Crane capacity is the heaviest total load a crane may lift in one set-up: at a given working radius, boom length and outrigger setting, as shown on its load chart. To calculate it for a real lift, you add up everything suspended under the boom tip, identify the rated load at the actual working radius, and determine what proportion of that rating the lift consumes, because the nameplate tonnage only applies in the most favourable position, close to the column.
We have built truck mounted cranes in Xuzhou since 2003. This guide shows how to calculate crane capacity step by step, with one formula, a blank worksheet you can print and fill in like a calculator, and three worked examples taken from our own load charts.
Key takeaways
- Gross load = the item + hook or hook block + slings + shackles + spreader beam + any attachment. Every kilogram under the boom tip counts.
- Use the rated load at the largest radius of the lift. If the radius falls between two chart rows, use the row further out. Never interpolate upward.
- Net capacity = rated load − rigging weight. It is the heaviest item you can hang on the hook.
- Percentage of chart capacity = gross load ÷ rated load × 100. Under US rules, a lift above 75 percent of the rated capacity at the longest radius triggers a weight check in some cases.
- Load × radius is a useful cross-check, but the load chart always decides.
The crane capacity formula
No single number describes what a crane can lift, since capacity changes continuously with radius, boom length, boom angle and outrigger configuration, so the crane capacity formula is really a comparison rather than a constant. These three lines cover almost every crane load chart calculation:
- Gross load = item weight + rigging weight (hook or hook block, slings, shackles, spreader, attachments).
- Net capacity = rated load at the radius − rigging weight.
- Percentage of chart capacity = gross load ÷ rated load at the radius × 100.
The lift is inside the chart only if the gross load is no more than the rated load, which means the percentage is 100 or less. OSHA’s construction crane rule defines rated capacity as “the maximum working load permitted by the manufacturer under specified working conditions”, such as “equipment configuration, radii, boom length, and other parameters of use” (29 CFR 1926.1401). That is why you never calculate capacity from the model name alone.
What the old “calculation methods” really mean
You may read that crane capacity can be determined by a load weight method, a radius method, a structural strength method or a stability method. These are not four alternative calculations but four separate limits, and the manufacturer has already incorporated every one of them into the load chart.
- Load weight: your job. You must know the gross weight of what you lift.
- Radius: your job too. The chart gives a different rating at each radius and boom set-up.
- Structural strength: the manufacturer’s job. Close to the column, the boom, cylinders and slewing base usually set the limit.
- Stability: also built into the chart. Further out, tipping usually sets the limit. The US general industry rule caps ratings for truck and wheel mounted cranes, on outriggers fully extended or without outriggers, at 85 percent of the tipping load (29 CFR 1910.180(c)(1)(i)).
Consequently, the operator or lift planner does not repeat the structural or stability calculations: you determine the gross load and the radius, then read the chart and compare. Independent structural analysis is reserved for exceptional lifts, and it belongs with the manufacturer or a qualified engineer.
How to calculate crane capacity in 6 steps
Work through the steps in this order for every lift, and if any step fails, change the configuration and begin again. Our guide on how to read a crane load chart explains the chart itself; this page is the arithmetic.
Step 1: Find the gross load
Start with the item itself, using the weight from a delivery note, a nameplate or a calculation from its dimensions, and then add the complete rigging. OSHA says the load means the object and “the load-attaching equipment, such as, the load block, ropes, slings, shackles, and any other ancillary attachment” (29 CFR 1926.1401). That includes a spreader beam, a grab, a basket or a lifting clamp. If the chart or manual tells you to deduct something else, such as a stowed jib, deduct it as well. Our crane hook guide shows why the hook block counts.
The US rule also says how the weight must be known. The operator must verify the load is within rated capacity, and one accepted way is a weight “determined from a source recognized by the industry (such as the load’s manufacturer), or by a calculation method recognized by the industry (such as calculating a steel beam from measured dimensions and a known per foot weight)” (29 CFR 1926.1417(o)(3)(i)). A guess is not one of them.
Step 2: Find the worst-case radius
Radius is the horizontal distance from the slewing centre of the crane to the hook. Measure it at the pick-up position, at the landing position and at any intermediate point where the load travels further out, and then use the largest value. A lift that starts at 3 m and lands at 7 m is a 7 m lift.
