Simple program to find resistance for a given resistivity in ohm metre on copper or aluminium wires given length and c.s.a.

rpla.py

#!/usr/bin/env python3
# Simple program to find resistance
# Author: Sam Saint-Pettersen, Feb 2026
# R = pL/A for a given resistivity in ohm metre (i.e. pCu or pAl).

import os
import sys

# Resistivities
resists: dict[str,float] = {
    'Cu': 1.72,
    'Al': 26.5
}

materials: dict[str,str] = {
    'Cu': 'Copper',
    'Al': 'Aluminium'
}

base = (10 ** -9)

def cls():
    _cls: str = 'cls' if os.name == 'nt' else 'clear'
    os.system(_cls)

def find_resistance() -> int:
    p: float = 0
    m: str = ''
    
    while (True):
        cls()
        print("Calculate resistance from length and c.s.a:")
        mat: int = int(input("Wire [1 = Copper (Cu) , 2 = Aluminium (Al), else quit]: "))
        match mat:
            case 1:
                p = resists['Cu']
                m = materials['Cu']
            case 2:
                p = resists['Al']
                m = materials['Al']
            case _:
                break

        print(f"Wire is {m} (p = {p} * 10^-9 ohm metre)")
        length: int = int(input("Length of wire in metres (m), 0 is quit: "))
        if length <= 0:
            break

        pL: float = ((p * base) * length)

        area: float = float(input("Enter c.s.a in mm squared (mm^2), 0 is quit: "))
        if area <= 0:
            break

        print()
        print("R = pL/A")

        a: float|int = area
        if a % 1 == 0: a = int(a)
        
        area = (area * (10 ** -6)) 

        print()
        print(f"(({p} * 10^-9 ohm metre) * {length} m) / ({a} * 10^-6 m)") 

        # Calculate the resistance, rounding the result to 3 decimal places.
        resistance: float = (round((pL / area) * 1000) / 1000)

        print()
        print(f"R = {resistance} ohms")

        if resistance < 1:
            print('OR')
            print(f'R = {int(resistance * 1000)} milliohms')

        print()
        input("Press Enter key to continue.")

    print("Exiting...")
    return 0

if __name__ == "__main__":
    sys.exit(find_resistance())

To use install Python and save this program (gist) to your computer. Run it on the command line or in Python's IDLE (comes with Python on Windows) or Thonny (for Linux) environment.