How to convert volts to ohms

How to convert electrical voltage in volts (V) to electric resistance in ohms (Ω).

You can calculate ohms from volts and amps or watts, but you can't convert volts to ohms since volt and ohm units do not measure the same quantity.

Volts to ohms calculation with amps

According to ohm's law, the resistance R in ohms (Ω) is equal to the voltage V in volts (V) divided by the current I in amps (A):

R_(Ω) = V_(V) /I_(A)

So ohms are equal to volts divided by amps:

ohms = volts / amps

or

Ω = V / A

Example

Calculate the resistance in ohms of a resistor when the voltage is 5 volts and the current is 0.2 amps.

The resistance R is equal to 5 volts divided by 0.2 amps, which is equal to 25 ohms:

R = 5V / 0.2A = 25Ω

Volts to ohms calculation with watts

The power P is equal to the voltage V times the currentI:

P = V × I

The current I is equal to the voltage V divided by the resistance R (ohm's law):

I = V / R

So the power P is equal to

P = V × V / R = V ² / R

So the resistance R in ohms (Ω) is equal to the square value of voltage V in volts (V) divided by the power P in watts (W):

R_(Ω) = V ²_(V) /P_(W)

So ohms are equal to the square value of volts divided by watts:

ohms = volts² / watts

or

Ω = V² / W

Example

Calculate the resistance in ohms of a resistor when the voltage is 5 volts and the power is 2 watts.

The resistance R is equal to square of 5 volts divided by 2 watts, which is equal to 12.5 ohms.

R = (5V)² / 2W = 12.5Ω

How to convert ohms to volts »

What is Ohm's law?

Ohm's Law is a formula used to calculate the relationship between voltage, current and resistance in an electrical circuit.

To students of electronics, Ohm's Law (E = IR) is as fundamentally important as Einstein's Relativity equation (E = mc²) is to physicists.

E = I x R

When spelled out, it means voltage = current x resistance, or volts = amps x ohms, or V = A x Ω.

Named for German physicist Georg Ohm (1789-1854), Ohm's Law addresses the key quantities at work in circuits:

Quantity	Ohm's Law symbol	Unit of measure (abbreviation)	Role in circuits	In case you're wondering:
Voltage	E	Volt (V)	Pressure that triggers electron flow	E = electromotive force (old-school term)
Current	I	Ampere, amp (A)	Rate of electron flow	I = intensity
Resistance	R	Ohm (Ω)	Flow inhibitor	Ω = Greek letter omega

If two of these values are known, technicians can reconfigure Ohm's Law to calculate the third. Just modify the pyramid as follows:

If you know voltage (E) and current (I) and want to know resistance (R), X-out the R in the pyramid and calculate the remaining equation (see the first, or far left, pyramid above).

Note: Resistance cannot be measured in an operating circuit, so Ohm's Law is especially useful when it needs to be calculated. Rather than shutting off the circuit to measure resistance, a technician can determine R using the above variation of Ohm's Law.

Now, if you know voltage (E) and resistance (R) and want to know current (I), X-out the I and calculate the remaining two symbols (see the middle pyramid above).

And if you know current (I) and resistance (R) and want to know voltage (E), multiply the bottom halves of the pyramid (see the third, or far right, pyramid above).

Try a few sample calculations based on a simple series circuit, which includes just one source of voltage (battery) and resistance (light). Two values are known in each example. Use Ohm's Law to calculate the third.