What This Calculator Solves
Enter any two of the four values — Voltage (V), Current (I), Resistance (R), or Power (P) — and the calculator solves the remaining unknowns instantly. All four relationships are solved simultaneously from whichever two inputs are provided, so there is no need to select a mode or rearrange a formula manually.
This is the general form of Ohm's Law applied to any resistive circuit element. It is distinct from the Voltage Drop Calculator, which is specifically scoped to cable and conductor runs using AWG resistivity data and NEC compliance limits.
The Formulas
- Voltage: V = I × R
- Current: I = V ÷ R
- Resistance: R = V ÷ I
- Power: P = V × I (also: P = I² × R, or P = V² ÷ R)
Unit reminder: volts (V), amperes (A), ohms (Ω), watts (W). Mixing milliamps with amps, or kilohms with ohms, is the most common source of wrong results.
Worked Examples
Example 1 — Find current from voltage and resistance:
V = 12 V, R = 470 Ω
I = 12 ÷ 470 = 0.0255 A (25.5 mA)
Typical small electronics circuit. Verify that 25.5 mA is within the rated current for the component in that branch.
Example 2 — Find resistance from voltage and current:
V = 9 V, I = 0.05 A (50 mA)
R = 9 ÷ 0.05 = 180 Ω
Use this to select the correct resistor value for a fixed supply and desired current limit.
Example 3 — Troubleshooting: verify expected voltage:
I = 0.1 A, R = 100 Ω (measured with multimeter)
Expected V = 0.1 × 100 = 10 V
If the measured voltage across that component is significantly lower, the resistance is higher than expected (corroded contact, failed component) or the current path is different from the assumed circuit.
When to Use This Calculator
- Resistor selection: find the resistance needed for a target current at a known supply voltage
- Current checking: verify a component is not exceeding its rated current
- Power estimation: check whether a component or supply needs to dissipate more watts than rated
- Troubleshooting: use measured voltage and current to back-calculate resistance, then compare to expected value
Ohm's Law vs Voltage Drop
These are related but answer different questions:
- Ohm's Law (this page): the relationship between V, I, and R for any resistive element. Solve for any unknown given the other two.
- Voltage Drop Calculator: loss of voltage specifically across a cable or wire run, calculated from AWG size, run length, material (copper/aluminum), and current. Used for wiring compliance against NEC 3% and 5% limits.
Common Mistakes
- Unit mismatch: entering milliamps (mA) where the calculator expects amps (A) gives a result 1000× too large. Convert first: 25 mA = 0.025 A.
- Leaving a non-target field partially filled: the calculator uses whichever two fields have values. If you accidentally leave a partial entry in a field, the result will use those two values rather than the ones you intended.
- Applying DC results to AC reactive circuits: in circuits with capacitors or inductors, total opposition to current is impedance (Z), not pure resistance. Ohm's Law still applies as V = I × Z, but Z must account for reactance. See the worked examples for DC resistive circuits only.
Important Limitations
- Linear resistance assumed: Ohm's Law applies to ohmic materials where resistance is constant. Semiconductors, diodes, and non-linear components do not follow a fixed V = IR relationship.
- Temperature effects: resistance increases with temperature in most conductors. The result reflects the resistance value you entered — not what it would be under thermal load.
- Ideal circuit assumed: the calculation treats each element in isolation. Real circuits include source impedance, wiring resistance, and contact resistance that shift the actual result.
