Ohms Law Calculator: Voltage, Current, Resistance and Power
How this Ohms law calculator works: V = I · R. Voltage equals current times resistance; together with P = V · I any two values give the other two. Example: 12 V across 6 Ω drives 2 A and dissipates 24 W. Choose below which two values you know.
Example: Which two values do you know: Voltage V and current I · Voltage V 12 V · Current I 2 A → Result: R = 6 Ω · P = 24 W. Source: OpenStax, OpenStax University Physics Vol. 2, 9.4 Ohm’s Law. Updated: .
How it is calculated
Ohm’s law
In 1826 Georg Simon Ohm found that the current through a conductor is proportional to the voltage across it. The constant of proportionality is the resistance:
V = I · R · I = V / R · R = V / I
Memory aid: in the Ohm’s law triangle V sits on top, I and R below – cover the value you want and the other two show the calculation.
Power and the formula wheel
Electrical power is P = V · I. Combined with Ohm’s law this gives all twelve formulas of the Ohm’s law wheel:
| To find | Formulas | ||
|---|---|---|---|
| Voltage V (volts) | I · R | P / I | √(P · R) |
| Current I (amps) | V / R | P / V | √(P / R) |
| Resistance R (ohms) | V / I | V² / P | P / I² |
| Power P (watts) | V · I | V² / R | I² · R |
Examples
- 1500 W space heater on 120 V: I = 1500 / 120 = 12.5 A, R = 120² / 1500 = 9.6 Ω
- 150 Ω LED resistor at 20 mA: V = 150 · 0.02 = 3 V, P = 0.02² · 150 = 60 mW
- 470 Ω resistor rated 0.25 W: I = √(0.25 / 470) = 23 mA, V = √(0.25 · 470) = 10.8 V
Limitations
Ohm’s law applies to ohmic resistors at constant temperature. Incandescent bulbs, diodes and motors are not linear, and in AC circuits with coils and capacitors the power factor also matters. Formulas: OpenStax University Physics Vol. 2, sections 9.4 and 9.5.
Frequently asked questions
What is the Ohm’s law formula?
V = I · R: voltage (volts) equals current (amps) times resistance (ohms). Rearranged: I = V / R and R = V / I.
How do you calculate watts with Ohm’s law?
P = V · I, or P = I² · R, or P = V² / R. Example: 120 V across 9.6 Ω gives 120² / 9.6 = 1500 W.
How many amps is 1000 watts at 120 volts?
I = P / V = 1000 / 120 ≈ 8.33 A. At 230 V it would be about 4.35 A.
How do you find resistance from voltage and current?
R = V / I. 12 V and 0.5 A give 24 Ω.
Does Ohm’s law work for AC?
For resistive loads such as heaters, yes, using RMS values. With motors, coils or capacitors you need impedance and the power factor.
Sources and legal basis
- OpenStax: OpenStax University Physics Vol. 2, 9.4 Ohm’s Law
- OpenStax: OpenStax University Physics Vol. 2, 9.5 Electrical Energy and Power (P = IV = I²R = V²/R)
As of:
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