Step 3: Read the rated load at that radius
Locate your radius on the chart for the boom configuration you will actually use. If it falls between two listed radii, round the radius up and apply the rating at the larger radius; never average two rows, and never read a higher value than the chart prints. Use the kilogram figures when you are operating close to the limit, because the pound values are frequently rounded.
Step 4: Work out the net capacity
Net capacity is the rated load minus all of the rigging, and it answers the practical question every buyer asks: how heavy can the item itself be? If the chart allows 1,600 kg and the rigging weighs 50 kg, the net capacity of the crane at that radius is 1,550 kg. Net capacity is always lower than the chart figure, and the gap matters most at long reach where ratings are small.
Step 5: Calculate the percentage of chart capacity
Divide the gross load by the rated load and multiply by 100. This crane capacity percentage indicates how much of the available rating the lift consumes, and anything above 100 is an overload that must not be lifted. The US rule says plainly that “the equipment must not be operated in excess of its rated capacity” (29 CFR 1926.1417(o)(1)).
One threshold in that rule is written as a number. If the operator does not have a reliable weight and instead begins hoisting with a load weighing device, load moment indicator or rated capacity limiter, the operator must check whether the load “exceeds 75 percent of the maximum rated capacity at the longest radius that will be used during the lift operation”. If it does, the lift stops until the weight is verified by the method in Step 1 (29 CFR 1926.1417(o)(3)(ii)).
Other planning margins are site practice rather than law. Many contractors require a written lift plan or additional verification when a lift approaches the chart limit, because wind, a swinging load or an uncertain weight all reduce the effective margin, and your company’s lifting procedure determines the exact trigger.
Step 6: Cross-check with the lifting moment
Multiply the gross load in tonnes by the radius in metres to obtain the lifting moment in t·m, which provides a quick plausibility check: an unexpectedly large result tells you to look again. However, do not divide the crane’s maximum moment by your radius to find capacity. That shortcut often gives a number higher than the chart, as the examples below show. Our article on the lifting moment of a crane explains why.
Crane capacity calculator worksheet
Print this worksheet or copy it into a spreadsheet, complete the entries from your load, your site survey and your crane’s chart, and then perform the calculations described on the right. It works like a crane capacity calculator, but it keeps every number visible so someone else can check it.
| Line | What to enter | How to get it |
|---|---|---|
| A | Item weight (kg) | Delivery note, nameplate or calculation from dimensions |
| B | Hook or hook block (kg) | Marked weight or crane manual |
| C | Slings, shackles, spreader, attachments (kg) | Marked weights or supplier data |
| D | Gross load (kg) | A + B + C |
| E | Worst-case radius (m) | Largest of pick-up, swing path and landing |
| F | Chart radius used (m) | E rounded up to the next listed radius |
| G | Rated load at F (kg) | Load chart, for your boom and outrigger set-up |
| H | Net capacity (kg) | G − B − C |
| I | Percentage of chart capacity | D ÷ G × 100 (must not exceed 100) |
| J | Lifting moment (t·m) | D ÷ 1,000 × E, a cross-check only |
| K | Outriggers and ground | Set as the chart assumes; ground firm and level; pads in place |
Worked examples from BOB-LIFT load charts
The rated loads below come directly from our own technical drawings, while the jobs are invented to demonstrate the method, so each one is labelled as an example scenario.
| Model | Boom type | Max lifting capacity | Max lifting moment | Rated loads from the chart (radius / kg) |
|---|---|---|---|---|
| SQ5ZA3 | Knuckle | 5,000 kg | 10.5 t·m | 2.1 m / 5,000; 4.45 m / 2,350; 6.2 m / 1,600; 8.05 m / 1,100 |
| SQ10SA4 | Telescopic | 10,000 kg | 25 t·m | Boom-length table, see Example 2 |
| SQ12ZA6 | Knuckle | 12,000 kg | 27 t·m | 2.25 m / 12,000; 4.7 m / 5,450; 6.4 m / 3,500; 8.19 m / 2,500; 10.1 m / 1,850; 12.2 m / 1,400; 14.5 m / 1,000 |
Example 1: SQ5ZA3 knuckle boom crane at 5.8 m
Example scenario (an illustration, not a customer record): a crew must unload a 1,250 kg pallet of paving stones from the truck bed at 3.5 m and set it on a footpath 5.8 m from the slewing centre. Two chain slings, shackles and the boom tip hook weigh 50 kg together.
- Gross load: 1,250 + 50 = 1,300 kg.
