Sling angle and sling tension calculator
Short answerTension in each sling leg equals the load divided by the number of legs, then by the sine of the leg's angle from horizontal. The load angle factor is 1 divided by that sine: 1.155 at 60 degrees, 1.414 at 45 degrees and 2.0 at 30 degrees. Compare the leg tension with the sling's rated load.
What the sling angle calculator works out
As sling legs move away from vertical, each leg carries more force than its share of the load, and the increase is steep at low angles. A sling rated for the load can be overloaded purely by geometry.
Enter the load, the number of legs and the angle of each leg measured from the horizontal. The calculator returns the tension per leg and the load angle factor, and flags angles below 45 and 30 degrees. It labels the load in kilograms, but the maths works in any unit.
The sling tension formula
- Tension per leg = Load / (number of legs x sin(angle from horizontal))
- Load angle factor = 1 / sin(angle from horizontal)
- Angle measured from vertical instead: factor = 1 / cos(angle from vertical)
- Factors: 90 degrees 1.000; 60 degrees 1.155; 45 degrees 1.414; 30 degrees 2.000
Worked example
A 2,000 lb load lifted on a two-leg bridle. At 60 degrees from horizontal each leg carries 2,000 / (2 x 0.866) = 1,155 lb. At 45 degrees it is 2,000 / (2 x 0.707) = 1,414 lb. At 30 degrees it is 2,000 / (2 x 0.5) = 2,000 lb, so each leg carries the full weight of the load.
In metric, a 1,000 kg load on two legs at 45 degrees puts about 707 kg on each leg.
How to read the result
Compare the tension per leg, not the weight of the load, with the rated load (working load limit) on each sling's tag at that hitch. OSHA requires that slings are not loaded beyond their rated capacity (29 CFR 1910.184(c)(4)). OSHA sling guidance says horizontal angles below 30 degrees should not be used unless the sling manufacturer or a qualified person recommends it. Where the angle falls between tabulated values, use the next lower angle or have a qualified person calculate the rating.
What the calculation does not tell you
- It assumes legs share the load equally, with the hook over the centre of gravity. For multi-leg slings on uneven or rigid loads, two legs may carry most of it; OSHA guidance calls for a qualified person to analyse nonsymmetrical lifts.
- It adds nothing for shock or dynamic loading. OSHA prohibits shock loading (1910.184(c)(11)).
- It does not reduce capacity for choker hitches, sharp edges, D/d ratio or sling condition.
- It does not replace a lift plan. Follow site procedures and a competent person.
Questions people ask
How do you calculate sling tension?
Divide the load by the number of legs, then divide by the sine of the sling angle measured from the horizontal. For a 2,000 lb load on two legs at 60 degrees: 2,000 / (2 x 0.866) = 1,155 lb per leg. Compare that figure with the sling's rated load.
What is the load angle factor?
It is the multiplier that converts each leg's share of the load into its real tension: 1 divided by the sine of the horizontal angle. It is 1.155 at 60 degrees, 1.414 at 45 degrees and 2.0 at 30 degrees, and it climbs quickly below 30 degrees.
What is the minimum sling angle?
OSHA sling guidance says horizontal sling angles below 30 degrees should not be used unless the sling manufacturer or a qualified person recommends it. Many sites set 45 degrees as the working minimum. Longer slings or a spreader beam open the angle and cut leg tension.
Is the sling angle measured from horizontal or vertical?
Both conventions exist. OSHA 1910.184 defines the angle of loading from the horizontal or vertical plane. This calculator uses the angle from horizontal. If you measure from vertical, the factor is 1 divided by the cosine of that angle; 30 degrees from vertical equals 60 degrees from horizontal.
Do four legs mean each leg carries a quarter of the load?
Not reliably. On a rigid load, small differences in leg length or attachment height mean two legs can end up carrying most of the weight. Many riggers rate a four-leg lift as if only two legs carry the load. Have a qualified person check uneven or nonsymmetrical lifts.
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