ProReckoner gives tradies free on-site calculators that cover all the essential maths — from concrete volumes and paint coverage to solar system sizing, roof pitch angles, pipe fall and electrical Ohm's Law. Built for builders, electricians, plumbers, painters, tilers, roofers, concreters and site managers. Works offline on your phone once loaded.
Materials & Area
4 toolsConcrete Calculator
Bags of premix or volume of ready-mix concrete for slabs, footings and posts. Handles metric and imperial dimensions.
Volume (m³) = Length × Width × Depth
Bags = Volume × 1000 ÷ (Yield per bag ≈ 13 L/20kg bag)
Always add 10% extra for waste and spillage. A standard 20 kg premix bag yields approximately 10–13 litres of concrete.
Paint Coverage Calculator
Litres of paint needed for walls, ceilings or exterior surfaces. Accounts for coats and coverage rate.
Litres = Area × Coats × Waste ÷ Coverage rate
Example: 40 m², 2 coats, 12 m²/L, 10% waste= 40 × 2 × 1.10 ÷ 12 = 7.3 litres
Area & Volume Calculator
Floor area, wall area or room volume for any shape. Essential for tiling, flooring, paint and HVAC estimates.
Rectangle: L × W
Triangle: ½ × Base × Height
Circle: π × (Diameter÷2)²
Volume: L × W × H
Tile & Flooring Estimator
Tiles or flooring boards needed for any room with waste allowance. Enter room and tile dimensions.
Tiles needed = (Room area ÷ Tile area) × Wastage factor
Always round up to whole boxes. Standard wastage: 5% straight lay, 10% diagonal, 15% complex patterns.
Roofing & Structure
2 toolsRoof Pitch & Rafter Calculator
Roof pitch angle, rafter length and rise from span and pitch. Essential for carpenters, roofers and builders.
Run = Span ÷ 2
Rise = Run × tan(pitch°)
Rafter length = Run ÷ cos(pitch°)
Common pitches: 15° (low), 25° (medium), 35° (steep). Most AU metal roofing requires minimum 5°.
Staircase Calculator
Number of risers, rise height and going from floor-to-floor height. Checks compliance with building code limits.
Risers = Round(Height ÷ Preferred riser)
Actual rise = Height ÷ Number of risers
Total run = (Risers − 1) × Going
Building code (AS 1657): riser 115–190 mm, going 240–355 mm, 2R+G = 550–700 mm.
Plumbing
1 toolPipe Fall & Gradient Calculator
Drainage pipe fall (drop) over a given distance. Checks compliance with minimum gradient requirements.
Fall (mm) = Length (m) × 1000 ÷ Gradient
Example: 6 m pipe at 1:60 → 6000 ÷ 60 = 100 mm fallA 1:60 gradient means 1 mm drop for every 60 mm of pipe length.
Job Quoting
1 toolJob Quote & Markup Calculator
Calculate your sell price from costs and desired margin. Includes labour, materials and overhead — essential for sole traders.
Total cost = Materials + Labour + Overhead
Sell price = Total cost ÷ (1 − Margin%/100)
Margin vs markup: A 20% margin means profit is 20% of sell price. A 20% markup means profit is 20% of cost. This calculator uses margin (the industry standard for quoting).
Electrical
3 toolsOhm's Law Calculator
Voltage, current, resistance and power — solve for any unknown. Essential for every electrician every day.
V = I × R (Voltage = Current × Resistance)
I = V ÷ R (Current = Voltage ÷ Resistance)
P = V × I (Power = Voltage × Current)
Example: 240V circuit, 10A load → R = 240÷10 = 24 ohms · P = 240×10 = 2,400W
Parallel Circuit Resistance
Total resistance of parallel resistors in a circuit. Used for load balancing and circuit protection calculations.
Total R = R per resistor ÷ Number of resistors
Example (Trades Math Workbook): 4 × 8 ohm resistors in parallel= 8 ÷ 4 = 2 ohms total Adding resistors in parallel always reduces total resistance and increases total current draw.
Duct Airflow Calculator
Airflow volume (CFM) for a duct from its dimensions and air velocity. Used by electricians and HVAC mechanics.
