Formulas

A quick reference of the formulas behind every TradeCalc calculator. Use these to verify calculations, study for your ticket, or just understand what's happening under the hood

BTU to CFM

CFM = BTU/hr ÷ (1.08 × ΔT)

Converts heating or cooling capacity in BTU/hr to airflow in CFM. Used when sizing air handlers or verifying airflow against equipment output.

Tonnage to CFM

CFM = Tons × 400

Convert AC tonnage to expected airflow using the standard 400 CFM/ton rule.

Air Velocity

Air Velocity (FPM) = CFM ÷ Area (sq ft)

Calculate air velocity in FPM from airflow and duct cross-sectional area.

Air Changes Per Hour

ACH = (CFM × 60) ÷ Room Volume

Estimate ACH based on room volume and airflow delivered.

Outside Air CFM

Outside Air (CFM) = (People × CFM/person) + (Area × CFM/ft²)

Calculate minimum outdoor air required for ventilation by occupancy and area.

Duct Area

Rectangular: Area (sq ft) = (Width × Height) ÷ 144

Round: Area (sq ft) = π × (Diameter ÷ 2)² ÷ 144

Calculate cross-sectional area of a round or rectangular duct.

Static Pressure

Static Pressure (in w.g.) = (Friction Rate × Duct Length) ÷ 100

Calculate total static pressure loss across supply and return sides.

Heat Loss

Heat Loss (BTU/hr) = Floor Area (ft²) × Heat Loss Factor

Estimate heat loss using floor area and a climate-based heat loss factor.

Pipe Flow Velocity

Velocity (ft/s) = Flow Rate (GPM) × 0.4085 ÷ Pipe ID (in)²

Calculate water velocity in a pipe using flow rate and internal diameter.

Pipe Sizing

Pipe ID (in) = √(Flow Rate (GPM) × 0.4085 ÷ Target Velocity (ft/s))

Calculate required pipe diameter based on flow rate and target velocity.

GPM to CFM

CFM = GPM × 0.1337

Convert water flow rate in GPM to equivalent airflow in CFM.

Hot Water Heater Sizing

BTU/hr = (GPH × 8.33 × ΔT) ÷ Efficiency

Estimate required hot water heater capacity based on demand and temperature rise.

Drain Slope

Drop (in) = Length (ft) × Slope (in/ft)

Calculate total vertical drop required over a horizontal drain run.