Center of Gravity Rigging Formula for a Two Point Lift

When you need it

Valves, pumps, skids and fabricated parts rarely have their center of gravity sitting in the middle. We rigged a lopsided skid with equal slings once, out of habit, and watched it hang nose-down the second it cleared the ground — the motor end was heavier and nobody had checked. Knowing each leg's load ahead of time lets you pick slings that actually match what they'll carry.

First, where is the center of gravity?

Many searches for calculating center of gravity for rigging are really about finding it. That comes from the manufacturer's drawings or data, a marked CoG on the equipment, or a calculation of the parts' weights and positions by an engineer. RigMaster does not locate the CoG: you tell it how far each lift point is from it, measured horizontally.

The center of gravity rigging formula

d1, d2 = horizontal distance from lift point 1 and lift point 2 to the CoG.
Vertical share of leg 1 = load × d2 ÷ (d1 + d2). Vertical share of leg 2 = load × d1 ÷ (d1 + d2).
With the hook over the CoG, the angles are linked: tan θ2 = d1 × tan θ1 ÷ d2. The far leg always ends up at the flatter angle.
Tension of each leg = its vertical share ÷ sin(its horizontal angle).

Worked example: 2,000 lb, lift points 3 ft and 5 ft from the CoG

RigMaster Unequal Legs result, leg 1 set at 60°
	Leg 1 (3 ft from CoG)	Leg 2 (5 ft from CoG)
Horizontal angle	60° (set)	46.1° (derived)
Vertical share	1,250 lb	750 lb
Tension	1,443 lb	1,041 lb

Leg 1 is closer to the CoG, so it takes 62.5% of the weight and the highest tension even though it hangs steeper. The same pick in metric, 1,000 kg with points 1 m and 1.5 m away and leg 1 at 60°, gives leg 2 at 49.1°, shares of 600 kg and 400 kg, and tensions of 693 kg and 529 kg.

Two point lift calculation in RigMaster

Open Unequal Legs / CoG Offset from Pro Calculators on the main screen.
Type the Total Load Weight.
Under Leg 1, type the Horizontal Distance to CoG and set the Horizontal Angle.
Under Leg 2, type its Horizontal Distance to CoG. The app derives leg 2's angle and shows it read-only.
Read Leg 1 Tension, Leg 2 Tension, Leg 1 Vertical Share and Leg 2 Vertical Share. The badge shows the worse of the two angles.
Tap Save or Lift Plan to record it.

Assumptions and common mistakes

The app assumes the hook is directly over the center of gravity and the load hangs level. A hook that is off to one side makes the load tilt until it is.
Distances are horizontal, not measured along the load's surface or along the slings.
Two legs only. For 3- or 4-leg picks on an off-center load, get an engineered plan.
Check the flatter leg's angle. If it falls under 45° or 30°, the badge turns yellow or red even when leg 1 looks fine.

Questions and answers

Which sling takes more load when the CoG is off center?

The one attached closer to the center of gravity. In the example above, the point 3 ft away carries 1,250 lb of a 2,000 lb load and the point 5 ft away carries 750 lb.

Can RigMaster calculate the center of gravity?

No. It needs each lift point's horizontal distance to the CoG, which comes from drawings, the manufacturer or a marked CoG.

Why are the two sling angles different?

Both legs hang from one hook over the CoG, so the leg farther away must lean over more. The app derives that angle for you.

Is this calculator free?

No, Unequal Legs / CoG Offset is part of RigMaster Pro.

4-leg sling load calculation, and why a rigid load may hang on only 2 legs · RigMaster

Off-center center of gravity: leg loads and tensions on a two-point lift