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How to Read a Crane Load Chart: A Practical Guide for Site Engineers
A crane's rated capacity — "100-tonne crane", "300-tonne crane" — tells you almost nothing useful on its own. What actually determines whether a crane can perform your lift safely is buried in a document called the Load Chart.
The Load Chart is the technical specification that defines what a crane can safely lift under any specific combination of operating conditions: how far from the crane the load is, how long the boom is, how the counterweight is configured, and whether the outriggers are fully deployed. Miss any of these variables, and your capacity calculation will be wrong — potentially fatally so.
This guide walks through the five key components of a crane load chart and the four-step method to extract the correct safe working load for any lift scenario.
What is a Crane Load Chart?
A crane load chart is a manufacturer-issued technical document that specifies the maximum safe lifting capacity of a crane under various operating configurations. It is not a single number — it is a matrix of numbers that changes based on working conditions.
The load chart serves as the legal and engineering reference for all crane lifting operations. When a crane supervisor or operator selects equipment for a lift, they must verify against the load chart that the crane can handle the job under the exact conditions present on site — not under ideal conditions in a catalogue.
Critically, the numbers in a load chart represent gross lifting capacity, which includes the weight of rigging equipment (hook block, slings, shackles). The net load you can actually lift is always less than what the chart shows.
The 5 Key Components of a Load Chart
Component 1: Maximum Lifting Capacity
This is the headline figure — the maximum weight the crane can safely handle under a given set of conditions. However, this number is only valid at a specific combination of boom length, boom angle, and lifting radius. Change any of these variables and the capacity changes.
Boom length effect: A shorter boom at the same angle has a higher rated capacity because it creates less bending moment on the structure and positions the load closer to the crane centre. As boom length increases, capacity decreases — sometimes dramatically.
Boom angle effect: At the same boom length, a higher boom angle (more vertical) reduces the lifting radius, which reduces the overturning moment, which increases the rated capacity. A lower boom angle extends the radius, increases overturning moment, and reduces capacity.
This is why you must always use the chart — not a rough estimate.
Component 2: Lifting Radius
The lifting radius (or working radius) is the horizontal distance from the crane's slewing centre to the point directly below the load hook. It is measured in metres, and it has an inverse relationship to capacity: the greater the radius, the lower the maximum safe load.
This is the parameter that trips up the most people. The rated capacity might be 100 tonnes, but that's probably at a radius of 3–4 metres with the boom nearly vertical. At a typical working radius of 12–15 metres, the same crane might be rated for 30–40 tonnes.
Always measure or calculate the actual working radius before finalising crane selection.
Component 3: Boom Extension Limits
Load charts specify the minimum and maximum permissible boom length, the minimum permissible boom angle, and any prohibited working zones. Operating outside these limits — attempting to extend the boom beyond its rated length or working at an angle below the chart minimum — creates structural loads the crane was not designed for.
Component 4: Counterweight Configuration
Most load charts present multiple tables corresponding to different counterweight configurations. More counterweight increases resistance to overturning, which increases lifting capacity — but it also increases the crane's total weight, which affects ground bearing requirements and travel capability.
Before reading any capacity figure from the chart, you must know exactly what counterweight is installed on the crane. Using a "full counterweight" table when the crane has partial counterweight fitted will give you a capacity figure that is higher than reality.
Similarly, the outrigger configuration — fully extended vs. partially extended vs. on tyres (for some crane types) — generates separate table sections. Always use the table that matches the actual site configuration.
Component 5: Deductions for Rigging Equipment
The gross capacity figure in the load chart is the total weight the crane can handle at the hook. This weight must be shared between the load itself and all the rigging equipment attached between the hook and the load.
Net Load = Gross Capacity − Rigging Weight
Rigging equipment includes the hook block, slings, shackles, spreader beams, and any other lifting attachments. On a 100-tonne capacity lift using a heavy hook block and spreader beam assembly that weighs 3.5 tonnes, the actual net load capacity is 96.5 tonnes — not 100 tonnes.
