies torque in N m because the project is metric. The technician has a torque wrench that reads in lb-ft. The technician does the job quickly and sets the wrench to the same number because it feels reasonable.
If the spec is 120 N m and the wrench is set to 120 lb-ft, the applied torque is about 163 N m. That can push a fastener beyond its intended preload range, especially for smaller bolts or for components with soft materials and limited thread engagement.
The reverse mistake is just as bad. If the spec is 120 lb-ft and the wrench is set to 120 N m, you apply only about 88.5 lb-ft. Under-torqued joints loosen, and then the joint sees movement, fretting, and fatigue that were not part of the original design assumptions.
Mistake 2: mixing torque with "angle tightening" assumptions
Many modern specifications use torque plus angle, or torque-to-yield methods for critical joints. In those cases, torque is often used as a seating step, followed by a defined rotation. If you change torque units incorrectly, you can alter the seating condition before the angle step. That changes preload and can affect joint performance.
The practical point is this: even if the final method is angle-based, the initial torque still matters. You should convert it correctly.
Mistake 3: confusing lb-in and lb-ft
In smaller fasteners, torque is sometimes specified in pound-inches (lb-in). This is a different unit, and the values differ by a factor of 12. This is an easy trap because the unit abbreviations look similar in handwritten notes.
A fastener spec of 60 lb-in is only 5 lb-ft. If you apply 60 lb-ft, you are off by a factor of 12. That is not a rounding error. That is a broken bolt.
Why torque matters: preload is the real design variable
Most bolted joints are designed around clamp force or preload. Torque is the field-friendly proxy. But torque does not directly equal preload. It depends on friction under the head, friction in the threads, lubrication, and surface condition.
This is why many torque specs include assumptions like "dry" or "lubricated." A conversion tool cannot tell you whether the bolt is lubricated. But it can at least ensure you are applying the correct unit system to the specified number.
A realistic example: torque spec from a datasheet
Suppose a motor mounting bolt spec lists 90 lb-ft for a particular grade and size. Your project documentation is in metric and your installation crew prefers N m. Convert:
90 lb-ft x 1.35582 = 122.0 N m
If you round to 120 N m, that is typically reasonable. If you round to 90 N m because you did not convert, you are under-torqued by about 26 percent. In bolted joints, that can be the difference between "works for years" and "mysteriously loosens after a few months."
A short checklist before you tighten anything important
- Confirm the unit printed on the torque wrench. Many wrenches can switch units. Some cannot.
- Confirm the unit in the specification. If it is not written, assume it is not known.
- Watch for lb-in versus lb-ft. If the number looks high for a small fastener, it probably is.
- Round sensibly. Over-precision looks impressive and does nothing useful in field torque work.
The humor here is that torque wrenches are extremely precise devices used in extremely imprecise environments. That is not a complaint. It is reality. Proper unit handling is one of the few controls you actually have.