The 1901 Definition That Won't Stop Showing Up
Standard gravity — 9.80665 m/s² — was not measured; it was chosen. At the 3rd General Conference on Weights and Measures in 1901, the world's metrologists picked a single reference value for the acceleration due to gravity, averaged from measurements around the globe, and declared it the standard. The kilogram-force, already in common use across Europe, was then defined against it: one kilogram-force is the force that one kilogram of mass experiences under standard gravity. The definition made the unit exact — 1 kgf = 9.80665 N with zero uncertainty — and simultaneously made it obsolete, because the whole point of SI was to remove gravity from force definitions entirely.
But exactness is a double-edged sword. The kgf is exactly convertible, which means it's easy to convert, which means nobody bothers to stop using it. A Japanese excavator spec sheet says breakout force "12,500 kgf." Any engineer can multiply by 9.80665 and get 122,583 N. Because the conversion is so trivial, it never becomes a barrier — so the unit survives in the documentation of every machine whose original design documentation was written before full SI adoption. This page exists to make that one multiplication automatic and auditable.
N = kgf × 9.80665
where 9.80665 is standard gravity (3rd CGPM, 1901)
The constant is exact by definition — this conversion has zero uncertainty.
The Tonne-Force Family Tree
The kgf has a bigger sibling that does most of the heavy lifting in industry: the tonne-force. 1 tf = 1,000 kgf = 9,806.65 N. It's the unit hiding behind every "t" on a crane chart, an excavator spec, or a press nameplate outside the US. When you see a Chinese, Japanese, or European machine rated "50 t," you're looking at 50,000 kgf = 490,332.5 N = 490.33 kN.
Note what this means for the US counterpart: a US crane rated "50 tons" means 100,000 lbf = 444,822 N = 444.82 kN. The same nominal rating "50 tons" means different forces depending on the country — 490.33 kN for tonne-force, 444.82 kN for US short-ton force, 508.02 kN for the British long-ton force. That 10% spread is exactly why international rigging documents now quote kilonewtons as the common language, and why converting the nameplate to N before any calculation is a non-negotiable habit. The conversion mistake chronicles on this site include more than one lift plan that died on this exact ambiguity.
Common Kilograms-Force to Newtons Conversions
| kgf | Newtons | Where you'd see this |
|---|---|---|
| 1 kgf | 9.807 N | The definitional anchor — one kilogram under standard gravity. |
| 5 kgf | 49.03 N | Bicycle lock clamp force. Light spring compression. |
| 10 kgf | 98.07 N | Handrail push load per person in some European codes. |
| 20 kgf | 196.13 N | Luggage scale reading for a checked bag in Asia. |
| 50 kgf | 490.33 N | One person's pull on a rope. Gym machine stack. |
| 100 kgf | 980.67 N | European machine cylinder force. 1 kN, roughly. |
| 500 kgf | 4,903 N | Compact car engine's weight force. |
| 1,000 kgf | 9,807 N | One tonne-force — the crane rating unit. |
| 5,000 kgf | 49,033 N | Small excavator bucket breakout force. |
| 25,000 kgf | 245,166 N | A 25-t crane's rated hook force. |
| 50,000 kgf | 490,333 N | Mid-size crawler crane capacity. |
Worked Examples
Excavator breakout force into the FEA model
The spec sheet for a Komatsu excavator says bucket breakout force "12,500 kgf." You're verifying the bucket linkage in a metric FEA package. 12,500 × 9.80665 = 122,583 N = 122.58 kN. If the FEA model receives 122,583 N instead of a mistaken 12,500 N (treating kgf as N), the stress results are a factor of 9.8 too low — the linkage passes a check it should fail. This specific error — dropping the 9.80665 — is the metric-world equivalent of the Mars Climate Orbiter's dropped 4.448, and it shows up in real design reviews whenever a legacy spec meets modern software.
Reading a European press nameplate
A used German press imported to a US shop is rated "80 t." The US maintenance engineer needs to compare it against an American press rated 90 tons. 80 tf = 80,000 kgf = 784,532 N = 784.5 kN. The US 90-ton press: 90 × 2,000 lbf = 180,000 lbf = 800,682 N = 800.7 kN. The two presses are nearly equal in force (784.5 kN vs 800.7 kN) despite the "80" vs "90" nameplates — the nominal difference (80 vs 90) is almost entirely eaten by the unit difference (tonne-force vs short-ton-force). Comparing nameplates without converting produces a 12.5% phantom gap.
Seat belt preload spec from a Japanese OEM
A Japanese automotive spec calls for seat belt pretensioner force "500 kgf." The European supplier building the component needs newtons: 500 × 9.80665 = 4,903 N = 4.9 kN. The same spec in a European OEM's document would be written 4,900 N or 4.9 kN directly. The 0.07% difference between the Japanese spec's exact value (4,903.3 N) and the European rounded value (4,900 N) is inside any test tolerance — but the difference between 500 kgf and 500 N is a factor of 10, which is not inside any tolerance. The component supplier's acceptance test must be run against the correct converted value, and that value is 4,903 N, not 500 N.
Engineering Context
The kilogram-force and its torque derivative kgf·m are the most common non-SI units in the machinery documentation of Japan (JIS B 7505, crane standards), Germany (DIN 1301 formally retired kp in the 1970s, but installed equipment predates it), and China (legacy GB standards before the 1990s SI harmonization). ISO 80000-4 permits their use with a footnote giving the exact conversion. The same 9.80665 constant powers the kgf·m to N·m torque converter — torque is force times lever arm, so the force-side conversion carries straight over. For the mass interpretation of kilogram ratings — what a 500-kg machine weighs in pounds — use the kg to lbs converter, but understand that mass and force are different quantities and the nameplate's kgf is always a force.
More: N to kgf · kgf to lbf · lbf to kgf · N to lbf · Force Guide
Related Unit Converters
Frequently Asked Questions
How do I convert tonne-force to kilonewtons?
Multiply tonne-force by 9.80665 to get kN. 1 tf = 9.80665 kN. A crane rated 25 t has a hook force of 245.17 kN. This is the single most useful conversion in international lifting work, because crane charts say tonnes and structural software says kilonewtons. The reciprocal: divide kN by 9.80665 to get tonne-force.
Why does the kgf survive in Japan when Japan uses SI everywhere else?
Japan adopted SI for legal metrology in 1993 (Measurement Law revision), but the construction and heavy machinery industries had decades of design documentation, test data, and nameplates denominated in kgf and tonne-force. Re-rating every installed crane and excavator would require revalidation, not just re-labeling — the structural certifications are tied to the original units. New JIS specifications use newtons, but the installed base will carry kgf for decades. Japanese engineers therefore grow up bilingual in force units, and this conversion is part of the job.
Can I use this converter for kgf·m torque values?
For the force part, yes — 1 kgf = 9.80665 N regardless of the lever arm. For the torque value itself, use the dedicated kg·m to N·m converter, which multiplies the kgf·m value by the same 9.80665. The two pages use the identical constant; the torque page just applies it to the full torque quantity. If you're converting engine torque from a Japanese manual (which typically quotes kgf·m) into the N·m used in modern spec sheets, the torque page is the one to bookmark.