Rigging
Weight of a load from volume and density (steel, concrete, timber)
- where
- volume (m³)
- length, width and thickness (m)
- number of identical pieces
- mass (kg)
- density (kg/m³)
- weight as a force (kN)
Explain this equation
Estimates the weight of a solid rectangular item from its dimensions and material density. Use it when there is no certified or weighed weight, to get a working figure for slings and crane checks.
L, b and H are the length, width and thickness. n is the number of identical pieces, so V is the total volume. ρ is the density: steel 7850 kg/m³, reinforced concrete about 2550 kg/m³ (25 kN/m³) and dry C24 softwood about 420 kg/m³. m is the mass in kg, and W_kN is the same weight as a force.
Mass is volume times density. Multiplying by 9.81 gives newtons, and dividing by 1000 gives kN.
Sense check: a 2 m × 1 m steel plate 20 mm thick is 314 kg, which is 157 kg per square metre. A cubic metre of reinforced concrete is about 2.55 t.
Watch out: wet timber, cast-in fixings and attached formwork add weight. Always prefer a weighed weight, and add a margin when unsure.
Estimates the weight of a solid rectangular load from its size and the material density, for when no certified weight is available.
Enter your values to see the result.
Good to know
Densities: structural steel 7850 kg/m³ (BS EN 1993-1-1); normal-weight concrete about 2450 kg/m³ plain and 2550 kg/m³ with normal reinforcement (BS EN 1991-1-1 Table A.1 gives 24 and 25 kN/m³; ÷ 9.81 and rounded up), plus 1 kN/m³ more for wet concrete; C24 softwood mean 420 kg/m³ dry (BS EN 338), but wet or treated timber can be 50% heavier. For hollow sections, beams and pipes use the maker’s mass per metre × length instead.
Common mistake
Using the dry density for timber that has been out in the rain, or forgetting cast-in items, lifting inserts and attached formwork on a precast unit.
Limits
Solid rectangular blocks only; add separate parts and rigging yourself. A calculated weight is an estimate: always prefer a weighed or certified weight, and add a margin when uncertain. For planning only: this does not replace a lift plan prepared by a competent person under LOLER 1998 (reg. 8) and BS 7121, and the manufacturer’s WLL markings, load charts and data always govern over this result.
Sources
- BS EN 1993-1-1 Eurocode 3, cl. 3.2.6: density of structural steel 7850 kg/m³; BS EN 338: C24 softwood mean density 420 kg/m³
- New Steel Construction, They’ve changed the density of concrete (BS EN 1991-1-1 Table A.1: normal-weight concrete 24 kN/m³, +1 kN/m³ for normal reinforcement)
- The Lifting Operations and Lifting Equipment Regulations 1998 (SI 1998/2307), reg. 8: every lifting operation properly planned by a competent person, appropriately supervised and carried out safely
- HSE L113 Safe use of lifting equipment. Lifting Operations and Lifting Equipment Regulations 1998. Approved Code of Practice and guidance
Results are calculations from the published methods and figures shown, and depend on the data you enter. For planning and checking only. Scaffolds must be erected to NASC TG20 or a bespoke design by competent persons, and lifting operations planned under LOLER by a competent person. Never rely on these results instead of a design, a lift plan or manufacturer ratings.