Balcony Load Calculator

Balcony Load Calculator — estimate balcony load for your project. Fast, accurate, free. No signup required.

A load estimate is only one part of a structural check. Compare the resulting stress with the Material Stress calculator, confirm roof geometry with the Roof Pitch and Area calculator, and use the Building Regulations calculator for the related compliance context.

The structural load calculator performs engineering checks for common construction scenarios: beam bending moments and required section modulus, foundation bearing pressure, rafter geometry, and general load-per-area calculations. These are the calculations that determine whether a structural element is the right size for the loads it must carry — getting them wrong leads to deflection, cracking, or catastrophic failure. Every structural element in a building is subject to dead loads (self-weight of materials), live loads (occupants, furniture, wind, snow), and sometimes seismic loads. This calculator handles the most common residential and light commercial structural checks using simplified first-principles formulas.

The results from this calculator are indicative guidance only. All structural elements in a building must be specified by a qualified structural engineer who considers the full loading scenario, material properties, connection details, and relevant design codes (Eurocodes EC1–EC5 in the UK, ASCE 7 in the US). This tool is for educational understanding and preliminary sizing only.

The mode is selected automatically based on your calculator topic: beam (bending moment and section modulus), footing (bearing pressure), rafter (length from span and pitch), or load (pressure from force and area).
For beam calculations: enter the uniformly distributed load (UDL) in kN/m, the span in metres, and the allowable bending stress of your material (mild steel ≈ 165 N/mm², structural timber Grade C24 ≈ 7.5 N/mm²).
For footings: enter the total column or wall load (kN) and the allowable ground bearing capacity (kN/m²) from your site investigation.
For rafters: enter the horizontal run and vertical rise — the calculator returns rafter length, pitch angle, and pitch ratio.
Cross-reference the section modulus or bearing area result against standard section tables (Blue Book for steel, span tables for timber) to select the appropriate member size.

Structural sizing formulas

Beam bending moment (midspan UDL): M = wL²/8 where w = UDL (kN/m), L = span (m); M in kNm. For a point load at midspan: M = PL/4.

Required section modulus: Z = M × 10⁶ / σ_allow (mm³) where σ_allow is allowable bending stress (N/mm²). Select a standard section with Z ≥ this value.

Footing bearing pressure: A_min = P / q_allow where P = total load (kN), q_allow = allowable bearing (kN/m²). For a square pad: side = √(A_min).

Rafter length: L = √(run² + rise²) (Pythagoras). Pitch angle = arctan(rise/run). Pitch ratio = rise/run.

Example (beam): 10 kN/m UDL on 5m span. M = 10×5²/8 = 31.25 kNm. For steel at 165 N/mm²: Z = 31.25×10⁶/165 = 189,394 mm³ = 190 cm³. A 254×102 UB 22 (Z = 262 cm³) would be adequate.

Interpreting structural results

Standard section selection guidance

For steel beams, the required section modulus (Z) identifies the minimum Universal Beam (UB) or Universal Column (UC) section. Use the SCI Blue Book (P363) or standard steel section tables to find the lightest section with Z greater than the required value — always choose the next size up. For timber rafters and joists, span tables in TRADA's 'Span Tables for Solid Timber Members' give approved joist sizes for common spans and spacings — use BS 5268 or BS EN 1995 (EC5) for structural timber design. For foundations, allowable bearing pressures: undisturbed firm clay = 75–150 kN/m²; medium dense sand = 100–200 kN/m²; dense gravel = 200–400 kN/m²; rock = 500–3,000 kN/m². Always confirm bearing capacity with a site investigation report — these are typical values only.

Construction tips and best practices

Always apply a safety factor — UK structural design uses partial factors from Eurocodes: γ_G = 1.35 for dead loads, γ_Q = 1.5 for live loads. The design load is typically 1.35×dead + 1.5×live.
Deflection often governs beam design before strength — check that the selected beam satisfies L/360 (for floors) or L/180 (for rafters) deflection limits in addition to bending strength.
For rafter sizing, remember that the rafter length is the slant length — timber length orders must use slant length, not the horizontal run.
Temporary props during renovation must carry the full dead load of the floors/walls above — never assume a temporary timber post is adequate without checking its slenderness ratio and bearing capacity.
Lintels over openings carry the load triangle above the opening (45° load dispersal) — for a 1.8m opening in a 100mm thick wall with two floors above, the lintel load can exceed 30 kN/m.

A typical domestic timber floor joist (50×200mm C16 at 400mm centres) can span up to approximately 4.2m — exceeding this span requires either deeper joists, closer centres, or structural steel.
The steel frame of The Shard (London) weighs approximately 7,500 tonnes — equivalent to about 1,000 double-decker buses.
A standard domestic brick (3.6 kg) exerts a dead load of approximately 35 N; a stack of 100 bricks (a half-pallet) imposes approximately 3.5 kN — the same as the weight of a small car.
Ground bearing capacity of firm clay (approximately 100 kN/m²) means a 1m × 1m foundation pad can safely support 10 tonnes of load.

Common mistakes to avoid

Using unfactored loads — always apply Eurocode partial safety factors (γ_G = 1.35, γ_Q = 1.5) before checking section capacity. Factored design loads are always higher than characteristic loads.
Ignoring self-weight of the beam — a steel UB section can weigh 20–100 kg/m (0.2–1.0 kN/m); this adds to the design UDL and must be included in the bending moment calculation.
Confusing span with clear span — the structural span is measured between bearing centres, not the clear opening. For a beam on 150mm padstones, the structural span is the clear opening plus 300mm.
Using bearing capacity values from general tables without a site investigation — bearing capacity varies enormously even within a single site; always confirm with a Ground Investigation Report.

Structural calculations must be carried out by a qualified structural engineer for any work requiring Building Regulations approval. This includes new buildings, extensions, loft conversions, removal of load-bearing walls, new openings, and changes to structural floors or roofs. Under the Building Regulations 2010 (England and Wales), structural calculations for such work must comply with the relevant Eurocodes and associated UK National Annexes. The results from this calculator are for preliminary guidance and educational purposes only and do not constitute a structural design, building regulations submission, or structural engineer's report. Always employ a qualified Chartered Structural Engineer (MIStructE or CEng MICE) for regulated structural work.