20x20 Concrete Slab Calculator

Brick Wall & Mortar Math:

Net wall volume is calculated by subtracting windows and doors deductions. Standard bricks are added with their respective mortar joint thicknesses to estimate brick volume and count.

Brick with Mortar Vol = (Brick L + Joint) × (Brick W + Joint) × (Brick H + Joint)
Brick Count = Net Wall Volume / Brick with Mortar Vol (includes 5% waste)
Actual Brick Volume = Brick Count × Single Brick Volume (without mortar)
Mortar Wet Volume = Net Wall Volume - Actual Brick Volume
Mortar Dry Volume = Mortar Wet Volume × 1.33 (for dry bulk shrinkage)
Cement & Sand are divided based on mix ratio (e.g. 1:6)

Plastering Volume Math:

Plaster thickness is multiplied by the plastering surface area to get the wet mortar volume. Mortar shrinks significantly when mixing, so a dry multiplier of 1.33 is applied, plus wastage.

Wet Mortar Volume = Surface Area × Plaster Thickness
Dry Mortar Volume = Wet Mortar Volume × 1.33
Cement Volume = Dry Mortar Volume × (1 / (1 + Sand Part))
Sand Volume = Dry Mortar Volume × (Sand Part / (1 + Sand Part))

Wall Painting & Putty Coverage Math:

Material estimations are based on standard coverage metrics per unit area for wall surfaces:

Wall Putty (2 coats): Coverage of ~1.2 kg per m² (~0.11 kg per sq ft)
Wall Primer (1 coat): Coverage of ~0.10 liters per m² (~0.009 liters per sq ft)
Emulsion Paint (2 coats): Coverage of ~0.25 liters per m² (~0.023 liters per sq ft)

Tile & Bedding Math:

Tile counts are determined by dividing the floor area by the single tile area. A 10% wastage allowance is automatically added for tile cutting. Bedding mortar lays underneath the tiles, which requires a wet mortar thickness of 25mm to 50mm.

Tile Count = (Floor Area / Tile Area) × 1.10 (Wastage Factor)
Bedding Wet Volume = Floor Area × Bedding Thickness
Bedding Dry Volume = Bedding Wet Volume × 1.33
Cement & Sand are split based on bedding mix ratio (e.g. 1:4)

Roofing Pitch & Materials Math:

A gable roof slope factor is calculated using sqrt(1 + (rise/run)^2). The total actual surface area is calculated by multiplying the footprint area (with overhang eaves) by this slope factor.

Roof Pitch Ratio = Rise / Run (e.g., 6:12 = 0.5)
Roof Slope Factor = √(1 + Pitch Ratio²)
Extended Roof Area = 2 × (Ground Length + 2×Overhang) × (Run + Overhang) × Slope Factor
Shingle Bundles = Roof Area × (1 + Waste) / Bundle Coverage (~3.1 m² or 33.3 sq ft)
Underlayment Rolls = Roof Area × (1 + Waste) / Roll Coverage (~40 m² or 430 sq ft)

Staircase Design & Concrete Math:

Stairs steps are designed based on rise constraints. The structural concrete volume is the sum of all individual step triangular prisms plus the waist slab running underneath them.

Number of Steps = Round(Total Rise / Target Riser Height)
Actual Riser Height = Total Rise / Number of Steps
Number of Treads = Number of Steps - 1
Total Run = Treads Count × Target Tread Depth
Staircase Angle = Arctan(Riser Height / Tread Depth)
Waist Slab Volume = Stringer Length × Stair Width × Waist Thickness
Steps Volume = Stair Width × Riser Height × Tread Depth × Steps Count / 2
Total Concrete Vol = Waist Slab Volume + Steps Volume

Gravel, Soil & Excavation Math:

Gravel base volume is calculated from area and depth. Because aggregates compact under weight, we multiply the base volume by a **Compaction Factor** (typically 10-20%) to find the necessary loose supply volume.

Wet Volume = Area Length × Area Width × Fill Depth
Total Volume = Wet Volume × (1 + Compaction Factor / 100)
Total Weight = Total Volume × Loose Density (approx. 1.5 - 1.7 tonnes/m³)

Lumber & Framing Math:

Stud count, joist quantities, or board feet volumes are calculated based on lengths, on-center spacing, and board sizing parameters, including a standard waste factor.

Stud Count = (Wall Length / Stud Spacing) + 1 + Corner/Plate Extras
Joist Count = (Span Width / Joist Spacing) + 1
Board Feet = (Thickness" × Width" × Length' / 12) × Board Count

Fence & Boundary Materials Math:

Total post counts, rail quantities, picket counts, and post-hole concrete bag quantities are computed based on boundary lengths and materials spacing presets.

Posts = Ceil(Fence Length / Post Spacing) + 1 + Corner/Gate Extras
Rails = (Posts - 1) × Rails per Panel
Pickets = Ceil(Fence Length × 12 / (Picket Width + Slat Gap)) × 1.05
Concrete Bags = Posts × 1.5 bags (80 lb Fast-Setting Concrete)

Drywall Sheets & Compound Math:

Sheet counts are computed from total wall and ceiling areas (minus window/door deductions). Finishing compound, tape, and screws are calculated using coverage constants.

Wall Area = 2 × (Length + Width) × Ceiling Height
Ceiling Area = Length × Width
Drywall Sheets = (Total Area - Deductions) / Sheet Area × 1.10 (Waste)
Compound Buckets = Total Area / 100 sq ft per 4.5-gal bucket
Drywall Screws = Total Area / 500 sq ft per 5 lb box

Insulation & R-Value Math:

Insulation package counts for rolls, batts, or loose-fill blown bags are calculated by comparing project square footage to standard coverage specifications.

Packs / Rolls = Area / Package Coverage Area × 1.05 (Waste)
Blown-In Bags = Area × Bags per 100 sq ft (for target R-value) × 1.05

Tile, Grout & Adhesive Math:

Tile piece counts are computed by dividing room square footage by individual tile square footage. Grout weight is determined by joint width, depth, and area.

Tile Area = (Width" × Height") / 144
Tile Count = (Tiling Area / Tile Area) × (1 + Waste / 100)
Grout (lbs) = (Length + Width) / (Length × Width) × Joint × Depth × 12.7 × Area
Thinset Bags = Tiling Area / 35 sq ft per 50 lb bag