How to use this calculator
- Get the real reaction. OEM outrigger-reaction program or chart for the planned pick — boom over the corner, max load, not the parked weight.
- Get the site allowable. Geotech report, site engineer or project criteria set the allowable ground pressure — never guess it from the surface.
- Size or check. Required area and square/round size — or enter the pad on the truck and read its actual ground pressure and utilization.
- Mind the mat itself. Stiffness, cribbing and anything buried (voids, utilities, frozen layers) are engineering checks beyond bearing area.
How it works
Pad sizing is bearing arithmetic — the same equation under every footing on every site:
A_req = R / q_allow · q_actual = R / A_pad · utilization = q_actual / q_allow
Both inputs are documents, not guesses: the reaction is the crane maker's number for the configured pick, the allowable is the site's number for the ground. The identical math holding up a building post is the deck footing calculator; the rigging above the hook lives in the sling angle load, hitch capacity and rigging CG calculators.
Worked example
Verified against the live calculator
A 250 kN outrigger reaction (≈25.5 t) on ground assessed
at 150 kPa:
A_req = 250 / 150 = 1.67 m² → 1.30 m square (or 1.46 m round)
Carry only a 1.0 m square pad and check mode shows why
it matters: 250 kN over 1.0 m² is 250 kPa — utilization
1.67, the ground overloaded by two-thirds. The honest
fixes are the ones the output names: a bigger mat, proven better
ground, or a smaller pick.
Frequently asked questions
How do you calculate outrigger pad size?
Required bearing area = outrigger reaction ÷ allowable ground bearing pressure. A 250 kN (~25.5 t) reaction over ground assessed at 150 kPa needs 1.67 m² — a 1.30 m square or 1.46 m round pad. The reaction comes from the crane's chart or outrigger-reaction program; the allowable from the site.
How do you calculate crane ground bearing pressure?
Reaction over pad contact area. The same 250 kN on a 1.0 m square pad puts 250 kPa into the ground — 1.67× the 150 kPa allowable, which is the calculator's danger flag telling you the pad is undersized for that ground.
Why not just divide the crane weight by four?
Because outrigger reactions are wildly uneven during a pick: boom over one corner can put well over half of crane-plus-load on a single outrigger. Use the OEM's outrigger-reaction program or chart values for the actual planned configuration — that's the number this calculator expects.
Does a bigger pad always mean lower ground pressure?
Only as far as the pad is stiff enough to engage its whole footprint. A thin plate bows and bears on a patch in the middle; engineered mats publish stiffness/spread ratings. This screen sizes contact area — pad structure, cribbing height and voids under the mat are site-engineering checks on top.
Method & assumptions
- Uniform bearing over the pad's full footprint — true only as far as the pad is rigid; engineered-mat stiffness ratings and cribbing design sit on top of this screen.
- Reaction and allowable ground pressure are user-entered (OEM program / site documents). No crane charts and no soil tables are embedded.
- Vertical bearing only: lateral loads, mats spanning voids or utilities, slopes, and frozen or layered ground are site-engineering questions.
- Crawler/track bearing and multi-crane mats follow their own OEM and engineering methods.