Tools
Units converter
Convert the units used in lifting, rigging and ground work, from t/m² to kN/m² and tonnes to kilonewtons, with every figure exact to its definition.
How it works
How the converter works
Every unit has an exact definition in terms of the SI unit of its quantity: the metre, the kilogram, the newton, the pascal and so on. The converter turns your figure into the SI unit and then into the unit you want, so every pair of units is converted by the same exact factors. Answers are given to 6 significant figures, with the figure in every other unit of the quantity alongside.
About the definitions
The imperial and US units come from the international yard and pound agreed in 1959: 1 inch is exactly 25.4 mm, 1 foot 0.3048 m, 1 yard 0.9144 m and 1 pound 0.45359237 kg. The UK gallon is 4.54609 litres, as the Weights and Measures Act 1985 defines it, and the US gallon is 231 cubic inches, 3.785411784 litres. A tonne is 1,000 kg; a UK (long) ton is 2,240 lb and a US (short) ton 2,000 lb. The factors follow NIST Special Publication 811, the US guide to SI units.
About force, weight and gravity
A mass (kilograms, tonnes) is how much there is of something. Its weight is a force (newtons, kilonewtons): the mass times the pull of gravity. The kilogram-force and tonne-force are defined with standard gravity, 9.80665 m/s², so 1 tonne-force is exactly 9.80665 kN, and the pound-force is the weight of a pound, 4.44822 N.
On site this is usually rounded to 9.81, so 1 t = 9.81 kN, and our other tools use that. The converter keeps the exact 9.80665 and says so under the answer: the difference is 0.03%. For Mass and weight you can change g, for example to 9.81, to match a calculation that uses it.
About pressure and ground bearing
A pascal is one newton on a square metre, so kN/m² and kPa are the same unit, and N/mm² is the same as MPa. A tonne-force on a square metre (t/m²) is 9.80665 kN/m². A bar is 100 kPa exactly, a standard atmosphere 101.325 kPa, and a pound-force per square inch (psi) is 6.89476 kPa. Ground assessments usually give kN/m², and older reports and many crane makers t/m² or kg/cm².
About load over an area
A load and a pressure are different kinds of figure, so kN can't be turned into kN/m² on its own: you need the area the load is spread over. Pressure = load ÷ area, and load = pressure × area. Choose Load over an area, enter the load (a force, or a mass, which is taken as its weight) and the area, and the converter gives the pressure in every unit; or start from a pressure to get the load. For an outrigger, our ground bearing pressure tool checks the pressure against what the ground can carry.
About angles and slopes
A slope can be given as an angle, as a gradient in per cent, or as 1 in X. The gradient is the rise for every 100 along the level: 100 × tan(angle). 1 in X is a rise of 1 for every X along the level, so X = 100 ÷ the gradient. A 30° slope is 57.7%, or 1 in 1.73; a 1 in 12 ramp is 8.33%, or 4.76°. The converter takes angles from level (0°) to vertical (90°).
About wind speeds and the Beaufort scale
1 mph is exactly 0.44704 m/s, and 1 knot is one nautical mile (1,852 m) an hour, 0.5144 m/s. For a speed the converter also gives the Beaufort force and its description, from the mean wind speed in whole knots on the Met Office's Beaufort scale. The scale describes the mean wind at 10 m. Crane wind limits are for gusts at the height of the lift, which are higher: our wind check gives the forecast at your height.
FAQs
Common questions
How many kN are in a tonne?
A tonne is a mass, and its weight under standard gravity is 9.80665 kN, so 1 t is about 9.81 kN, the figure used on site. A 10 t load weighs about 98.1 kN. Going the other way, 100 kN is the weight of about 10.2 t.
What is the difference between kN and kN/m²?
kN is a force: the load itself. kN/m² is a pressure: that load spread over each square metre it sits on. 100 kN on a mat of 2 m² gives 50 kN/m². To go from one to the other you need the area, which is what Load over an area in the converter does.
How do I convert t/m² to kN/m²?
Multiply by 9.80665, or 9.81 on site. 10 t/m² is 98.07 kN/m², about 98 kN/m². To go back, divide by 9.80665: 150 kN/m² is about 15.3 t/m².
Is a tonne the same as a ton?
No. A tonne is 1,000 kg. A UK long ton is 2,240 lb, 1,016 kg, and a US short ton is 2,000 lb, 907 kg. A US spec sheet that says 50 tons means short tons, about 45.4 t, so check which ton a document means before you use its figures.
How do I convert kg to kN?
Multiply the mass in kilograms by 9.80665 and divide by 1,000. A 2,500 kg load weighs 24.5 kN, and rounding g to 9.81 gives the same 24.5 kN. The converter does this under Mass and weight.
What is 1 psi in kPa or bar?
1 psi is 6.895 kPa, or 0.0689 bar. 100 psi is 689.5 kPa, or 6.895 bar, and 1 bar is 14.5 psi. Hydraulic pressures on older or American kit are often in psi.
How do I turn a gradient % into an angle?
The angle is the arctangent of the gradient ÷ 100. A 10% gradient is 5.71°, or 1 in 10; 100% is 45°, or 1 in 1. A 1 in 12 ramp is 8.33%, or 4.76°.
What is a tonne-metre?
A moment: a tonne-force acting 1 m from a point, such as a load times its radius from the crane's centre of slewing. 5 t at 10 m is 50 t·m, about 490 kN·m, as 1 t·m is 9.80665 kN·m. What a crane can lift comes from its load chart, not a moment alone.
How do I convert mph to m/s for wind limits?
Multiply mph by 0.44704. A 20 mph limit is 8.94 m/s, and crane limits given in m/s go back by dividing by 0.44704: 9.8 m/s is 21.9 mph and 13 m/s is 29.1 mph. Check whether a limit is for gusts or the mean wind, and at what height.
Planning guidance only. The converter changes a figure from one unit to another. Check figures that matter against the source document; the appointed person confirms the figures used in a lift plan.
© Synergy Lifting Ltd. The units converter and its code are our own work. You are welcome to use it to plan your lifts, but please do not copy, reproduce or reuse the tool, its code or its results without our written permission.
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