Rigging Calculator
Sling angle to tension per leg, a load-angle factor table, a WLL/hitch check, top-block and gin-pole sheave load, capstan hoist pull, and a simple center-of-gravity — the numbers a tower rigger needs on site, free and instant, no account.
Free account unlocks the full tower Rigging Plan Calculator (equipment checks, obstruction clearance, printable PDF) — see the signup card below the calculators.
Sling Angle → Tension per Leg
2, 3 or 4-leg bridle — tension climbs fast below 45°.
Load Angle Factor Table
Factor = 1 / sin(angle). Below 30° is the danger zone — each leg can near the full load.
| Angle from horizontal | Load angle factor | 1,000 lb load → tension/leg* |
|---|---|---|
| 90° | 1 | 1,000 lb |
| 75° | 1.04 | 1,035 lb |
| 60° | 1.15 | 1,155 lb |
| 50° | 1.31 | 1,305 lb |
| 45° | 1.41 | 1,414 lb |
| 40° | 1.56 | 1,556 lb |
| 30° | 2 | 2,000 lb |
| 20° | 2.92 | 2,924 lb |
| 15° | 3.86 | 3,864 lb |
*2-leg bridle, full load on one leg (worst-case single-leg reference). Use station 01 above for your exact leg count.
Sling WLL / Hitch Check
Tag rating × hitch factor vs. the tension the leg actually sees.
The manufacturer's tag always governs. Choker/basket factors are typical values — verify against the actual sling tag.
Top Block / Gin Pole Sheave Load
At 0° (line folds straight back) the block sees 2× the line tension.
Size the block, its sling and shackle for this resultant — not for the line tension alone.
Capstan Hoist Pull
Line pull the capstan must develop, accounting for sheave bearing friction.
Capstan and anchorage should be load-tested to 1.5× the maximum anticipated line pull, or engineered with a safety factor ≥ 2.0 — see the full Rigging Plan Calculator.
Simple Center of Gravity
Weighted average along one axis — spreader bar / multi-point pick balance point.
Single-axis statics (sum of moments about the reference point). For an irregular load, the true CoG may be off-axis — verify with a trial lift.
This tool assists a qualified/competent rigger. It does not replace engineering judgment or review by a licensed professional engineer, and it is not an OSHA-certified or OSHA-approved product — no training program or certification is implied. Verify every input and result against the equipment's actual rated tags and your company's rigging plan before a lift. Calculations follow engineering methods consistent with ANSI/ASSP A10.48 and ASME B30.9 methodology.
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Questions tower crews actually ask
Is this rigging calculator really free?
Yes. The sling-angle, load-factor, WLL/hitch, block-load, capstan-pull and center-of-gravity calculators on this page work instantly with no account. Creating a free account additionally unlocks the full tower Rigging Plan Calculator — equipment checks, obstruction clearance and a printable PDF plan — at no cost while we build out the product.
How do I calculate tension per leg on a sling?
Tension per leg = (load ÷ number of legs carrying it) ÷ sin(angle from horizontal). A 2-leg bridle at 60° carries each leg at about 1.15× its share of the load; at 45° that climbs to 1.41×, and at 30° each leg is carrying the full load. For 3- and 4-leg bridles, standard conservative practice assumes only 2 legs actually share the load — fabrication tolerances make perfectly even sharing unlikely.
What is a safe sling angle for rigging?
Keep sling angles at 45° or steeper from horizontal whenever the geometry allows. Below 30° the load angle factor accelerates sharply (tension can exceed the load itself), and below 15° the math is considered unbounded and rigging at that angle is unsafe — this calculator blocks that case rather than returning a number.
Why does a block or gin pole sheave see more force than the rope pull?
A block or sheave feels both rope legs passing over it. When the rope folds straight back on itself (0° between the legs) the block sees close to 2× the line tension. As the angle between the legs opens up toward 180° (rope running straight through), the force on the block drops toward the line tension itself.
How do you calculate the pull needed on a capstan hoist?
Capstan line pull equals the load on the line, multiplied by a friction factor for each sheave the rope passes over before reaching the capstan (roughly 1.02 per steel roller bearing sheave, higher for bronze or plain bushings). The capstan and its anchorage should then be verified — operationally load-tested to 1.5× the maximum anticipated pull, or engineered to a safety factor of at least 2.0.
Is this the same rigging plan calculator used for A10.48 plans?
The instant calculators on this page run the exact same calculation library (src/lib/rigging) as our full Rigging Plan Calculator, which builds an ANSI/ASSP A10.48 and ANSI/TIA-322-style tower rigging construction plan (Class II/III/IV) with a printable PDF. A free account unlocks that full tool.
Take it further
This tool assists a qualified/competent rigger and does not replace engineering judgment or review by a licensed professional engineer. It is not an OSHA-certified or OSHA-approved product and does not issue a certification. Calculations follow engineering methods consistent with ANSI/ASSP A10.48 and ASME B30.9 methodology; verify every input and result against the equipment's actual rated tags before a lift.