Construction Trade Formulas
Quick reference: All formulas used by the 8 calculators on this site. Hazen-Williams for pipe sizing, NEC 125% rule for breakers, concrete volume, roof pitch conversion, tile waste factors, paint coverage and stud framing.
1. Pipe Sizing — Hazen-Williams
Hazen-Williams Equation
Q = 0.2785 × C × d^2.63 × S^0.54Q = flow rate (GPM) · C = roughness coefficient · d = internal diameter (in) · S = head loss per unit length
C-values by pipe material
| Material | C value | Best for |
|---|---|---|
| PVC / CPVC | 150 | Cold water, irrigation |
| Copper Type L | 130 | Domestic hot/cold supply |
| PEX | 140 | Flexible domestic supply |
| Galvanized Steel | 120 | Older installations |
| Cast Iron | 100 | Mains, commercial |
Worked example: 10 GPM through 100ft of ¾" copper pipe (ID = 0.811", C = 130).
Pressure drop = 3.49 psi per 100ft. Velocity = 6.02 ft/s.
Use Pipe Size Calculator →
Pressure drop = 3.49 psi per 100ft. Velocity = 6.02 ft/s.
2. Concrete Volume
Volume Formulas
Slab: V = length × width × depth (all in feet = cubic feet)Column: V = π × (diameter/2)² × height
1 cubic yard = 27 cubic feet = 0.765 m³
Bags (60lb) = V (cu ft) ÷ 0.45 · Bags (80lb) = V (cu ft) ÷ 0.60
Always add 8–10% waste
Worked example: 10×10ft slab at 4" thick.
V = 10 × 10 × (4÷12) = 33.3 cu ft = 1.23 cu yd → 74 × 60lb bags. Add 8% = 80 bags.
Use Concrete Calculator →
V = 10 × 10 × (4÷12) = 33.3 cu ft = 1.23 cu yd → 74 × 60lb bags. Add 8% = 80 bags.
3. Electrical Load — NEC 125% Rule
Load Calculation (NEC 2023)
Single phase: I = P ÷ VThree phase: I = P ÷ (V × √3)
Breaker size = I × 1.25 → round up to next standard
Standard sizes: 15, 20, 25, 30, 40, 50, 60, 70, 80, 100A
Worked example: 7,200W EV charger on 240V.
I = 7200 ÷ 240 = 30A. Breaker = 30 × 1.25 = 37.5A → 40A breaker. Use 8 AWG wire.
Use Electrical Calculator →
I = 7200 ÷ 240 = 30A. Breaker = 30 × 1.25 = 37.5A → 40A breaker. Use 8 AWG wire.
4. Voltage Drop
Voltage Drop Formula
V_drop = (2 × L × I × R) ÷ 1000L = one-way length (ft) · I = current (A) · R = resistance (Ω/1000ft)
12 AWG copper: R = 1.588 · 10 AWG: R = 0.999 · 8 AWG: R = 0.628
Drop % = V_drop ÷ V_supply × 100 · Max: 3% lighting, 5% power
AWG reference table
| AWG | mm² | Max amps | Typical use |
|---|---|---|---|
| 14 AWG | 2.5 mm² | 15A | Lighting |
| 12 AWG | 4 mm² | 20A | General outlets |
| 10 AWG | 6 mm² | 30A | Dryer, AC |
| 8 AWG | 10 mm² | 40A | EV charger, range |
| 6 AWG | 16 mm² | 55A | Large appliances |
5. Roof Pitch Conversion
Pitch Formulas
Degrees = arctan(rise ÷ run)Percentage = (rise ÷ run) × 100
Rafter length = √(rise² + run²)
Rafter factor = √(1 + (rise/run)²)
True roof area = plan area × rafter factor
| Pitch | Degrees | % | Rafter factor |
|---|---|---|---|
| 4:12 | 18.4° | 33% | 1.054 |
| 6:12 | 26.6° | 50% | 1.118 |
| 8:12 | 33.7° | 67% | 1.202 |
| 12:12 | 45.0° | 100% | 1.414 |
6. Tile & Grout
Tile Quantity
Base tiles = room area ÷ tile areaTotal = base tiles × (1 + waste factor)
Waste: 10% straight · 15% diagonal · 20% herringbone
Grout (kg) = area × (joint_w × thickness) ÷ (tile_L × tile_W) × 1.9 × 1.1
7. Paint Coverage
Paint Quantity
Wall area = 2 × (length + width) × ceiling heightSubtract: doors ~1.8m² each · windows ~1.2m² each
Paint (L) = total area × coats ÷ coverage rate
Standard emulsion: 10–12 m²/L (400–500 sq ft/gal)
Textured surfaces: reduce coverage by 30–40%
8. Stud Framing
Stud Count Formula
Studs = ceil(wall length ÷ spacing) + 1Add 2 king + 2 trimmer studs per door/window opening
Plate length = wall length × 2 (top + bottom)
16" OC (400mm): load-bearing walls · 24" OC (600mm): partitions (~25% fewer studs)