A crane's lifting capacity changes with every job. Boom length, working radius, ground conditions, and lift angle all affect what a crane can safely handle. A wrong calculation puts lives, equipment, and the entire site at risk.
A crane size calculator helps operators confirm the right crane for the job before the lift begins. In the UK, LOLER 1998 makes accurate load planning a legal requirement on every site.
What Is Crane Load Capacity and Why Does It Matter
Crane load capacity is the maximum weight a crane can safely lift under specific conditions. Two numbers matter: the maximum load and the Safe Working Load (SWL).
The maximum load is the absolute limit under perfect conditions. The SWL is the practical figure. It includes a safety buffer for dynamic forces, weight estimation errors, and shifting site conditions.
Capacity decreases as the radius increases. A crane rated for 50 tonnes at 3 metres may only handle 18 tonnes at 12 metres. An overloaded crane can tip, collapse, or drop its load. That means this can result in injuries and serious legal consequences under HSE regulations.
The Role of Load Charts
Every crane comes with a load chart from the manufacturer. It shows the rated capacity at different boom lengths and working radii. No other document on site is more important.
Load charts are model-specific and configuration-specific. A chart for a 30-metre boom is useless when a jib extension is fitted. Reading the wrong chart is just as dangerous as doing no calculation at all.
A crane capacity calculator uses the same data, but operators must always verify against the physical chart for the machine being used.
The Lifting Capacity Formula
The key concept is load moment. This is the rotational force the crane structure has to resist.
Formula: Load Moment = Load Weight × Radius
A bigger radius means a bigger load moment, which means less safe lifting capacity. When the load moment on the working side exceeds what the counterweight can balance, the crane tips. Load charts mark the safe boundary for every configuration.
Never plan a lift above 75 to 85 percent of rated capacity. That buffer covers heavier-than-expected loads, dynamic forces mid-lift, and outrigger settlement.
Step-by-Step: Calculating Crane Load Capacity
1. Determine the Load Weight
Weigh everything being lifted: the load itself, slings, shackles, hook block, and spreader beams.
Guessing the total weight is one of the most common mistakes in lift planning. Use certified scales or manufacturer specs.
2. Measure the Load Radius
The load radius runs horizontally from the crane's centre of rotation to a point directly above the load's centre of gravity.
A small error here can significantly affect permitted capacity. Measure to the load's centre, not its edge. Use a tape or laser tool.
3. Check the Boom Angle and Length
Boom length and angle decide which column of the load chart applies. A longer boom cuts capacity at any radius.
A lower angle adds structural stress and cuts it further. Use the shortest boom length the job allows, and record exact figures from the site, not from the original plan.
4. Refer to the Load Chart
Match the radius, boom length, and configuration to the load chart. Find the rated capacity for that setup.
The total lifted weight must sit within 75 to 85 percent of that figure. If it does not, move the crane closer or bring in a bigger machine. The chart limit must not be exceeded.
5. Factor in Attachments
Fly jibs, extensions, and heavy hook blocks all add weight. That weight comes off the available lifting capacity.
Check the crane manual for attachment weights and subtract them from the chart figure before planning the lift.
6. Consider External Factors
Load charts are built around ideal conditions. Wind puts lateral force on any suspended load.
Soft ground can cause outrigger pads to sink and shift the effective radius. Before lifting, confirm the crane is level, outriggers are fully deployed, wind is within limits, and the ground can carry the outrigger loads.
7. Apply a Safety Margin
Never lift at 100 percent of rated capacity. Keep the working load between 75 and 85 percent of the chart figure.
When the numbers sit close to that ceiling, reposition the crane, shorten the boom, or step up to a larger machine.
Daily Maintenance and Its Impact on Capacity
Rated capacity only holds when the crane is in the condition the manufacturer designed it for. Worn wire ropes, cracked boom sections, or failing hydraulics cut safe capacity without any visible warning.
Before every shift: Check wire ropes for broken wires and corrosion, inspect hydraulic lines for leaks, look over boom sections for cracks or bending, and confirm the load moment indicator is working.
A crane that clears inspection performs in line with the load chart. One that does not is a risk waiting to become an incident.
Tools That Support Accurate Calculations
A crane size calculator lets operators input radius, boom length, and load weight to get a fast capacity check.
Onboard load monitoring systems track weight and radius in real time and alert operators before limits are reached. Lift planning software runs full simulations for complex or high-risk jobs.
These tools reduce errors and save planning time. They are especially useful before mobilization, when picking the right crane upfront avoids expensive swaps once work starts.
Final Thoughts
Every safe lift comes down to four things: accurate measurements, the right load chart, a proper safety margin, and a well-maintained crane. Use the load moment formula. Match the chart to the exact configuration on site.
Stay within 75 to 85 percent of rated capacity. When the numbers are borderline, a larger crane should always be used.
Contact the Velebit Lifting team if you have any questions about the lifting capacity of a crane.