Area (ft²) = Width (in) × Height (in) ÷ 144
Airflow (CFM) = Area (ft²) × Velocity (ft/min)
Example (Trades Math Workbook Q13): 8" × 24" duct at 1,200 ft/minArea = 192 ÷ 144 = 1.33 ft² · CFM = 1.33 × 1,200 = 1,596 CFM
Tiling & Grouting
2 toolsGrout Calculator
Kg of grout needed for a tiled area. Based on tile size, joint width and depth — covers floor and wall tiling.
Grout (kg/m²) = (Tile perimeter ÷ Tile area) × Joint width × Depth × Grout density
Always add 10% extra for wastage. Standard grout density ≈ 1.6 kg/L. Check manufacturer's coverage rate on the bag.
Tile Adhesive Calculator
Kg of tile adhesive needed for a given area. Coverage rate varies by adhesive type and notched trowel size.
Adhesive (kg) = Area × Coverage rate × Wastage factor
Example (from Trades Math Workbook Q3): 9.2m × 7.8m = 71.76m². At 5kg/m² with 10% waste = 394.7 kgAlways check your specific adhesive manufacturer's recommended coverage rate.
Site, Plumbing & Quoting
3 toolsWater Pressure Calculator
Static water pressure from pipe height. Used by plumbers to check pressure at fixtures and size pressure reducers.
Pressure (kPa) = Height (m) × 9.81
Pressure (psi) = Height (ft) × 0.433
Example (Trades Math Workbook Q12): 28 ft pipe → 28 × 0.433 = 12.12 psiAS/NZS 3500 requires minimum 100 kPa at most fixtures. Maximum recommended: 500 kPa.
Material Weight Estimator
Weight of steel plates, concrete panels and aluminium sheets from dimensions. Used by site managers and crane operators.
Area = Length × Width
Weight = Area × Thickness factor × Material density
Example (Trades Math Workbook): 2ft × 18ft steel, ¾" thickArea = 36ft² · ¾ × 40 lb/ft² = 30 lb/ft² · Weight = 1,080 lb
Markup & Discount Calculator
Calculate selling price with markup, or sale price with discount. Includes tax. Essential for all sole traders.
Selling price = Cost × (1 + Markup%/100)
Sale price = Regular price × (1 − Discount%/100)
Example (Trades Math Workbook): Wholesale $97.25, 10% markup= $97.25 × 1.10 = $106.98
Solar
4 toolsSolar System Size Calculator
How many kW of solar you need based on your daily energy use and local sun hours. Starting point for every solar quote.
System size (kW) = Daily usage (kWh) ÷ (Sun hours × Efficiency)
Example: 20 kWh/day ÷ (4.5 hrs × 0.85) = 5.2 kW systemPeak sun hours vary by location and season. Always size slightly larger to account for cloudy days and future energy growth.
Solar Panel Estimator
Number of panels and roof area needed for a given system size. Essential for roofers and electricians planning an installation.
Panels needed = System size (W) ÷ Panel wattage
Roof area = Panels × Panel area (m²) × 1.25 (spacing)
Example: 6.6 kW ÷ 400W = 16.5 → 17 panelsRoof area = 17 × 2.0 m² × 1.25 = 42.5 m² (inc. spacing)
Always allow 25% extra roof area for panel spacing and access clearance.
Solar Savings & Payback
Estimated annual savings and years to pay off a solar system. The number every customer asks before signing.
Annual generation (kWh) = System kW × Sun hrs × 365 × 0.85
Savings = (Self-use kWh × Tariff) + (Export kWh × Feed-in rate)
Payback (years) = System cost ÷ Annual savings
Example: 6.6kW × 4.5hrs × 365 × 0.85 = 9,224 kWh/yr50% self-use at 30¢ + 50% export at 8¢ = ~$1,753/yr savings
Inverter Sizing Calculator
Correct inverter size from total panel wattage. Includes derating factor and Australian grid connection rules.
Min inverter size (kVA) = Panel kWp ÷ Derating factor
Max panel/inverter ratio = 133% (AS/NZS 4777.1)
Example: 6.8 kWp ÷ 1.1 = 6.18 kVA inverter → select 6.0 or 6.6 kVASingle phase grid limit is typically 10 kVA. Three phase allows up to 30 kVA. Always confirm with your DNSP (local network operator).