If you think of an electrical device as a piece of plumbing, voltage is the amount of water that you send down into the pipe, resistance is the pipe's relative width or narrowness, and current is the speed with which the water flows. Power measures the water's relative difficulty or ease making its way through the pipe. You relate all these values to one another using a common set of physics equations known as Ohm's law. If you need to calculate electricity's current flow, you'll need to have at least two of the three values -- voltage, resistance or power -- listed above.
Calculate current flow using voltage and resistance. According to Ohm's law, you can express electricity's current in amps as a ratio of its voltage in volts to the resistance of the device it's flowing through in ohms -- I = E/R, respectively. For example, if you want to know the current flow of 220 V of electricity as it flows through a laptop computer with 80 ohms of resistance, you would simply plug these values into the equation as follows: I = 220/80 = 2.75 amps.
Calculate current flow using power and resistance. Ohm's law also states that electrical current, "I," is equal to the square root of the power dissipated as it travels through the device divided by that device's resistance. If a light bulb dissipates 80 watts of power and has a resistance of 55 ohms, you can calculate the electricity's current as follows: I = sqrt(80/55) = sqrt(1.4545) = 1.20 amps.
Calculate current flow using power and voltage. If you have a space heater which dissipates 420 watts of power when it takes in 120 V of electricity, Ohm's law states you can calculate this electricity's current using the equation "I = P/E." For this example, compute current like so: I =420/120 = 3.5 amps.