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Mobile crane outrigger on spreader mat
Lifting & Rigging Calculator

Ground Bearing Pressure Calculator

Calculate outrigger ground bearing pressure in tons per square meter for mobile crane lifts, spreader mats and critical lift planning.

Lift Parameters
Enter crane, load, and mat details below.
t

Total crane weight in tonnes

t

Weight of the load being lifted

Outrigger Mat Dimensions

m

Length of one outrigger mat

m

Width of one outrigger mat

Ground Bearing Pressure Results

Enter parameters and calculate to see results.

How the Ground Bearing Pressure Calculator Works

Ground bearing pressure is calculated using a simple formula: Load ÷ Contact Area. When a crane lifts a load, the combined weight of the crane, the load, and all rigging is transferred through the outriggers to the ground.

  • One-outrigger case (Worst Case): This assumes 100% of the total load is concentrated over a single outrigger. This is the highest pressure scenario and must always be checked for safety.
  • Two-outrigger case: When lifting over the rear or side, the load is spread over two outriggers. This reduces the pressure because the total contact area doubles.
  • Mat size matters: Larger mats spread the load over a wider area, reducing the pressure on the ground. Smaller mats increase the pressure.
Note: This calculator provides planning estimates only. Final approval must come from a site assessment, an engineered lift plan, or a competent lifting person.

What Is Included in Total Considered Load

To get an accurate pressure reading, every piece of weight that transfers to the ground must be accounted for. The total considered load includes:

Crane Weight

The total weight of the crane, including all counterweights installed for the lift.

Lifted Load

The actual weight of the object being lifted.

Hook Block & Rigging

The weight of the hook block, slings, shackles, and any spreader bars used.

Outrigger Mats

Large steel or timber mats can weigh several tonnes and add to the total ground force.

Why it matters: Forgetting items like a heavy hook block or spreader beam leads to underestimated ground pressure, which can cause ground failure. Always use actual weights when available.

Common Mistakes

Forgetting to add the hook block or lifting gear weight to the total load.
Using the wrong mat dimensions (e.g., using the outrigger float size instead of the spreader mat size).
Ignoring the one-outrigger worst-case scenario and only planning for shared loads.
Assuming this calculator replaces actual crane manufacturer outrigger reaction data.
Using the pressure results without comparing them to site-approved allowable ground bearing values.
Assuming that using mats automatically makes weak ground safe without a proper site assessment.

Worked Example

Crane weight: 48 t

Load weight: 28.5 t

Hook block: 1.5 t

Lifting gear: 0.54 t

Mat size: 2 m × 2 m = 4 m² per mat

Total considered load48 + 28.5 + 1.5 + 0.54 = 78.54 t

Worst case over one outrigger:

Pressure = 78.54 ÷ 4 = 19.64 t/m²

Load shared over two outriggers:

Pressure = 78.54 ÷ 8 = 9.82 t/m²

The worst-case pressure is 19.64 t/m² when the full load is on one mat. When spread over two mats, the pressure drops to 9.82 t/m² because the total contact area doubles.

Safety Note

  • For critical lifts, actual manufacturer outrigger reactions or engineer-approved lift data should take priority when available.
  • This tool provides planning estimates and does not replace a competent lifting person, engineer, or site ground assessment.
  • Always verify calculations with the crane load chart, lift plan, and actual site ground conditions before commencing any lift.