- Worst-case radius: 5.8 m, the landing point.
- Chart radius: 5.8 m sits between 4.45 m and 6.2 m, so use 6.2 m. Rated load: 1,600 kg.
- Net capacity: 1,600 − 50 = 1,550 kg.
- Percentage: 1,300 ÷ 1,600 × 100 = about 81.
- Moment check: 1.3 t × 5.8 m = 7.5 t·m, below the 10.5 t·m maximum.
The lift is comfortably inside the chart. Notice, however, the trap described in Step 6: dividing 10.5 t·m by 5.8 m gives about 1,810 kg, which is more than the 1,600 kg the chart allows at 6.2 m. The chart wins.
Example 2: SQ10SA4 telescopic crane with a spreader beam
A telescopic crane chart lists ratings by boom length and boom angle. Here is part of the SQ10SA4 table from its drawing, with each cell giving working radius (m) / lifting capacity (kg):
| Boom angle | 5.6 m boom | 9.02 m boom | 12.44 m boom | 15.87 m boom |
|---|---|---|---|---|
| 30° | 4.3 / 5,700 | 7.6 / 2,700 | 10.5 / 1,810 | 13.5 / 1,300 |
| 40° | 3.8 / 6,300 | 6.8 / 3,100 | 9.3 / 1,950 | 12.0 / 1,500 |
| 50° | 3.2 / 8,700 | 5.7 / 3,600 | 7.8 / 2,450 | 10.0 / 1,850 |
| 60° | 2.4 / 10,000 | 4.4 / 4,300 | 6.1 / 3,000 | 7.8 / 2,170 |
Example scenario (an illustration, not a customer record): four steel beams, each 6 m long at 95 kg per metre, must be placed 7.2 m from the slewing centre. The beams weigh 4 × 6 × 95 = 2,280 kg, worked out from dimensions as the US rule allows. The hook block, two slings and a spreader beam add 180 kg.
- Gross load: 2,280 + 180 = 2,460 kg.
- Worst-case radius: 7.2 m.
- Chart radius: with the 9.02 m boom, 7.2 m sits between the 6.8 m and 7.6 m rows, so use 7.6 m. Rated load: 2,700 kg.
- Net capacity: 2,700 − 180 = 2,520 kg.
- Percentage: 2,460 ÷ 2,700 × 100 = about 91.
- Moment check: 2.46 t × 7.2 m = 17.7 t·m, below the 25 t·m maximum.
The lift is within the chart but close to it. If the truck parks so that the radius is no more than 6.8 m, the rating rises to 3,100 kg and the lift uses about 79 percent of chart. Now suppose a wall forces the crew to use the 12.44 m boom instead. At 7.8 m that boom is rated 2,450 kg, which is less than the 2,460 kg gross load. The lift fails by 10 kg, and it fails only because the spreader beam was counted. That is why rigging belongs in every calculation.
Example 3: SQ12ZA6 knuckle boom crane at long reach
Example scenario (an illustration, not a customer record): a 1,100 kg bundle of roof trusses, weight taken from the supplier’s delivery note, must go onto a roof 11.6 m from the slewing centre. Slings, shackles and the hook weigh 60 kg.
- Gross load: 1,100 + 60 = 1,160 kg.
- Worst-case radius: 11.6 m.
- Chart radius: 11.6 m sits between 10.1 m and 12.2 m, so use 12.2 m. Rated load: 1,400 kg.
- Net capacity: 1,400 − 60 = 1,340 kg.
- Percentage: 1,160 ÷ 1,400 × 100 = about 83.
- Moment check: 1.16 t × 11.6 m = 13.5 t·m, only half of the 27 t·m maximum.
This is where the moment shortcut misleads most. Dividing 27 t·m by 11.6 m suggests about 2,330 kg. The chart allows 1,400 kg, because at full reach the boom’s own weight uses much of the crane’s strength. A 12 t crane is a 1.4 t crane at 12.2 m. For more on how knuckle boom ratings fall with reach, see our page on knuckle boom crane lifting capacity.
Outrigger setting and ground
A load chart is only valid for the configuration it was calculated for. If the crane stands on partially extended outriggers, on soft ground or on a slope, the chart figure no longer applies, and your calculation is invalid before it begins. The US general industry rule says the stability margins are influenced by “track, wind, or ground conditions”, among other factors, and adds: “All of these shall be taken into account by the user” (29 CFR 1910.180(c)(1)(iv)).
