16 October, 2026 | 
Admin
Outrigger Pad Sizing Methodolo
Read more
Outrigger Pad Sizing Methodology — The Complete Technical Guide
16 October, 2026 | Admin
Outrigger pad sizing comes down to one relationship: ground bearing pressure equals load divided by contact area, and that pressure has to stay below what the ground can safely bear. Everything else — safety factors, ground type, equipment class — feeds into that one calculation. This guide walks through the full methodology step by step, with worked examples, and pairs with our free online calculator for a fast answer once you understand the logic.
The Core Formula
Required pad area (m²) = (Outrigger load × safety factor) ÷ allowable ground bearing pressure
Load is taken from your equipment’s load chart in tonnes, converted to kilonewtons by multiplying by 9.81. Ground bearing pressure is expressed in kilopascals (kPa), and varies significantly by ground type.
Step 1: Determine Your Actual Load
Use the maximum outrigger reaction force from your equipment’s load chart — not the machine’s total weight, and not a rough estimate. This figure changes with boom angle, radius, and configuration, so identify the worst-case value across the working range you’ll actually use on this job. For crane work specifically, this is usually listed directly on the load chart as the maximum outrigger force per leg.
Step 2: Assess Ground Bearing Capacity
Ground bearing capacity varies enormously by soil type. The table below gives typical, indicative reference ranges — always confirm with site-specific geotechnical data for critical or unusual lifts.
| Ground Type | Typical Bearing Pressure | Description |
| Rock / compacted aggregate | ~300 kPa | Hard-standing, well-compacted base |
| Compacted fill | ~200 kPa | Prepared, compacted general site ground |
| Sandy / loose soil | ~120 kPa | Desert sand, uncompacted ground |
| Sabkha / soft / reclaimed | ~75 kPa | Reclaimed land, salt-flat, weak ground |
If you’re not certain which category your site falls into, treat it as the more conservative (lower bearing capacity) category — oversizing a pad is a minor cost; undersizing is a safety risk.
Step 3: Apply a Safety Factor
A safety factor accounts for uncertainty in actual ground conditions, dynamic loading during a lift, and the consequences of an error. Standard practice for most jobs is 1.5×. Less-understood or higher-risk ground conditions warrant the more conservative end, up to 2.0×.
Step 4: Calculate Required Area and Choose a Pad Size
With load, ground bearing pressure, and safety factor established, the formula gives you the minimum required contact area. From there, match to the nearest standard pad size that meets or exceeds that area — square pads are sized directly by side length; round pads by equivalent-area diameter.
Worked Example 1: Mobile Crane on Sandy Ground
A mobile crane’s load chart shows a maximum outrigger reaction of 18 tonnes at the planned working radius. The site is desert sand — sandy/loose soil, ~120 kPa. Using a standard 1.5× safety factor:
- Load in kN: 18 × 9.81 = 176.6 kN
- Required area: (176.6 × 1.5) ÷ 120 = 2.21 m²
- Equivalent square side: √2.21 = ~1.49m → round up to the next standard size, 1500×1500mm
Worked Example 2: Concrete Pump Truck on Compacted Fill
A concrete pump truck’s outrigger load is 8 tonnes on a compacted fill site (~200 kPa), using a standard 1.5× safety factor:
- Load in kN: 8 × 9.81 = 78.5 kN
- Required area: (78.5 × 1.5) ÷ 200 = 0.59 m²
- Equivalent square side: √0.59 = ~0.77m → round up to 800×800mm
Worked Example 3: Heavy Crane on Reclaimed Land
A larger mobile crane’s outrigger load is 45 tonnes on reclaimed land (~75 kPa) — a more conservative 2.0× safety factor is applied given the uncertain ground condition:
- Load in kN: 45 × 9.81 = 441.5 kN
- Required area: (441.5 × 2.0) ÷ 75 = 11.8 m²
- This exceeds any practical single pad size — this is a signal to use interlocking mats rather than a single oversized pad, or to get a site-specific geotechnical assessment before proceeding.
Common Sizing Mistakes
- Using the crane’s total weight instead of the outrigger-specific reaction force
- Skipping the safety factor entirely, or using too low a factor on uncertain ground
- Assuming a pad size that worked on a previous job will work on this one, without checking this site’s specific ground condition
- Not recognizing when the required area exceeds what a single pad can practically provide — that’s a signal to switch to interlocking mats, not to just accept a smaller, undersized pad
Try the Calculator
Now that you understand the methodology, our free online calculator runs this exact formula and matches the result against our tested Saferig pad range — enter your load and ground type, and get a specific recommended size, product code, and rated capacity in under a minute.
FAQ
1. What safety factor should I use for a standard construction lift?
1.5× is standard practice for most jobs on reasonably well-understood ground. Increase to 2.0× for less-certain ground conditions or higher-consequence lifts.
2. What do I do if the calculated area is bigger than any standard pad size?
This means a single pad isn’t the right tool for the job — consider interlocking ground mats, which distribute load across a much wider continuous area, or get a site-specific geotechnical assessment to confirm whether the ground’s actual bearing capacity is higher than the general reference value used.
3. Is a calculator result the same as a certified engineering calculation?
No — both this methodology and the online calculator give an indicative sizing reference using typical ground bearing values. For critical, heavy, or non-standard lifts, site-specific geotechnical data and engineering sign-off is recommended.
Recent Posts
High Visibility Reduce Trip Hazards
See our range of high-visibility safety mats and markers designed to reduce trip hazards across your site.
Related Blogs
