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Estimator's Formulas

Essential construction estimating calculations. All formulas use metric units with [variable] placeholders for easy reference.

Area & Perimeter

Rectangle Area m²

Calculate the area of a rectangular space.

[Area] = [Length] × [Width]
Example: Room 5.4m × 3.6m = 19.44 m²

Rectangle Perimeter m

Calculate the perimeter of a rectangular space.

[Perimeter] = 2 × ([Length] + [Width])
Example: Room 5.4m × 3.6m = 2 × (5.4 + 3.6) = 18 m

Triangle Area m²

Calculate the area of a triangle.

[Area] = [Base] × [Height] ÷ 2
Example: Base 6m, Height 4m = 6 × 4 ÷ 2 = 12 m²

Circle Area m²

Calculate the area of a circular space.

[Area] = π × [Radius]²
Example: Radius 3m = 3.14159 × 3² = 28.27 m²

Circle Circumference m

Calculate the circumference of a circle.

[Circumference] = 2 × π × [Radius]
Example: Radius 3m = 2 × 3.14159 × 3 = 18.85 m

Trapezoid Area m²

Calculate the area of a trapezoid shape.

[Area] = ([Top] + [Bottom]) × [Height] ÷ 2
Example: Top 4m, Bottom 6m, Height 3m = (4 + 6) × 3 ÷ 2 = 15 m²

Wall Area (with openings) m²

Calculate net wall area after deducting openings.

[Net Area] = [Gross Area] - [Openings]
Example: Wall 12m² - Door 1.8m² - Window 1.2m² = 9 m²

Volume

Rectangular Volume m³

Calculate volume of a rectangular prism.

[Volume] = [Length] × [Width] × [Depth]
Example: Slab 10m × 8m × 0.15m = 12 m³

Cylinder Volume m³

Calculate volume of a cylinder (columns, pipes).

[Volume] = π × [Radius]² × [Height]
Example: Column r=0.25m, h=3m = 3.14159 × 0.25² × 3 = 0.59 m³

Cone Volume m³

Calculate volume of a cone.

[Volume] = π × [Radius]² × [Height] ÷ 3
Example: r=2m, h=3m = 3.14159 × 2² × 3 ÷ 3 = 12.57 m³

Sphere Volume m³

Calculate volume of a sphere.

[Volume] = 4/3 × π × [Radius]³
Example: r=1.5m = 4/3 × 3.14159 × 1.5³ = 14.14 m³

Trapezoidal Prism Volume m³

Calculate volume for trenches with battered sides.

[Volume] = ([Top Width] + [Bottom Width]) ÷ 2 × [Depth] × [Length]
Example: Trench top 1.2m, bottom 0.6m, depth 1m, length 20m = 18 m³

Concrete

Concrete Slab m³

Calculate concrete volume for ground slabs.

[Volume] = [Length] × [Width] × [Thickness]
Example: Slab 15m × 12m × 0.1m = 18 m³
Typical thicknesses:
  • Domestic slab: 85-100mm
  • Industrial: 150-200mm
  • Heavy duty: 200-300mm

Strip Footing m³

Calculate concrete for continuous strip footings.

[Volume] = [Width] × [Depth] × [Length]
Example: Footing 0.45m × 0.3m × 40m = 5.4 m³

Pad Footing m³

Calculate concrete for isolated pad footings.

[Volume] = [Length] × [Width] × [Depth] × [Quantity]
Example: 8 pads @ 1.2m × 1.2m × 0.4m = 4.61 m³

Concrete Column m³

Calculate concrete for rectangular columns.

[Volume] = [Width] × [Depth] × [Height] × [Quantity]
Example: 6 columns @ 0.4m × 0.4m × 3.5m = 3.36 m³

Concrete Beam m³

Calculate concrete for beams.

[Volume] = [Width] × [Depth] × [Length]
Example: Beam 0.3m × 0.6m × 8m = 1.44 m³

Concrete Steps m³

Calculate concrete for stairs.

[Volume] = [Avg Thickness] × [Width] × [Slope Length]
Example: Stair 0.15m avg × 1.2m × 4m = 0.72 m³
Average thickness = Waist + (Riser ÷ 2)

Formwork Area m²

Calculate formwork for slab edges.

