Boom truck size estimator

R 03 mHow it works

The estimator works through a lift in the order a rental desk asks about it: what the hook really carries, how far out it has to reach, how high it has to go, and how much capacity to look for at that radius.

W = load + rigging + jib
R = max(pick radius, set radius)
H = landing height + clearance + load height + rigging length
L = √((R − foot offset)² + (H − foot pin height)²)
A = atan2(H − foot pin height, R − foot offset)
C = W × (1 + margin ÷ 100), ÷ 0.80 for duty-cycle work
M = C × R ÷ 1000   (tonne-metres)

Weight on the hook. Everything below the boom tip counts. IHSA states that a hoist's rated load does not account for hook blocks, hooks, slings or equalizer beams, so their weight is added to the load². A fly jib left on the boom head is added too when you lift on the main hook, unless the chart allows for it; that rule comes from Queensland's mobile crane code, a non-Canadian reference³.

Radius. R is the larger of the pick radius and the set radius, because the chart figure has to hold at the worst point of the swing. Radius is the horizontal distance from the crane's centre of rotation to the hook with the boom loaded².

Height. Hook height is the landing height plus your clearance, the height of the load and the length of the slings and hook block. The tool treats tip height as equal to hook height, so add an allowance of your own on a tight job.

Boom length and angle. For a straight boom of length L at angle A from horizontal, radius is about L × cos A plus the horizontal offset of the boom foot, and tip height about L × sin A plus the foot pin height². Solving those two for L and A gives the shortest straight boom that reaches the point and the angle it would sit at. The truck's boom has to reach at least that far at that angle, and its chart has to allow the load there.

Capacity to look for. The weight on the hook is raised by your working margin; IHSA recommends selecting a crane with at least a 5% working margin over the capacity required for every lift². For repetitive work it is divided by 0.80, because IHSA notes manufacturers usually require ratings to be reduced by 20% for duty-cycle work such as concrete placing, or supply a separate chart².

Load moment. Capacity times radius, in tonne-metres. Knuckle-boom (articulating) cranes are rated this way: the chart load multiplied by the radius at which it can be lifted. Northern Territory WorkSafe in Australia, a non-Canadian reference, gives the example 2.89 m × 4.55 t = 13.145 metre-tonnes⁴.

R 06 mAssumptions and sources

• The geometry treats the boom as a straight, rigid line. IHSA notes that the radius grows under load because of boom and machine deflection², so the real radius at the hook is larger than the drawing.
• Boom angle indicators can be off by as much as 2 degrees and should not be relied on for critical lifts; IHSA prefers a measured radius². The angle shown here is geometry, not a reading to set the boom by.
• Level matters. In IHSA's example table, one crane lost 10% of chart capacity with a short boom at minimum radius when it was 1 degree out of level², and a long boom lost 30% at 1 degree².
• Chart ratings come from two different limits. CSA Z150:20 separates ratings governed by crane stability (clause 4.1.1) from ratings governed by structural competence (clause 4.1.2)⁵. This tool cannot tell which one governs at your radius; the chart can.
• The weight of the load must be known. In British Columbia, each load's weight must be determined and communicated to the operator and rigger, and if it cannot be determined accurately, the crane needs a load weight indicator or overload prevention system¹.
• Values marked "Example" (0.5 m clearance, 0 m foot offset, 2.5 m foot pin height) are illustrative, not typical. Take the offset and pin height from the dimension sheet of the truck you are quoting.

R 09 mReading the result

The big number is the capacity to look for on a load chart at your radius. It is not the truck's headline rating. A headline rating is the best case on the chart, close in and on fully extended outriggers, and capacity falls fast as the radius grows. The example knuckle-boom chart published by Northern Territory WorkSafe, a non-Canadian reference, drops from 4,550 kg at 2.89 m to 730 kg at 12.55 m⁴.

So when you call a supplier, give them three numbers: the weight on the hook, the radius and the tip height. Ask for the chart figure at that radius. Ask which quadrant it applies over: capacity is normally greatest over the rear, less over the side and may be nil over the front². Ask about the outriggers: full "on outriggers" ratings apply only when every beam is fully extended and the tires are clear of the ground². If the site will not let the outriggers go out fully, the chart for that setup is the one that counts.

If you already have the chart, enter its capacity at your radius. The tool divides the weight on the hook by that figure and flags three thresholds:

• Over 100%: the configuration cannot make the lift.
• Over 75%: Alberta requires a lift calculation for any lift exceeding 75% of a crane's rated capacity⁶.
• Over 90% at more than half the chart's maximum permitted radius: British Columbia defines this as a critical lift for a mobile crane or boom truck¹.

"Get this priced" carries the weight on the hook, the radius and the tip height into a quote request. Only those numbers go into the link (they show in the address bar), and nothing is saved until you submit the request form.