ton-force (Short/UK) to Kilogram-force

A long ton-force (UK) is based on 2,240 lb (≈ 9,964 N), while a short ton-force (US) is based on 2,000 lb (≈ 8,896 N). The long ton-force is about 12% larger. Confusing the two is a common source of error when interpreting crane ratings or press capacities from British versus American documentation.

The long ton (2,240 lb) was the standard UK weight unit before metrication, rooted in the medieval practice of measuring goods in multiples of 20 hundredweight (112 lb each). Legacy shipbuilding, mining, and heavy engineering documents still reference long tons. Modern British engineering has largely switched to metric tonnes, but older equipment and archived specs remain in long ton-force.

Multiply long ton-force by 9.96402 to get kilonewtons. So 10 long ton-force ≈ 99.6 kN. For a rough estimate, 1 long ton-force is very close to 10 kN — a convenient approximation for quick conversions when reviewing older British engineering documents.

Older British crane certifications, Royal Navy vessel displacement figures, and pre-1970s structural steel test reports commonly use long ton-force. Maritime salvage operations and heritage railway maintenance also reference long tons. When refurbishing Victorian-era bridges or machinery, engineers must convert these legacy ratings to modern SI units for compliance with current codes.

The Royal Navy measured displacement in long tons for centuries, and major warship classes are historically known by their long-ton figures — HMS Dreadnought at 18,120 long tons, HMS Hood at 46,680 long tons. Modern Royal Navy vessels are specified in metric tonnes, but naval history, treaty references (e.g., the Washington Naval Treaty's 35,000 long-ton capital ship limit), and ship recognition databases retain long-ton figures because changing them would break continuity with a vast body of historical documentation.

Many countries in Europe and Asia adopted kgf in engineering and industry before SI became dominant. Crane ratings, hydraulic press capacities, and elevator load limits were standardized in kgf and remain labelled that way on existing equipment. Rewriting decades of documentation and re-stamping machinery is costly, so kgf persists in practice even where SI is officially mandated.

Multiply kilogram-force by 9.80665 to get newtons. So 1 kgf = 9.80665 N, and 100 kgf = 980.665 N. For quick estimates, multiply by 9.81 or roughly 10. This conversion factor is simply the standard acceleration due to gravity in m/s².

They are the same unit. Kilopond (kp) was the name used in Germany, Scandinavia, and other European countries, while kilogram-force (kgf) is the internationally recognized term. Both equal the gravitational force on 1 kg at standard gravity. ISO 80000-4 deprecates both in favor of the newton, but kgf remains common in Asian and European industrial contexts.

Using kgf conflates mass and force, leading to errors when gravity varies or when applying F = ma. Engineers must insert a gravitational constant (gₙ = 9.80665 m/s²) to convert between mass and force, which SI avoids by using separate units — kilograms for mass and newtons for force. Mistakes in this conversion have caused real-world structural miscalculations.

A 70 kg adult weighs about 70 kgf. A firm handshake exerts roughly 10–15 kgf. A bicycle tire inflated to 6 bar exerts about 4.5 kgf per cm² on the rim. These values feel intuitive because they numerically match familiar masses, which is precisely why kgf remains popular in non-technical contexts despite SI deprecation.