[Area] = [Perimeter] × [Slab Depth]
Example: Edge 54m × 0.1m depth = 5.4 m²

Masonry

Bricks per m² No.

Calculate number of standard bricks required.

[Bricks] = [Wall Area] × 50
Example: Wall 25m² × 50 = 1,250 bricks
Standard brick 230×76×110mm with 10mm joints = ~50/m²

Blocks per m² No.

Calculate number of concrete blocks required.

[Blocks] = [Wall Area] × 12.5
Example: Wall 40m² × 12.5 = 500 blocks
Standard block 390×190×190mm with 10mm joints = ~12.5/m²

Mortar for Brickwork m³

Calculate mortar volume for brick walls.

[Mortar] = [Wall Area] × 0.03
Example: Wall 50m² × 0.03 = 1.5 m³
Approximately 30 litres mortar per m² of single-leaf brickwork

Mortar for Blockwork m³

Calculate mortar volume for block walls.

[Mortar] = [Wall Area] × 0.012
Example: Wall 100m² × 0.012 = 1.2 m³

Wall Ties No.

Calculate wall ties for cavity walls.

[Ties] = [Wall Area] × 2.5
Example: Wall 80m² × 2.5 = 200 ties
Minimum 2.5 ties/m² at 450mm vertical × 900mm horizontal centres

Roofing

Roof Pitch (Degrees) °

Calculate roof pitch angle from rise and run.

[Pitch] = arctan([Rise] ÷ [Run])
Example: Rise 2m, Run 4m = arctan(0.5) = 26.6°

Rafter Length m

Calculate rafter length from rise and run.

[Rafter] = √([Rise]² + [Run]²)
Example: Rise 2m, Run 4m = √(4 + 16) = 4.47 m

Roof Area (Simple) m²

Calculate roof area from plan area and pitch factor.

[Roof Area] = [Plan Area] ÷ cos([Pitch])
Example: Plan 100m², 25° pitch = 100 ÷ 0.906 = 110.4 m²

Roof Sheets No.

Calculate number of roofing sheets required.

[Sheets] = [Roof Length] ÷ [Cover Width]
Example: Length 12m ÷ 0.762m cover = 16 sheets
Common cover widths: Trimdek 762mm, Klip-Lok 700mm, Custom Orb 762mm

Roof Tiles No.

Calculate number of roof tiles required.

[Tiles] = [Roof Area] × [Tiles/m²]
Example: Area 150m² × 10.5 tiles/m² = 1,575 tiles
Concrete tiles: ~10.5/m², Terracotta: ~12-15/m²

Ridge Capping m

Calculate ridge and hip capping requirements.

[Capping] = [Ridge Length] + [Hip Lengths]
Example: Ridge 8m + 4 hips @ 5m = 28 m

Earthworks

Excavation Volume m³

Calculate excavation for rectangular pit.

[Volume] = [Length] × [Width] × [Depth]
Example: Pit 15m × 12m × 0.5m = 90 m³

Bank to Bulk (Swell) m³

Calculate bulked volume after excavation.

[Bulk Vol] = [Bank Vol] × (1 + [Swell %])
Example: 90m³ bank × 1.25 (25% swell) = 112.5 m³ bulk
Typical swell factors:
  • Sand: 10-15%
  • Clay: 20-40%
  • Rock: 40-80%

Compaction Factor m³

Calculate compacted volume from loose fill.

[Compacted] = [Loose Vol] × [Compaction Factor]
Example: 100m³ loose × 0.85 = 85 m³ compacted
Typical compaction: 0.80-0.90 (80-90% of loose volume)

Trench Volume m³

Calculate trench excavation volume.

[Volume] = [Width] × [Depth] × [Length]
Example: Trench 0.6m × 1.2m × 50m = 36 m³

Cut & Fill Balance m³

Calculate net earthworks balance.

[Balance] = [Cut Vol] - [Fill Vol]
Example: Cut 500m³ - Fill 350m³ = 150 m³ export

Truck Loads No.

Calculate number of truck loads for spoil removal.

[Loads] = [Bulk Volume] ÷ [Truck Capacity]
Example: 112.5m³ ÷ 10m³ truck = 12 loads
Common truck sizes: 6-wheeler 6m³, 8-wheeler 10m³, Semi 14m³

Steel & Rebar

Rebar Weight kg

Calculate weight of reinforcement bars.

