12V voltage-drop calculator
Estimate DC copper cable voltage drop, load voltage, and power loss for 12V and other low-voltage systems using feet, meters, AWG, or mm².
Estimate inverter running current first if you are checking an inverter cable. For a boat, use the separate marine wire planning tool with its stated marine assumptions.
Enter your system assumptions
Common copper areas: 14 AWG = 2.08 mm²; 12 = 3.31; 10 = 5.26; 8 = 8.37; 6 = 13.3; 4 = 21.2; 2 = 33.6; 1/0 = 53.5; 2/0 = 67.4; 4/0 = 107.2. Enter the area for your selected cable; this table is not an ampacity recommendation.
Formula and assumptions
loop R = 2 × length m × 0.017241 × [1 + 0.00393 × (temperature °C − 20)] ÷ area mm²; drop V = A × R; loss W = A × drop VThe return conductor is included automatically: enter one-way length. Both conductors are assumed copper, equal size, and at the entered temperature. 75°C is a conservative warm-conductor scenario, not an insulation or terminal rating.
Worked example
At 12 V and 20 A, a 10 ft (3.048 m) one-way run of 10 AWG copper (5.26 mm²), at 75°C, loses approximately 0.486 V or 4.05%. The load sees about 11.514 V; cable loss is about 9.72 W.
Frequently asked questions
Does low voltage drop mean the wire is safe?
No. This tool does not check ampacity, terminal limits, insulation, routing, bundling, fuse coordination, or fault interruption. There is no universal passing percentage; compare the equipment’s minimum operating voltage.
Can I use this for chassis returns or aluminum?
No. The model assumes two equal copper conductors and excludes connection, fuse, switch, and chassis resistance. Use measured resistance or a qualified system design for other paths.