Why does a shallower sling angle increase the tension so sharply?

Because the vertical component of a leg's tension is what actually holds the load, and that component is the tension multiplied by the sine of the angle. As the angle falls, the sine falls with it, so the tension has to rise to keep the vertical component the same. At thirty degrees the sine is a half, so each leg carries twice the vertical load it is supporting — and the horizontal pull it exerts on the load exceeds the lift.

Why should I not credit all four legs of a four-leg sling?

Because on a rigid load with fixed-length legs, the arrangement is statically indeterminate: manufacturing tolerance, a slightly out-of-square load and a few millimetres of stretch decide which legs take the weight, and it is rarely all four evenly. Common rigging practice, and the manufacturers' own guidance, is to assume two legs carry unless the assembly can demonstrably equalise. The breakdown gives you both numbers so the assumption is a choice rather than an accident.

What does the sideways pull line mean for the load itself?

It is the horizontal force each leg applies to its attachment point, pulling inward toward the centre of the load. On a fabricated item with lifting lugs, that force tries to bend the lugs and crush the item between them; on a bundle it tries to close the bundle up. Shallow angles make it large, and it is the reason a spreader beam is sometimes the only sensible answer rather than a refinement.

Does this account for the weight of the rigging itself?

No. Enter the weight of the load, and remember separately that the whole rigging assembly — slings, shackles, spreader, chain blocks, tag lines — is a deduction from the crane's chart capacity along with the hook block, the rope below the boom head and any jib fitted. The omitted spreader beam is the classic deduction people forget, and it is often the heaviest single item in the assembly.

Heavy Civil & Infrastructure

Sling Leg Tension and Angle Factor Calculator

What each leg of a multi-leg sling carries at the angle it hangs, and the share the heaviest leg takes when the centre of gravity is off the middle.

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Every formula citedNamed published standards
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Imperial · sales tax

Weight of the Load

The weight hanging below the hook, as weighed or as certified.

Use a weighed figure wherever one exists. A weight carried forward from a similar item, taken from a drawing that predates a design change, or estimated from a material list is the classic late discovery on a lift, and it is discovered at the worst possible moment — when the load is already off the ground.

Legs Sharing the Load

How many legs of the sling assembly are attached to the load.

Attaching four legs does not mean four legs carry. On a rigid load with fixed-length legs the assembly is statically indeterminate, and manufacturers and rigging practice both say to credit only two unless the arrangement demonstrably equalises. The breakdown reports that case alongside the equal-share one so the difference is visible rather than assumed away.

Sling Angle to the Horizontal (degrees)

The angle between a sling leg and the horizontal plane of the load.

Measured from the horizontal, so ninety degrees is a straight vertical pull and small angles are shallow, wide-spread legs. Tension climbs steeply as the angle falls, and common rigging practice sets a hard floor at thirty degrees. If a sling has to be shallower than that, the answer is a longer sling or a spreader beam, not a bigger sling.

Centre-of-Gravity Offset (% of the distance between lift points)

How far the centre of gravity sits from the midpoint, as a share of the span between attachment points.

Zero means the centre of gravity is exactly between the lift points and the load hangs level. Twenty percent means it sits a fifth of the span toward one side, which puts seventy percent of the weight on that side rather than fifty. Motors, transformers and anything with a heavy end are never symmetric, and a load that hangs visibly out of level on the first trial lift is telling you this number is not zero.

Tension in the most heavily loaded leg

1,270 lbf

Medium confidence

With more than two legs the equal-share figure is the optimistic case. Compare it against the two-leg line in the breakdown, and use the two-leg figure unless the arrangement genuinely equalises.

Load angle factor at this sling angle: 1.15 multiplier
Vertical share carried by the most heavily loaded leg: 1,102.5 lbf
Tension if only two legs are credited: 2,546.11 lbf
Minimum rated capacity needed for each leg: 2,546.11 lb
Sideways pull into the load at each attachment: 636.53 lbf

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

ASME B30.9, Slings — sling capacity is rated for the angle of use, and a multi-leg sling on a rigid load is not credited with sharing equally across more than two legs unless the arrangement is known to equalise
Leg tension = vertical share ÷ sin(angle to horizontal); the load angle factor is 1 ÷ sin(angle), so a leg at 30 degrees carries twice the vertical load it supports
Centre-of-gravity share: for lift points in symmetric pairs, the heavier side carries (0.5 + offset) of the load, where the offset is expressed as a fraction of the distance between the pairs