[Weight] = [Length] × [kg/m]
Example: N12 bar 6m × 0.888 kg/m = 5.33 kg
Rebar weights (kg/m):
  • N10: 0.617
  • N12: 0.888
  • N16: 1.58
  • N20: 2.47
  • N24: 3.55
  • N28: 4.83
  • N32: 6.31

Mesh Weight kg

Calculate weight of reinforcement mesh.

[Weight] = [Area] × [kg/m²]
Example: SL82 mesh 50m² × 3.64 kg/m² = 182 kg
Common mesh weights:
  • SL62: 2.19 kg/m²
  • SL72: 2.96 kg/m²
  • SL82: 3.64 kg/m²
  • SL92: 4.53 kg/m²
  • SL102: 5.56 kg/m²

Rebar Lap Length mm

Calculate minimum lap length for reinforcement.

[Lap] = [Bar Dia] × [Factor]
Example: N16 bar × 40 (typical) = 640 mm
Typical factors: 25-50 × bar diameter depending on concrete grade and position

Steel Section Weight kg

Calculate weight of structural steel sections.

[Weight] = [Length] × [kg/m]
Example: 200UB25.4 beam × 8m = 25.4 × 8 = 203.2 kg

Bar Cutting Waste %

Calculate cutting waste allowance for rebar.

[Total] = [Net Weight] × (1 + [Waste %])
Example: 1000kg × 1.05 (5% waste) = 1,050 kg
Typical waste: 3-5% for standard bars, 8-12% for complex shapes

Finishes

Paint Coverage L

Calculate paint required for walls/ceilings.

[Litres] = [Area] ÷ [Coverage Rate] × [Coats]
Example: 200m² ÷ 12m²/L × 2 coats = 33.3 L
Typical coverage: 10-14 m²/L depending on surface and paint type

Floor Tiles No.

Calculate number of floor tiles required.

[Tiles] = [Floor Area] ÷ [Tile Area]
Example: 50m² ÷ (0.6 × 0.6) = 139 tiles
Add 10% for cutting waste on straight layouts, 15% for diagonal

Tile Adhesive kg

Calculate adhesive for floor/wall tiles.

[Adhesive] = [Area] × [kg/m²]
Example: 50m² × 4 kg/m² = 200 kg
Typical: 3-5 kg/m² depending on tile size and trowel notch

Grout kg

Calculate grout for floor/wall tiles.

[Grout] = [Area] × [kg/m²]
Example: 50m² × 0.5 kg/m² = 25 kg
Varies with tile size and joint width: 0.3-1.0 kg/m²

Plasterboard Sheets No.

Calculate plasterboard sheets required.

[Sheets] = [Wall Area] ÷ [Sheet Area]
Example: 85m² ÷ (2.4 × 1.2) = 30 sheets
Standard sheets: 2400×1200mm, 2700×1200mm, 3000×1200mm

Carpet m²

Calculate carpet with waste allowance.

[Carpet] = [Floor Area] × 1.10
Example: Room 24m² × 1.10 = 26.4 m²
10% waste for simple rooms, 15% for irregular shapes

Waste Factors

Standard Waste Allowances

Typical waste factors to add to net quantities for ordering materials.

Material Waste % Notes
Concrete (ready-mix) 2-5% Higher for complex pours
Reinforcement (bars) 3-5% 8-12% for complex bending
Reinforcement (mesh) 5-10% Cutting waste
Bricks 5% Breakage and cutting
Blocks 5% Breakage and cutting
Timber (framing) 10-15% Cutting waste
Plasterboard 10% Cutting and damage
Floor tiles (straight) 10% Cutting waste
Floor tiles (diagonal) 15% Higher cutting waste
Carpet 10-15% Seaming and fitting
Roofing sheets 5-10% Laps and cutting
Roof tiles 5% Breakage and cutting
Insulation batts 5% Compression and fitting
Paint 5-10% Touch-ups and coverage variation
Waterproofing membrane 10-15% Overlaps and turns
Applying waste factor:
[Order Qty] = [Net Qty] × (1 + [Waste %])

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