Part of the CalcProMaster guides library · Electrical, mechanical & structural math
Engineering calculations punish small errors — a wrong voltage-drop figure means undersized cables, a wrong gear ratio means a drivetrain that never reaches design speed. These guides show the formula, the assumptions behind it, and a worked example you can check by hand.
Every engineering formula hides assumptions, and knowing them matters more than memorizing the algebra. Voltage drop = 2 × L × I × ρ ÷ A assumes a resistive load, a conductor at operating temperature, and length L counted along both conductors — hence the 2. Ignore any of these and the number is precise but wrong.
A worked check: 20 m run, 10 A load, 2.5 mm² copper (ρ ≈ 0.0175 Ω·mm²/m). Drop = 2 × 20 × 10 × 0.0175 ÷ 2.5 = 2.8 V — about 1.2% of a 230 V supply, comfortably inside the 3–5% usually tolerated. If your real circuit shows double that, the likely culprits are temperature-derated conductors or an undersized return path, not the formula.
Awg, mm², psi, bar — converting the units engineering specs mix freely.
Load-bearing material math for structural and civil work.
VD = 2 × L × I × R per conductor pair, where L is one-way cable length in metres, I the load current, and R the conductor resistance in ohms per metre. Keeping drop under 3% of supply voltage is the common design target for branch circuits.
It is driven-teeth ÷ driver-teeth: a 4:1 ratio cuts speed to a quarter while multiplying torque fourfold (minus friction losses). The gear ratio calculator converts ratio, RPM and torque in both directions.
Voltage Drop Calculator Gear Ratio Calculator Torque Calculator