MasaApps360
English
RigMaster

How to calculate sling tension in each leg of a 2-leg sling

Divide the load by the number of legs carrying it, then multiply by the sling angle factor, 1 ÷ sin of the horizontal angle. A 2,000 lb load on two legs at 60° puts 1,155 lb on each leg. RigMaster's free Sling Tension calculator does this live and warns you as the angle drops.

3 min read · Updated 2026-09-16

Download on the App Store

Why sling tension is higher than the load share

When a sling hangs straight down, each of two legs carries half the load. Spread the legs out and they start pulling sideways against each other as well as holding the load up. The tension climbs faster than it looks like it should as the angle drops, and the increase gets especially steep once the angle goes below 45°.

A sling rated for half the load's weight can still end up overloaded for exactly that reason. What matters is the tension in the leg, not the load's weight divided by two — compare it against the vertical rating stamped on the tag.

The sling tension formula for lifting

Tension per leg = load ÷ legs carrying × angle factor, where the angle factor is 1 ÷ sin(θ) and θ is the horizontal angle: the angle between the sling leg and the horizontal.

Worked example: 2,000 lb on a 2-leg sling

2-leg sling, 2,000 lb total load, as shown in RigMaster
Horizontal angleAngle FactorTension per LegBadge
60°1.1551,155 lbAngle OK
45°1.4141,414 lbAngle OK
30°2.0002,000 lbyellow caution

At 30° each leg carries the full weight of the load, not half of it. Going from 60° to 30° raises the tension by about 73%.

Same pick in metric: 1,000 kg total load, 2 legs
Horizontal angleAngle FactorTension per Leg
60°1.155577 kg
45°1.414707 kg
30°2.0001,000 kg

How to calculate sling tension in RigMaster

  1. Open Sling Tension from the Calculators list on the main screen.
  2. Under Load, type the Total Load Weight (the app starts from 1,000 lb or 500 kg the first time).
  3. Keep Sling Legs on 2.
  4. Under Sling Angle, leave Input on Angle and drag the Horizontal Angle slider, or tap one of the 30°, 45°, 60° and 90° buttons. If you can measure the sling instead, pick Length & Height.
  5. Read the Result: Tension per Leg, Angle Factor and Legs Assumed Carrying, with the diagram showing the angle and the tension on each leg.
  6. Tap Save to keep the calculation in History.

The slider runs from 15° to 90° in 1° steps. Switch between pounds and kilograms from the Units menu in the toolbar; every field converts at once.

What do the colour warnings mean?

  • 45° and above: Angle OK.
  • 30° up to just under 45°: a yellow Below 45° — tension rises fast. Verify sling ratings. An angle of exactly 30° falls in this band.
  • Below 30°: a red Below 30° — NOT permitted by standard rigging practice. The number stays on screen so you can see how bad it is, but the placard tells you to re-rig.
  • Below 5°: no result.

Tips and common mistakes

  • Compare the tension with each leg's own tag. Every leg needs a vertical rated capacity at least equal to the Tension per Leg shown.
  • Do not guess the angle. A 10° error near 30° changes the factor far more than the same error near 60°. Measuring sling length and height is more reliable.
  • Use the real load weight, including anything bolted to or stored in the load.
  • Longer slings raise the angle and cut the tension. If the numbers are high, a longer sling or a spreader arrangement is usually the fix, planned by a competent person.

Questions and answers

What is the sling tension formula?
Tension per leg = load ÷ number of legs × 1 ÷ sin(horizontal angle). For 2,000 lb on two legs at 45°, that is 1,000 × 1.414 = 1,414 lb per leg.
Is there a sling tension calculator in metric?
Yes. RigMaster works in kilograms and metres as well as pounds and feet. 1,000 kg on two legs at 60° gives 577 kg per leg.
Is the sling tension calculator free?
Yes for 2-leg slings, by angle or by length and height, with ads. 3- and 4-leg bridles need the optional Pro upgrade.
Why does the tension equal the whole load at 30 degrees?
Because sin 30° is exactly 0.5, so the factor is 2. Half the load times 2 is the full load on each leg, which is why flatter angles are avoided.

You might also like

More RigMaster guides