Educational explanation, not professional design advice. Last reviewed 24 October 2026. Disclaimer
In plain English
A building has to carry its own weight all the time — the slabs, beams, walls and floor finishes. That is the dead load. It also has to carry things that come and go: people, furniture, stored goods and vehicles. That is the live load. Engineers estimate both and add safety factors before sizing beams, columns and foundations.
Technical definition
Dead load (permanent action, G, in Eurocode terms) is the weight of the structure and of permanently fixed elements, such as finishes, partitions treated as permanent and fixed equipment. Live load (imposed load, Q) is the load produced by the use and occupancy of the building, which varies in magnitude and position. Codes such as ASCE 7 and EN 1991-1-1 give minimum live loads for different types of use.
How it works and where it is used
Dead load is calculated from dimensions and unit weights of materials (for example kN/m³ for concrete). Live load is taken from the code for the occupancy category, as a uniformly distributed load (kN/m²) and sometimes a concentrated load. Loads are then multiplied by load factors and combined; for example, ASCE 7 includes the strength combination 1.2D + 1.6L, and EN 1990 uses 1.35G + 1.5Q for a common persistent design situation. Other actions — wind, snow, earthquake — are added as the code requires.
Formula
Self-weight of a slab (per m²) = thickness × unit weight
Example factored load (ASCE 7): wu = 1.2 D + 1.6 L (EN 1990): wEd = 1.35 G + 1.5 Q
The combinations and factors shown are examples; the governing combinations depend on the code, the national annex and the other actions present.
Worked example
A 150 mm reinforced concrete slab, using a unit weight of 25 kN/m³ (the value given for reinforced concrete in EN 1991-1-1), has a self-weight of 0.15 × 25 = 3.75 kN/m². With 1.0 kN/m² for finishes, the dead load is 4.75 kN/m². If the code gives an imposed load of 2.0 kN/m² for this use, the EN 1990 combination is 1.35 × 4.75 + 1.5 × 2.0 = 9.41 kN/m².
In the workshop and in practice
Load estimates are the first step in sizing any structural member. In workshops they matter when adding mezzanines, storage racks, heavy machines or lifting points to an existing building — all of which add dead or live load that the original design may not have allowed for.
Common mistakes and misconceptions
- Forgetting finishes, services and partitions in the dead load.
- Treating a heavy machine as “live load” without checking the code’s concentrated or equipment loads, vibration and impact.
- Mixing load factors from different codes. Use one code consistently.
- Using unfactored loads with factored resistances, or the reverse.
Safety note
Structural changes and heavy new loads on existing buildings must be checked by a qualified structural engineer before work begins.
Related calculators
- Concrete Volume Calculator — Concrete volume for slabs, footings, columns and beams, with optional wastage.
- Rebar Weight Calculator — Weight of reinforcing steel bars (rebar / sariya) per metre, per bar and in total.
- Plate Weight Calculator — Weight of a rectangular plate or sheet in steel, aluminium, copper and more.
Related topics
- Water–Cement Ratio — The mass of water divided by the mass of cement in a concrete mix.
- Stress and Strain — Internal force per unit area, and the stretch per unit length it causes.
References
- ASCE/SEI 7: Minimum Design Loads and Associated Criteria for Buildings and Other Structures
- EN 1991-1-1 (Eurocode 1): Actions on structures — Densities, self-weight, imposed loads for buildings
- EN 1990 (Eurocode 0): Basis of structural design (load combinations)