ft lbs calculator

What is a foot-pound (ft-lb)?

A foot-pound (ft-lb) is a unit commonly used for torque and energy/work in the U.S. customary system. In plain language, 1 ft-lb means applying one pound of force through one foot of distance. You’ll see this unit everywhere from automotive torque specs (lug nuts, spark plugs, axle bolts) to ballistics data (muzzle energy).

Even though torque and energy share the same dimensional unit, context matters. Mechanics often write torque as lb-ft to distinguish it from energy values listed as ft-lb. In conversation, many people still use the terms interchangeably.

How this ft lbs calculator works

1) Force × Distance (Torque/Work)

The core formula is simple:

ft-lb = force (lbf) × distance (ft)

If you apply 75 lbf at the end of a 2 ft wrench, the resulting torque is 150 ft-lb.

2) Bullet Muzzle Energy

For ballistics, this page uses the common grain-and-velocity formula:

ft-lb = (bullet weight in grains × velocity² in ft/s) ÷ 450240

This gives a quick estimate of muzzle energy in foot-pounds.

Why people use an ft-lb calculator

• Automotive repairs: convert torque specs between ft-lb and N·m.
• Tool setup: choose correct torque wrench settings.
• Engineering and fabrication: estimate rotational loading.
• Ballistics: compare ammunition energy by caliber/load.
• General conversion: switch between in-lb, ft-lb, N·m, J, and kgf·m.

Quick reference values

• 1 ft-lb = 12 in-lb
• 1 ft-lb = 1.355817948 N·m
• 1 ft-lb = 1.355817948 J
• 1 N·m ≈ 0.737562149 ft-lb
• 1 kgf·m ≈ 7.233013852 ft-lb

Example calculations

Example A: Torque on a breaker bar

You push with 120 lbf at the end of a 1.5 ft bar:

120 × 1.5 = 180 ft-lb

Example B: Convert torque spec

A fastener calls for 90 ft-lb. In metric that is approximately:

90 × 1.3558 ≈ 122.0 N·m

Example C: Muzzle energy

A 124 grain bullet at 1150 ft/s has energy:

(124 × 1150²) ÷ 450240 ≈ 363.8 ft-lb

Common mistakes to avoid

• Mixing up in-lb and ft-lb (factor of 12 difference).
• Using wrench length in inches without converting to feet.
• Assuming torque and energy are always interchangeable in meaning.
• For muzzle energy, entering bullet weight in grams instead of grains.
• Rounding too aggressively for precision torque work.

FAQ

Is ft-lb the same as lb-ft?

Numerically, yes. Both represent the same magnitude. Conventionally, mechanics prefer lb-ft for torque, while ft-lb is often seen for energy/work.

Can ft-lb values be negative?

Yes, in directional analysis (clockwise vs. counterclockwise torque, or signed work), values can be negative. For everyday specs and comparisons, magnitude is typically shown as a positive number.

Should I use N·m or ft-lb?

Use whatever your service manual or project standard requires. This calculator makes it easy to switch between systems quickly and consistently.

Bottom line

If you work on cars, machinery, or ballistics data, a reliable ft lbs calculator saves time and prevents mistakes. Use the calculator above for instant results, then verify final specs against your manufacturer documentation when precision matters.