Voltage Drop Calculator

Ensure all fields are completed before calculating, otherwise you may get an error message.

Terms of use

Electrical Apps for iPhone
Android Electrical Apps

Voltage Drop Calculator

A free Voltage Drop Calculator to calculate the volt drop for cables by inputting your desired Load, Voltage and Cable Length and then select from the pre-set options. In some countries Voltage Drop might be referred to as Voltage Loss.

Voltage Drop Formula

How to calculate Voltage Drop = The voltage drop calculation formula is expressed as Voltage Drop = ( mV/A/m ) x L x Ib / 1000.


  • mV = millivolts
  • A = Amps
  • L = Cable Length in meters
  • Ib = Design Current in Amps
  • / = Divide by 1000 to convert the result from Millivolts to Volts
  • mV/A/m = millvolts per Amp per Meter.


To find the mV/A/m for a type of cable, the volt drop per Amp per meter values are listed in Appendix 4 of BS 7671, however we can save you the hassle and time, as we have already programmed these values directly in to this Voltage Drop Calculator (including a correction factor for temperature).


Not only will this voltage drop calculator calculate the voltage drop in Volts, it will also calculate the voltage drop percent (%).

Voltage Drop Cable Size Calculations

A voltage drop calculator will only calculate the voltage drop of a specific cable type and size combination that you select, it will not calculate the selected cable and size is suitable for the circuit as other parameters also apply, such as installation method, maximum demand etc.. for this you should use a Cable Calculator.

Voltage Drop Regulations

525.1 – In the absence of any other consideration, under normal service conditions the voltage at the terminals of any fixed current-using equipment shall be greater than the lower limit corresponding to the product standard relevant to that equipment.

525.201 –  Where fixed equipment current-using equipment is not subject of a product standard the voltage at the terminals shall be such as not to impair the safe functioning of that equipment.

525.202 – The above requirements are deemed to be satisfied if the voltage drop between the origin of the installation (usually the supply terminals) and a socket-outlet or the terminals of fixed current-using equipment does not exceed that stated in Appendix 4, section 6.4.
(Source: BS 7671)

Voltage Drop Example

The volt drop example below might be an example for a cooker or electric shower circuit voltage drop calculation.

Cable Type: Flat Twin & Earth
Cable Size: 6 mm² / 2.5 mm² (cpc)
Cable Length: 15 Meters
Design Current: 32 Amps (7.36 kW)

A Flat Twin & Earth 6 mm² volt drop mV/A/m = 7.3, so 7.3 x 32 Amps x 15 meters / 1000 = 3.5 Volts.

Voltage Drop Calculator

Voltage Drop Demo

Voltage Drop Calculator for iPhone and Android, note the demo may not fully reflect the current version of the app.

Android Electrical Apps
Electrical Apps for iPhone

For more great electrical software’s like this voltage drop calculator take a look at our full version cloud electrical software or our mobile electrician apps.