Our drawings give the outrigger span as a range from retracted to fully extended: 2,272–5,122 mm on the SQ5ZA3, 2,290–5,990 mm on the SQ10SA4 and 2,316–5,706 mm on the SQ12ZA6. Plan for the fully extended figure unless your crane has a separate chart for a reduced span. The construction rule also says the crane must not be used unless ground conditions are “firm, drained, and graded” enough to meet the manufacturer’s support and levelling requirements (29 CFR 1926.1402(b)).
The ground underneath each outrigger pad requires a separate calculation. Our guide to crane outriggers and ground bearing pressure works through pad sizing. Slewing matters too, because some cranes have different ratings over the rear and over the side; our article on truck mounted crane slewing covers the working zones.
Common crane capacity calculation mistakes
- Using the nameplate: a 10 t crane lifts 10 t only at its shortest radius.
- Forgetting the rigging: a spreader beam or grab can turn a pass into a fail at long reach, as Example 2 shows.
- Measuring the radius at pick-up only: the landing point is often further out.
- Interpolating upward: reading a value between two rows instead of rounding the radius up.
- Dividing max moment by radius: it overstates capacity at long reach.
- Ignoring the boom set-up: on a telescopic crane, a longer boom at the same radius is rated lower.
- Calculating for a set-up you will not use: outriggers partly out, a slope or soft ground.
In the UK, the Lifting Operations and Lifting Equipment Regulations require every lifting operation to be “properly planned by a competent person”, “appropriately supervised” and “carried out in a safe manner” (LOLER 1998, regulation 8). HSE’s guidance on planning and organising lifting operations lists overload, overturning and derating among the risks a plan should address. A filled-in worksheet is a simple way to show that planning was done.
Frequently asked questions
What is the formula for crane capacity?
Gross load must be no more than the rated load at the working radius. Gross load is the item plus the hook block, slings, shackles and attachments. The rated load comes from the crane’s load chart for that radius, boom set-up and outrigger setting.
What is the net capacity of a crane?
Net capacity is the rated load at the radius minus the weight of all rigging. It is the heaviest item you can lift. On the SQ12ZA6 at 12.2 m, a 1,400 kg rating with 60 kg of rigging gives a net capacity of 1,340 kg.
How do you calculate crane capacity percentage?
Divide the gross load by the rated load at the chart radius you are using, then multiply by 100. A 1,300 kg gross load against a 1,600 kg rating uses about 81 percent of chart capacity. Above 100 is an overload.
What percentage of crane capacity is safe?
The legal limit is the chart itself: never above 100 percent. US rule 29 CFR 1926.1417(o)(3)(ii) uses 75 percent of the rated capacity at the longest radius as the point where a weight must be verified, when the operator relies on a weighing device instead of a known weight. Any lower planning margin is set by your company or site, not by a universal rule.
Is there a crane capacity calculator I can use?
The worksheet on this page is one. Enter the item weight, rigging, radius and chart rating, and it gives gross load, net capacity and percentage. Any calculator is only as good as the chart you feed it, so always use the chart supplied with your own crane.
How we compiled this guide
This is an update of our earlier article on ways to calculate crane capacity. We compiled the rated loads, lifting moments and outrigger spans from the BOB-LIFT dimension drawings and load charts of the SQ5ZA3, SQ10SA4 and SQ12ZA6. The worked examples are illustrations, and the percentages and moments in them were calculated by us from those charts. The quotations come from the official eCFR text of 29 CFR 1910.180, 1926.1401, 1926.1402 and 1926.1417, from legislation.gov.uk and from the HSE website. The photos were taken at our Xuzhou factory. Achord Chan, Product Operations Director, reviewed the guide in November 2026. It does not replace your crane manual, a lift plan by a competent person or the law where you work.
Before shipping, every BOB-LIFT crane is mounted on our test bay and run through lifting, slewing and outrigger functions under load. Our crane load test guide explains testing after delivery, and the truck mounted crane inspection checklist covers the daily checks that keep the chart valid.
Get the load chart for your lift
Send us your heaviest item, your rigging and the radius you need, and we will run the worksheet against the right load chart and recommend a crane. Browse our knuckle boom truck cranes and telescopic boom truck cranes, each with its drawing and chart. Every crane carries a 12-month warranty on the whole machine, common spare parts leave the factory within 48 hours, and our service team answers within 12 hours. Contact us for a quotation.
