Ohm’s Law Formula Chart (V, I, R, P)
All twelve Ohm’s law and power formulas on one sheet, with series and parallel formulas and a current lookup by load.
Ohm’s Law and Power Formulas Chart
Ohm’s law links voltage, current and resistance: if you know any two of V, I, R and P, you can find the other two. This sheet puts all twelve forms on one table, with a current lookup for common loads and a unit prefix table, so you can solve most basic circuit questions without a calculator.
V in volts, I in amperes, R in ohms (Ω), P in watts
All twelve formulas
| Find | From two of V, I, R, P (1) | (2) | (3) |
|---|---|---|---|
| Voltage V (volts) | V = I × R | V = P ÷ I | V = √(P × R) |
| Current I (amps) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| Resistance R (ohms) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| Power P (watts) | P = V × I | P = I² × R | P = V² ÷ R |
Read across: pick the row for the quantity you want, then use the column that matches the two values you already know (V and I, I and R, or P and a second value).
Series and parallel resistors
| Circuit | Total resistance |
|---|---|
| Series | Rtotal = R1 + R2 + R3 + … |
| Parallel | 1 ÷ Rtotal = 1 ÷ R1 + 1 ÷ R2 + … |
| Two in parallel | Rtotal = (R1 × R2) ÷ (R1 + R2) |
| Voltage divider | Vout = Vin × R2 ÷ (R1 + R2) |
Current drawn by a load (I = P ÷ V)
Use this to check wire and fuse sizes for resistive loads such as heaters and lamps. Motors and electronics draw more current than P ÷ V suggests because of power factor and efficiency.
| Power | At 12 V (A) | At 24 V (A) | At 48 V (A) | At 230 V (A) |
|---|---|---|---|---|
| 10 W | 0.83 | 0.42 | 0.21 | 0.04 |
| 50 W | 4.17 | 2.08 | 1.04 | 0.22 |
| 100 W | 8.33 | 4.17 | 2.08 | 0.43 |
| 200 W | 16.7 | 8.33 | 4.17 | 0.87 |
| 500 W | 41.7 | 20.8 | 10.4 | 2.17 |
| 1000 W | 83.3 | 41.7 | 20.8 | 4.35 |
| 2000 W | 166.7 | 83.3 | 41.7 | 8.70 |
Unit prefixes
| Prefix | Symbol | Multiply by | Example |
|---|---|---|---|
| mega | M | × 1,000,000 | 1 MΩ = 1,000,000 Ω |
| kilo | k | × 1,000 | 4.7 kΩ = 4,700 Ω |
| (base) | – | × 1 | 1 A = 1 A |
| milli | m | × 0.001 | 250 mA = 0.25 A |
| micro | µ | × 0.000001 | 100 µA = 0.0001 A |
| nano | n | × 0.000000001 | 10 nF = 0.00000001 F |
2. Resistance when hot: R = V ÷ I = 12 ÷ 5 = 2.4 Ω.
3. A 7.5 A fuse would suit this circuit, and the wire must carry at least 5 A without excessive voltage drop (see the voltage drop quick table).
2. Power: P = V × I = 5 × 0.0227 = 0.114 W.
3. A 1/4 W (0.25 W) resistor has more than twice the margin, which is the usual rule: rate the resistor for at least double the power it dissipates.
- Mixing units. Convert mA to A and kΩ to Ω before using the formulas, or the answer is off by 1,000.
- Using Ohm’s law on AC reactive loads. For motors, transformers and capacitors use impedance and power factor, not R alone.
- Forgetting that power grows with the square. Double the current through a resistor and the heat is four times.
- Treating a lamp’s cold resistance as its working resistance. A filament lamp is much lower in resistance when cold.
Frequently asked questions
What is Ohm's law? Ohm's law says the current through a conductor is proportional to the voltage across it: I = V ÷ R.
How do I find power from voltage and current? Multiply them: P = V × I. If you know current and resistance, use P = I² × R; if you know voltage and resistance, use P = V² ÷ R.
What is the unit of resistance? The ohm (Ω). One ohm is the resistance that lets one ampere flow when one volt is applied.
Safety note. These figures are reference values and not a substitute for your local electrical code or a qualified electrician.