Tension per sling leg, from a tape measure.
Sling tension climbs silently as your angle drops. At 30° each leg carries double the share of the load. Enter the height and leg length you already have on your tape. Read tension per leg, the tension factor, and a color-coded danger warning in under 30 seconds.
The silent multiplier
Drop the angle and per-leg tension does not rise gently. It accelerates. The legs are not just holding the load up, they are pulling against each other. At 30° from horizontal, each sling leg carries DOUBLE the share of the load weight.
Where α is the angle from horizontal. The flatter the sling, the closer sin(α) creeps toward zero, and the tension factor toward infinity.
| Angle | Factor | Zone |
|---|---|---|
| 90° (vertical) | 1.00× | Safe |
| 60° | 1.15× | Safe |
| 45° | 1.41× | Caution |
| 30° | 2.00× | Caution |
Sling tension reference
Tension per sling leg is set by one ratio: how far the legs lean off vertical. It climbs far faster than the geometry looks like it should. This reference tabulates the numbers a rigger reaches for around that fact: the angle-to-factor chart, the height-and-leg-length ratios that reach the same factor without a protractor, hitch and leg-count derates, and the standards that govern the call.
Sling angle factor chart
Tension factor is 1/sin(angle from horizontal). Multiply the per-leg share of the load (total weight divided by number of legs) by the factor to get the actual tension in one leg. Below 45° the factor accelerates; below 30° it passes 2.0 and keeps climbing.
| Angle from horizontal | Tension factor (1/sinθ) | Per-leg tension, 2,000 lb on 2 legs | Zone |
|---|---|---|---|
| 90° (vertical) | 1.000 | 1,000 lb | Safe |
| 75° | 1.035 | 1,035 lb | Safe |
| 60° | 1.155 | 1,155 lb | Safe |
| 50° | 1.305 | 1,305 lb | Safe |
| 45° | 1.414 | 1,414 lb | Caution |
| 40° | 1.556 | 1,556 lb | Caution |
| 35° | 1.743 | 1,743 lb | Caution |
| 30° | 2.000 | 2,000 lb | Caution |
| 25° | 2.366 | 2,366 lb | Danger |
| 20° | 2.924 | 2,924 lb | Danger |
| 15° | 3.864 | 3,864 lb | Danger |
Most rigging programs treat 30° as a hard floor: at that angle each leg already carries the entire load weight. Many cap field picks at 45°.
Reading tension from height and leg length
When you have a tape but no inclinometer, skip the angle entirely. With vertical height H from the load up to the hook and sling leg length Lg, the tension factor is Lg ÷ H with no trig at all. The chart below turns that ratio straight into a factor and the angle it implies (arcsin H/Lg).
| Lg ÷ H ratio | Tension factor | Implied angle | Zone |
|---|---|---|---|
| 1.00 | 1.00 | 90° | Safe (vertical) |
| 1.15 | 1.15 | 60° | Safe |
| 1.31 | 1.31 | 50° | Safe |
| 1.41 | 1.41 | 45° | Caution |
| 1.74 | 1.74 | 35° | Caution |
| 2.00 | 2.00 | 30° | Caution |
| 2.92 | 2.92 | 20° | Danger |
Formulas:
Worked example, a 6,000 lb beam on a 2-leg bridle with 4 ft of height to the hook and 8 ft slings:
Each leg holds the full 6,000 lb at that geometry. A height larger than the leg length is impossible, so recheck the tape if the tool rejects it.
Hitch and leg-count load factors
Angle is one derate. Hitch type and how legs actually share the load are two more the per-leg factor never captures on its own. Rated sling capacity itself changes with the hitch:
| Hitch type | Capacity vs. single vertical | Note |
|---|---|---|
| Vertical (straight) | 1.00× | Baseline rated capacity |
| Choker | 0.75–0.80× | A choke angle under 120° drops it further |
| Basket, legs vertical | 2.00× | Only when both legs run truly vertical |
| Basket, legs angled | 2.00× × sin(angle) | Same angle penalty as a bridle |
| Legs | Assumed share | Field reality |
|---|---|---|
| 2 | 50% each | Close on a rigid, level load |
| 3 | 33% each | Rarely even; treat as a floor |
| 4 | 25% each | Two opposite legs often carry it all; design for two |
On three- and four-leg picks the equal-share figure is a planning floor, not a guarantee. The load can ride on two legs if the others go slack or the load is stiff.
Standards, codes, and limits
| Reference | Scope |
|---|---|
| ASME B30.9 | Slings: design factors, sling-angle effects, inspection, tagging |
| OSHA 1926.251 | Rigging equipment for material handling (construction) |
| OSHA 1910.184 | Slings (general industry) |
| ASME B30.10 | Hooks |
| Design factor 5:1 | Standard ratio of breaking strength to working load limit for slings |
The working load limit (WLL) stamped on the tag already folds in the design factor. The angle factor on this page is applied on top of the load, never in place of the WLL. Verify per-leg tension against the tag's rated capacity before every lift.
Related concepts
- Working load limit (WLL): the maximum load a sling may carry, its breaking strength divided by the design factor (typically 5:1). Stamped on every tag.
- D/d ratio: the bend-diameter to rope-diameter ratio; sharp bends over small pins cut a wire-rope sling's effective capacity.
- Center of gravity: an off-center COG shifts more load onto the nearer legs, breaking the equal-share assumption before angle even enters.
- Sling angle vs. included angle: field charts use the angle from horizontal; some load charts state the included angle between the two legs. They are not interchangeable.
- Shock loading: sudden starts, stops, or snatch picks spike tension well past the static factor. Rig for a smooth lift.