Inputs used

Weight of the Load: 4410 lb
Legs Sharing the Load: Four legs
Sling Angle to the Horizontal (degrees): 60
Centre-of-Gravity Offset (% of the distance between lift points): 0

Intermediate steps

Load angle factor at this sling angle: 1.15 multiplier
Vertical share carried by the most heavily loaded leg: 1,102.5 lbf
Tension if only two legs are credited: 2,546.11 lbf
Minimum rated capacity needed for each leg: 2,546.11 lb
Sideways pull into the load at each attachment: 636.53 lbf

Final result1,273.06 lbf

Confidence note: With more than two legs the equal-share figure is the optimistic case. Compare it against the two-leg line in the breakdown, and use the two-leg figure unless the arrangement genuinely equalises.

What this calculation does not cover

Sizes the sling legs only. Shackles, the master link, the lifting points on the load and the structure behind them each have their own rating, and the assembly is governed by the smallest of them.
Applies no design factor. A sling's working load limit already includes one, so the tension here is compared against the rated capacity at the angle of use, never against a breaking strength.
Static only. Snatching a load, freeing one that is stuck, hoisting fast or swinging hard all put forces through the rigging that this figure does not contemplate.
Says nothing about the crane. Rigging weight is a deduction from the chart capacity, and this page does not compute what remains for the load.

Add the equipment this sizes

This result is a specification — 1,270 lbf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

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Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations3

ASME B30.9, Slings — sling capacity is rated for the angle of use, and a multi-leg sling on a rigid load is not credited with sharing equally across more than two legs unless the arrangement is known to equalise
Leg tension = vertical share ÷ sin(angle to horizontal); the load angle factor is 1 ÷ sin(angle), so a leg at 30 degrees carries twice the vertical load it supports
Centre-of-gravity share: for lift points in symmetric pairs, the heavier side carries (0.5 + offset) of the load, where the offset is expressed as a fraction of the distance between the pairs

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How to calculate sling leg tension and angle factor in 5 steps

Weight of the LoadThe weight hanging below the hook, as weighed or as certified.
Legs Sharing the LoadHow many legs of the sling assembly are attached to the load.
Sling Angle to the Horizontal (degrees)The angle between a sling leg and the horizontal plane of the load.
Centre-of-Gravity Offset (% of the distance between lift points)How far the centre of gravity sits from the midpoint, as a share of the span between attachment points.
Tension in the most heavily loaded legThe tool computes the tension in the most heavily loaded leg from those figures and shows the formula, its sources, and a confidence rating alongside it.

Tension in the most heavily loaded leg by weight of the load

Page defaults, not your figures above.

Weight of the Load	Tension in the most heavily loaded leg (lbf)
2,000 lb	577
3,000 lb	866
4,000 lb	1,155
5,000 lb	1,443
6,000 lb	1,732
7,000 lb	2,021
8,000 lb	2,309

Frequently asked questions

Why does a shallower sling angle increase the tension so sharply?: Because the vertical component of a leg's tension is what actually holds the load, and that component is the tension multiplied by the sine of the angle. As the angle falls, the sine falls with it, so the tension has to rise to keep the vertical component the same. At thirty degrees the sine is a half, so each leg carries twice the vertical load it is supporting — and the horizontal pull it exerts on the load exceeds the lift.
Why should I not credit all four legs of a four-leg sling?: Because on a rigid load with fixed-length legs, the arrangement is statically indeterminate: manufacturing tolerance, a slightly out-of-square load and a few millimetres of stretch decide which legs take the weight, and it is rarely all four evenly. Common rigging practice, and the manufacturers' own guidance, is to assume two legs carry unless the assembly can demonstrably equalise. The breakdown gives you both numbers so the assumption is a choice rather than an accident.
What does the sideways pull line mean for the load itself?: It is the horizontal force each leg applies to its attachment point, pulling inward toward the centre of the load. On a fabricated item with lifting lugs, that force tries to bend the lugs and crush the item between them; on a bundle it tries to close the bundle up. Shallow angles make it large, and it is the reason a spreader beam is sometimes the only sensible answer rather than a refinement.
Does this account for the weight of the rigging itself?: No. Enter the weight of the load, and remember separately that the whole rigging assembly — slings, shackles, spreader, chain blocks, tag lines — is a deduction from the crane's chart capacity along with the hook block, the rope below the boom head and any jib fitted. The omitted spreader beam is the classic deduction people forget, and it is often the heaviest single item in the assembly.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.