Sling angle calculator
Enter the load, the number of sling legs and the included angle between them. The calculator works out the tension in each leg, checks it against the BS EN mode-factor rating and the sling's marked WLL, and warns you when the angle is unsafe.
Free to use — no signup; everything stays in your browser. Use as a planning aid, then review against the actual site.
Mode-factor maths, live
watch the angle
result updates as you add entries
Lift details
Enter the load and how it's slung. As the legs spread, the tension in each one rises far faster than most people expect.
Wider legs = sharper angle = far higher tension in each leg.
Why the sling angle matters so much
When a load hangs on two or more sling legs, the legs do not simply share the weight equally downwards. As the legs spread apart, each one has to pull both up and inwards — so the tension along the leg climbs steeply as the included angle (the angle measured at the top, between opposite legs) widens. Misjudging this is a common cause of sling overload and dropped loads.
The two checks this tool shows
1. Leg tension (the physics). For a symmetrical lift the load is taken by two legs, and the tension in each is (load ÷ 2) ÷ cos(½ × included angle). The multiplier on the even share is the tension factor:
| Included angle | Tension factor | Effect |
|---|---|---|
| 30° | 1.04 | Barely more than the even share |
| 60° | 1.16 | Recommended working zone |
| 90° | 1.41 | Recommended maximum |
| 120° | 2.00 | Each leg carries the whole load — absolute maximum |
Beyond 120° the figures run away: at about 150° a single leg carries more than the entire load. That is why the included angle must never exceed 120°, and 90° is the recommended working maximum.
2. The mode-factor rating check. Riggers rate an assembly from the single-leg Working Load Limit (WLL) using a mode factor: a two-leg sling up to 90° included is rated at 1.4 × the single-leg WLL, dropping to 1.0 between 90° and 120°. Three- and four-leg slings are conventionally rated the same as two legs, because an offset centre of gravity means only two legs can be relied on to carry the load. The tool shows both the actual leg tension and the minimum single-leg WLL the lift needs.
What still has to happen on site
This is a planning aid, not the lift plan. The sling's own tag marking governs, and the lift must be planned, supervised and carried out by a competent appointed person under LOLER. Confirm the load weight and centre of gravity, keep the tag legible, and only use accessories that are within their thorough-examination date (six-monthly for lifting accessories).
Sources: HSE, Lifting Operations and Lifting Equipment Regulations 1998 (LOLER); BS 7121 Safe use of cranes; BS EN 1492-1 (textile slings) and BS EN 818-4 (chain slings) mode-factor convention; and Lifting Equipment Engineers Association (LEEA) guidance.
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