Enter any two of voltage, current, resistance and power to get the other two.
You enterCalculated
Press a formula to work from the two values it needs.
Edit a boxed value, or press a reading to work from it instead.
Resistance
12Ω
From voltage 24 V and current 2 A.
Check the values to see a result.
For a resistive load or DC. On AC with a power factor below 1, V ÷ I is the impedance, not the resistance.
V = I × RP = V × I = I2 × R = V2 ÷ R
24 V and 2 A give 12 Ω and 48 W.
Tip: A 250 Ω receiver on a 4-20 mA loop drops 5 V at 20 mA: I × R, the same law.
Ohm’s law says the voltage across a resistance equals the current through it times the resistance: V = I × R. With the power law, P = V × I, any two of voltage, current, resistance and power give the other two. 24 V across 12 Ω drives 2 A and dissipates 48 W.
Pick the two values you know. Each quantity has three formulas, one for each pair of the other three, so these twelve cover every case.
V = I × R = P ÷ I = √(P × R)
Use volts, amps and ohms together. For milliamps, divide by 1,000 first: 20 mA is 0.02 A. Pushing 8 A through 3 Ω takes 8 × 3 = 24 V.
I = V ÷ R = P ÷ V = √(P ÷ R)
A load of 15 Ω on 240 V draws 240 ÷ 15 = 16 A. A 60 W lamp on 12 V draws 60 ÷ 12 = 5 A.
R = V ÷ I = V2 ÷ P = P ÷ I2
Watts alone do not give ohms: you also need the voltage or the current at that power. A 2,000 W heater rated at 230 V has a hot resistance of 2302 ÷ 2,000 = 26.45 Ω. A load that takes 1,440 W at 12 A is 1,440 ÷ 122 = 10 Ω.
P = V × I = I2 × R = V2 ÷ R
Ohms alone do not give watts either: a resistance takes power from the voltage across it or the current through it. 16 Ω on 48 V takes 482 ÷ 16 = 144 W. I2 × R is why cable losses rise with the square of the current: twice the current in the same cable heats it four times as much.
On an AC circuit with a resistive load, such as a heater, the same formulas hold with RMS values. Where the load has inductance or capacitance, V ÷ I is the impedance, and the real power is V × I × PF. The kW to amps calculator covers AC circuits with a power factor.
At full scale, 20 mA flows through the 250 Ω input: 0.02 × 250 = 5 V. The transmitter needs that much supply voltage on top of its own minimum, which is the check the 4-20 mA loop voltage calculator makes for the whole loop.
Voltage 5 V Open in the calculator
I = P ÷ V = 3,000 ÷ 230 = 13.04 A, and the element’s hot resistance is V2 ÷ P = 17.63 Ω. A heater is close to a pure resistance, so Ohm’s law applies directly.
Current 13.04 A Open in the calculator
A quarter-watt 120 Ω resistor can carry up to √(0.25 ÷ 120) = 45.64 mA before it reaches its rating, with 5.477 V across it. An RS-485 terminator sees far less than that, which is why small resistors are enough there.
Current 45.64 mA Open in the calculator
Current in mA for standard resistor values at three common DC voltages, from I = V ÷ R.
| Resistance (Ω) | 5 V (mA) | 12 V (mA) | 24 V (mA) |
|---|---|---|---|
| 10 | 500 | 1,200 | 2,400 |
| 47 | 106.4 | 255.3 | 510.6 |
| 100 | 50 | 120 | 240 |
| 120 | 41.67 | 100 | 200 |
| 220 | 22.73 | 54.55 | 109.1 |
| 250 | 20 | 48 | 96 |
| 470 | 10.64 | 25.53 | 51.06 |
| 1,000 | 5 | 12 | 24 |
| 2,200 | 2.273 | 5.455 | 10.91 |
| 4,700 | 1.064 | 2.553 | 5.106 |
| 10,000 | 0.5 | 1.2 | 2.4 |
Power in watts that a resistance takes at four common voltages, from P = V2 ÷ R.
| Resistance (Ω) | 12 V (W) | 24 V (W) | 120 V (W) | 230 V (W) |
|---|---|---|---|---|
| 1 | 144 | 576 | 14,400 | 52,900 |
| 2 | 72 | 288 | 7,200 | 26,450 |
| 3 | 48 | 192 | 4,800 | 17,633 |
| 4 | 36 | 144 | 3,600 | 13,225 |
| 6 | 24 | 96 | 2,400 | 8,817 |
| 8 | 18 | 72 | 1,800 | 6,613 |
| 10 | 14.4 | 57.6 | 1,440 | 5,290 |
| 12 | 12 | 48 | 1,200 | 4,408 |
| 16 | 9 | 36 | 900 | 3,306 |
| 20 | 7.2 | 28.8 | 720 | 2,645 |
| 50 | 2.88 | 11.52 | 288 | 1,058 |
| 100 | 1.44 | 5.76 | 144 | 529 |
V = I × R: voltage in volts equals current in amps times resistance in ohms. Rearranged, I = V ÷ R and R = V ÷ I.
Multiply them: P = V × I. 12 V at 2 A is 24 W. This holds for DC and for resistive AC loads; for other AC loads, multiply by the power factor as well.
Not directly. An LED’s current rises steeply above its forward voltage, and a motor’s current depends on its load and back-EMF, not a fixed resistance. Ohm’s law still describes the resistors and cables around them.
Its resistance turns I2 × R into heat. A cable run of 0.1 Ω carrying 30 A dissipates 90 W along its length. The conductor resistance calculator gives R for a cable size and length.
P = I2 × R = 52 × 5 = 125 W, with 5 × 5 = 25 V across the resistance.
P = V2 ÷ R = 2402 ÷ 12 = 4,800 W, and the load draws 240 ÷ 12 = 20 A.
Volts: V = I × R, so 2 A through 12 Ω is 24 V. Milliamps times ohms gives millivolts and milliamps times kilohms gives volts: 20 mA through 250 Ω is 5,000 mV, or 5 V. In the other direction, volts divided by milliamps gives kilohms.
Yes. With watts and volts, I = P ÷ V and R = V2 ÷ P; with watts and amps, V = P ÷ I and R = P ÷ I2. Choose the pair you know in the calculator and it gives the other two.
A ZIO reads 4-20 mA and 0-10 V signals, where Ohm’s law sets the loop voltage and the load a transmitter can drive.
