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Sling angle and rigging load calculator

Updated October 18, 20262 min tool
1

The load

Units
m
m
m
kg

From the drawing, the nameplate or a load cell. Never a guess.

Centre of gravity, from the middle of the load

m
m
m
2

The rigging

Where the slings take hold, and how long they are. The view and the numbers follow every change.

Hitch
Legs
m
Size the legs by
m

Sling and hardware, pick point to hook.

Leg lengths
Loading the 3D view…

Most in one leg

840 kg

Within ratingBelow 45°, or close to the ratingOver the rating or too shallowCentre of gravity

3

Tension in the slings

Most loaded leg

840 kg

21% of its 3,992 kg rating.

Smallest angle

46°

From horizontal. Load factor 1.39.

Load and rigging

1,241 kg

Add the hook block and anything else the crane's chart counts.

Out of level

0.0°

Headroom

3.81 m

Hook above the top of the load.

Shackles

3,250 kg

5/8 in, 3¼ t

LegAngleLengthTensionRatingUsed
146.2°5.20 m840 kg3,992 kg21%
246.2°5.20 m840 kg3,992 kg21%

The angle is everything: tension per leg for this load, 2 legs sharing

90°

606 kg

× 1.00

60°

700 kg

× 1.15

45°

857 kg

× 1.41

30°

1,212 kg

× 2.00

20°

1,772 kg

× 2.92

The rules · Alberta

  • Rigging is used at no more than 20% of its breaking strength (5:1), or 25% where the maker fatigue rates it to EN 1677-1, and 10% when a worker is raised or lowered (OHS Code s.292). Alloy chain catalogued at 4:1 relies on that fatigue rating.
  • Slings made since July 2009 meet ASME B30.9-2006 (s.297); the rating is marked on them (s.293) and they are inspected before each period of use in a shift (s.294).
  • Protect slings from sharp edges (s.296). The load’s weight is known before the lift (s.68), and a lift over 75% of the crane’s rated capacity needs a lift calculation (s.68.1).
  • Sling legs at 30° from horizontal or more, unless the sling maker or a qualified person approves (ASME B30.9 and the makers’ instructions).
OHS Code Part 21 Part 6

A summary to plan with. The regulation and your employer's procedures govern.

Plan the lift, then check it

Confirm the weight and the centre of gravity, inspect every sling and piece of hardware before use, keep everyone out from under the load, and use tag lines. A calculation is a plan: the slings' tags, the crane's chart and your lift plan govern.

The tension in a sling leg depends on its angle as much as on the load. This calculator builds the lift in 3D, from a steel beam on a chain bridle to a pipe bundle in two baskets, and works out the tension in every leg, how far the load tilts if its centre of gravity is off centre, and whether every sling and shackle is within its rating.

The angle is everything

A sling leg holding a load straight up carries its share and no more. Lean it over and it has to pull sideways as well, against the other legs, so the tension rises:

Tension per leg = (load ÷ number of legs) ÷ sin(angle from horizontal)

  • At 90° (vertical) the factor is 1.000.
  • At 60° it is 1.155: each leg carries 15.5% more than its share.
  • At 45° it is 1.414.
  • At 30° it is 2.000: each leg carries twice its share, and that is the lowest angle the standards allow.
Below 30°, stop and re-rig
Use longer slings, pick closer together, or use a spreader or lifting beam. A leg below 30° from horizontal needs the sling maker's or a qualified person's approval, and in BC an engineered lift.

Centre of gravity, tilt and leg lengths

A load turns until its centre of gravity hangs straight under the hook. Off-centre equipment, like the motor end of a pump skid, pulls the load out of level when the legs are one length, and the legs nearest the centre of gravity carry more. Set “Adjusted to hang level” to see the leg lengths that hold it level and how the load divides between them.

Before the lift

  • Know the weight and the centre of gravity: from the drawing, the nameplate or a load cell, never a guess.
  • Inspect every sling and piece of hardware before use, and check its tag. Rated capacity is on the tag, not in a chart.
  • Protect synthetic slings from sharp edges, and keep chokers above 120°.
  • Keep everyone out from under the load, and control it with tag lines.
  • Over 75% of the crane's capacity in Alberta, the lift needs a lift calculation (OHS Code s.68.1).

Our training covers rigging and lift planning, and our safety manuals include hoisting and rigging procedures.

Sources: ASME B30.9 as adopted by Alberta (OHS Code s.297), BC (OHS Regulation Part 15), Manitoba, Nova Scotia and Newfoundland and Labrador; BC Table 15-3; Crosby Spectrum 10 Grade 100 chain and G-209 shackle ratings; Kennedy Type 115 wire rope slings; Lift-All Webmaster web slings and Tuflex round slings. Ratings are the makers' published values for new slings; the sling's own tag governs. The statics are exact for one to three legs; four legs are rated as three.

Frequently asked questions

How do you calculate sling tension from the sling angle?
Divide the load by the number of legs sharing it, then divide by the sine of the angle each leg makes with the horizontal: tension per leg = (load ÷ legs) ÷ sin(angle). A 2,000 kg load on two legs at 60° puts 1,000 ÷ 0.866 = 1,155 kg in each leg. At 30° it is 2,000 kg in each leg, twice the vertical share.
What is the minimum sling angle?
Keep legs at 30° from horizontal or more. ASME B30.9, which every Canadian province that names a sling standard adopts, and the sling makers do not allow less without the maker’s or a qualified person’s approval, and BC’s OHS Regulation (s.15.34) does not permit it unless the lift is engineered. Aim for 45° to 60°: the tension climbs quickly below 45°.
How many legs of a four-leg bridle carry the load?
On a stiff load, usually not all four: small differences in leg length and the load’s stiffness let it rock onto two or three. The sling makers’ charts and BC’s regulation (s.15.33) rate a four-leg bridle as if three legs carry the load. The calculator checks it that way and shows the worst leg if the load rocks onto two.
Why does a load tilt on slings?
A hanging load always turns until its centre of gravity is straight under the hook. With legs of one length the hook sits over the middle of the pick points, so a centre of gravity off that middle tilts the load until it is under the hook. Shorten the legs on the heavy end, or move the pick points, to bring it level.
How much does a choker reduce a sling’s capacity?
A choker hitch is rated at about 75% of the vertical rating for wire rope and 80% for alloy chain, web and round slings. A choke tighter than 120° reduces it further: to 87% of the choker rating at 90° to 120°, 74% at 60° to 89°, 62% at 30° to 59° and 49% below 30° (the ASME B30.9 table as published by the sling makers).

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