Choosing the right outrigger pad comes down to one question: is the pad big enough to spread the load without exceeding the ground's bearing capacity? This guide shows you how to calculate the minimum pad size for any crane, MEWP, or plant equipment. No specialist software needed.
What You Need Before You Start
Two pieces of information: the maximum outrigger load in tonnes and the ground type at the outrigger positions.
The outrigger load comes from the crane duty chart, the outrigger reaction force table, or the manufacturer's load data. For MEWPs and smaller plant, it is usually in the machine specification or IPAF operator's guide. If you cannot find it, divide the gross vehicle weight by the number of outriggers as a conservative starting point.
The ground type is assessed on site. Walk each outrigger position. Look at the surface, check for soft spots, recent excavation, underground services, or areas where the ground may have been backfilled.
The Calculation — Step by Step
Ground Pressure (kN/m²) = Load (kN) ÷ Pad Area (m²)
Where Load (kN) = Tonnes × 9.81
Step 1: Convert the outrigger load to kilonewtons. Multiply tonnes by 9.81. Example: 15 tonnes × 9.81 = 147.2 kN.
Step 2: Identify the ground bearing capacity for your ground type. Use the reference below to find your ground condition:
Concrete / Tarmac / Rock
400 kN/m²
Dense Gravel or Hardcore
400 kN/m²
Compact Sand / Stiff Clay
300 kN/m²
Loose Gravel / Firm Clay
150 kN/m²
Medium Sand
100 kN/m²
Soft Clay / Silt / Topsoil
75 kN/m²
Loose Sand / Made Ground
50 kN/m²
Unknown
50 kN/m² (conservative default)
Step 3: Calculate the minimum pad area. Divide load (kN) by ground capacity. Example: 147.2 / 75 = 1.96 m².
Step 4: Add a 25% safety margin. Example: 1.96 × 1.25 = 2.45 m².
Step 5: Select a pad that meets or exceeds the required area.
Pad Areas for Quick Reference
Square Pads
400×400 = 0.16m² | 500×500 = 0.25m² | 600×600 = 0.36m² | 800×800 = 0.64m² | 1000×1000 = 1.00m² | 1200×1200 = 1.44m² | 2000×1200 = 2.40m²
Round Pads
800mm = 0.50m² | 900mm = 0.64m² | 1000mm = 0.79m² | 1200mm = 1.13m²
Worked Examples
Mobile Crane on Grass
40 tonne crane, 22 tonne outrigger load, grass and topsoil (75 kN/m²). Load: 215.8 kN. Minimum area: 2.88m². With 25% margin: 3.60m². The 1200mm square (1.44m²) is not enough. The 2000×1200mm large format (2.40m²) is still below the required area with margin. This lift needs engineered ground improvement or timber cribbing.
MEWP on Compacted Gravel
3.5 tonne scissor lift, 3 tonne outrigger load, gravel (400 kN/m²). Load: 29.4 kN. Minimum area: 0.07m². A 300×300mm Essential pad meets the requirement. The 400×400mm pad is the sensible everyday choice.
Crane on Firm Clay
18 tonne outrigger load, firm clay (150 kN/m²). Load: 176.6 kN. Minimum area: 1.18m². With 25% margin: 1.47m². The 1200×1200×80mm square (1.44m²) just passes on firm conditions. Step up to the 2000×1200mm if there is any doubt.
Common Mistakes to Avoid
Using GVW Instead of Outrigger Load
Load distribution varies depending on boom radius and configuration. Always use the duty chart figure.
Ignoring Ground Variability
One outrigger may be on hard standing while another is on soft fill. Assess each position individually.
Assuming Good Weather Means Good Ground
Clay can look firm on the surface but have poor bearing capacity underneath after rain.
Forgetting the Safety Margin
The minimum calculation does not account for dynamic loading or ground deterioration. Always add 25%.
Free Tool
Pad Size Calculator
The calculator does this automatically. Enter load and ground type, get an instant recommendation from the CPS range.
Free Tool
Ground Bearing Calculator
Already have a pad? Check whether it keeps ground pressure within safe limits for your specific conditions.
Run the Calculation Now
Use the free Pad Size Calculator for any crane or MEWP. Takes 30 seconds and links directly to the product page.
