Enter any two of voltage, current, resistance and power to get the other two.
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 and use the matching pair of formulas.
V = I × R, I = V ÷ R, R = V ÷ I
Use volts, amps and ohms together. For milliamps, divide by 1,000 first: 20 mA is 0.02 A.
P = V × I
The power in watts that the resistance turns into heat, or that the load draws from a DC supply.
P = I2 × R, P = V2 ÷ R
The same power without the third quantity. 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 |
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.
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.
