What this calculator does
A lift plan goes wrong in one of two places: the crane picks up more than its rating at the radius you are working at, or the rigging multiplies the load by geometry nobody wrote down. The second is the one that surprises people. A sling pair hung at 60° carries twice the load that the same pair carries vertically, and the lift ticket will not tell you that.
How to use it
- Enter the weight being lifted and the rigging weight separately, or enter the gross weight and leave the rigging at zero. Do not count the rigging twice.
- Enter the number of legs and the angle of each leg FROM VERTICAL — not from horizontal. A vertical lift is 0°; the usual 45° bridle is already a 1.41× rise before any leg is added.
- Enter the crane rating AT YOUR RADIUS from the load chart. Rating at minimum radius is not the number to plan a lift with.
- Read the rigging check first. A sling at its working limit drops the load; a high crane utilisation is a planning problem, not a failure.
Governing formula
T_leg = W / (n · cos β) → with rigging efficiency: T = W / (n · cos β · η) Where:
-
W= Gross lifted weight, N -
n= Number of sling legs -
β= Angle of each leg from the vertical, degrees -
η= Rigging efficiency, applied for n ≥ 3 only
Derived Equations:
angle rise = 1 / cos β per-leg share = 1 / (n · cos β) plan radius = h · tan β Worked example — 2,000 kg on a 2-leg bridle at 30°
The figures below were produced by running this page's own engine with the example inputs (Weight being lifted (kg), Rigging weight (kg), Number of sling legs (no), Sling angle from vertical (°), Rigging efficiency (3 legs or more) (–), Working radius at the hook (m), Crane rated capacity at this radius (kg), Required sling safety factor (–), One sling leg capacity (kg), Hook height above the load (m), Clearance to nearest obstruction (m)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Tension in one leg | 11328 N |
|---|---|
| Force rise from sling angle | 1.15 × |
| Share of the load per leg | 0.58 % |
| Rigging within its working limit | — |
| Crane within its rating | — |
| Sling utilisation | 38.49 % |
| Crane utilisation | 40 % |
| Spread the geometry needs | 1.15 m |
| Fits the available clearance | — |
| Sling length required | 2.31 m |
| Safety factor achieved | 2.6 – |
| Gross lifted weight | 2000 kg |
| What this means | The rigging is the governing problem: each leg sees 11.3 kN against a 3000 kg leg, which is 38 % of its capacity and short of your 5:1 requirement. Steepen the sling angle, add legs, or use bigger gear. At 30° from vertical the angle alone multiplies the force in every leg by 1.15×, and with 2 legs each one carries 57.7 % of the load — 1155 kg against a gross weight of 2000 kg. That geometry needs 2.3 m of sling per leg and puts the load 1.2 m out from the hook. These are STATIC figures. Wind on the load, the snatch when a sling comes tight, and swing are not included, and a real lift plan applies a dynamic factor on top. |
- The angle rise is 1/cos(30°) = 1.155, so the 2,000 kg becomes 2,309 kg of pull.
- Each of two legs carries half of that: 1,155 kg, against a 3,000 kg leg — inside the limit, and the rigging efficiency is correctly ignored for a 2-leg lift.
- The crane sees 2,000 kg against 5,000 kg rated at 10 m, so 40 % of capacity.
- The geometry puts the load 2 · tan(30°) = 1.15 m out from the hook, inside the 3 m available.
Assumptions & limitations
Engineering calculation. Static equilibrium for an n-leg bridle, with the 1/cos(β) angle rise kept separate from the per-leg share, and the rigging efficiency applied for 3 legs or more only.
- Crane rated capacity at radius, sling leg capacity, sling angle, rigging efficiency and safety factors are all USER inputs. This tool holds no load chart, no crane directory and no sling catalogue, and looks nothing up.
- The capacity entered is a chart value at the radius you entered. Real charts also depend on boom length, jib configuration, counterweight and outrigger spread, none of which are modelled here.
- Everything is STATIC. Dynamic amplification, wind on the load, snatch loading and swing are excluded, and a real lift plan applies a dynamic factor on top of the static result.
- The load must already include the rigging weight, or be entered separately in the rigging field. The hook block, shackles and sling mass are not known to this tool.
- A capacity utilisation under 100 % is not an approval. Ground bearing pressure, outrigger pads, access, overhead lines, exclusion zones and the lift supervisor are outside what this tool can check.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS 7121-1 — safe use of cranes, as the framework for lift planning.
- EN 13414 — steel wire rope slings, safety factor context.