This deduction is frequently omitted in rush situations. It should never be.
Step-by-Step: How to Read a Load Chart
Follow these four steps for any lift:
Step 1: Confirm Configuration and Select the Correct Table
Identify the three setup parameters that determine which table to use:
- Outrigger position: fully extended, partially extended, or on tyres
- Counterweight mass: what is actually installed on the crane
- Boom assembly: main boom only, with fly jib, with offset jib, etc.
Select the table that matches all three parameters exactly. Using the wrong table invalidates everything that follows.
Step 2: Determine Boom Length and Configuration
Identify the actual boom length (in metres) that will be used for the lift, accounting for any jib extensions. On the load chart, locate the column or curve corresponding to that boom length.
Step 3: Measure the Lifting Radius and Read the Capacity
Measure or calculate the actual horizontal distance from the crane's slewing centre to the load pickup point. Find this radius on the chart (typically on the left column or horizontal axis) and cross-reference with the boom length column or curve to read the gross lifting capacity.
Step 4: Calculate the Net Load Capacity
Subtract the total weight of all rigging equipment from the gross capacity:
Net Capacity = Gross Capacity − (Hook block + Slings + Shackles + Spreader beam)
The result is the maximum weight of the actual load (not including rigging) that the crane can safely lift in this configuration.
If the load weighs more than this net capacity, you cannot perform the lift with this crane in this configuration. Options: reposition the crane to reduce the radius, use a larger crane, or plan a tandem lift.
Common Mistakes That Cause Accidents
Using the wrong table: Applying the "full counterweight, outriggers fully extended" table when the crane is operating with partial counterweight and partially extended outriggers gives a capacity figure that may be 30–40% higher than reality.
Confusing angle with radius: Some people calculate the boom angle and use it as if it were the radius. These are different parameters. Always measure the actual horizontal distance to the load.
Ignoring rigging weight: On large lifts where rigging assemblies weigh several tonnes, this omission directly causes overload.
Exceeding boom length limits: Attempting to extend the boom beyond the chart's maximum, or working below the chart's minimum angle, is operating outside the manufacturer's structural validation.
Not accounting for terrain slope: Load charts assume level, stable ground. On sloped terrain, the effective radius increases and capacity decreases. If the crane is not level, the chart cannot be applied as printed.
Frequently Asked Questions
Q: Does a "200-tonne crane" lift 200 tonnes?
Only at minimum radius with maximum counterweight and fully extended outriggers on level, adequate ground. In typical field configurations at working radii of 10–20 metres, the same crane might safely lift 60–100 tonnes. Always verify against the chart for your specific conditions.
Q: What does SWL mean on a load chart?
SWL (Safe Working Load) is equivalent to Net Capacity in our discussion above — the load chart's gross capacity minus the weight of all rigging attachments. Some manufacturers use SWL directly; others show gross capacity and require the user to deduct rigging weight.
Q: What is the bold line or shaded area on some load charts?
This typically marks the boundary between structural capacity limits and stability (tipping) limits. Above the line or inside the shaded zone, the crane is limited by its structural strength. Below the line or outside the zone, the crane is limited by tip-over risk. The lower of the two limits always applies.
Q: Who is responsible for verifying the load chart calculation on site?
The crane supervisor is legally and professionally responsible. For complex or critical lifts, the Lifting Plan (a written document) should include the load chart calculation, verified and signed by the supervisor or a lifting engineer.
Practical Support for Your Lifting Project
Understanding load charts is the foundation of safe crane selection. It is also the reason why selecting the right crane for a project requires more than a phone call asking "what's your biggest crane?" — it requires specific data: load weight, load dimensions, lifting radius, height, and site configuration.
At S.K. Kunatham Group, our team performs load chart verification and Lifting Plan preparation for every project that requires it. For complex lifts, we provide full engineering support before a crane arrives on site.
Get in touch:
- LINE: @skgroup
- Phone: 074-333-074
- Free site assessment and quote for any project in Southern Thailand