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Ohm's law says current through a conductor is proportional to the voltage across it: V = I × R. Pick the quantity to solve for and the calculator applies the matching formula: V = I × R, I = V / R, or R = V / I. Combined with the electrical power formula P = V × I, every one of the four quantities can be found from any two known values (power also comes from P = I2 × R or P = V2 / R). The results always show the complete set: voltage, current, resistance, and power together, so you can check a whole circuit at a glance. For learning and electronics planning; mains wiring work belongs to a qualified electrician.
Ohm's law is the fundamental relationship in electric circuits: the current through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance. In symbols, V = I × R, where V is voltage in volts, I is current in amperes (amps), and R is resistance in ohms.
Double the voltage across a fixed resistor and the current doubles; double the resistance at fixed voltage and the current halves. This one idea underlies nearly all of DC circuit analysis, and combined with the power formula P = V × I it tells you how much heat a component must handle.
Choose what to solve for: voltage, current, resistance, or power. Enter the known values in volts, amps, ohms, and watts (leave the solved field alone; it is recomputed from the others). For voltage, enter current and resistance; for current, voltage and resistance; for resistance, voltage and current; for power, voltage and current.
The calculator applies the exact formula for your target and returns all four values (V, I, R, P) together, so one calculation checks the whole circuit. The formula used is shown with the result, which makes it easy to follow the math by hand.
Every quantity can be reached from several pairs, which is why the classic Ohm's law wheel shows 12 formulas. To find voltage: V = I × R, V = P / I, V = sqrt(P × R). To find current: I = V / R, I = P / V, I = sqrt(P / R). To find resistance: R = V / I, R = V2 / P, R = P / I2. To find power: P = V × I, P = I2 × R, P = V2 / R.
Notice the pattern: the V = IR family handles the voltage-current-resistance triangle, while the power formulas trade one of those for watts. Knowing all three power forms means you can work with whichever pair you measured.
Electronics spans enormous ranges, so units carry prefixes: micro (u, 0.000001), milli (m, 0.001), kilo (k, 1,000), mega (M, 1,000,000). A 20 mA LED current is 0.02 A, a 4.7 kohm resistor is 4,700 ohms, and a 1 kW heater is 1,000 watts.
Watch the case: lowercase m is milli, uppercase M is mega, and a milliohm and a megohm differ by a factor of a billion. Mixing up prefixes is the most common source of wrong answers, so convert every value to base units (V, A, ohm, W) before calculating, or double check the prefix after.
In series, resistances simply add: 100 ohm + 220 ohm = 320 ohm. The same current flows through each resistor and the voltage divides between them in proportion to resistance.
In parallel, add reciprocals: 1/R = 1/R1 + 1/R2, so two 100 ohm resistors in parallel make 50 ohm. The same voltage appears across each and the current divides. A voltage divider is two resistors in series tapped at the middle: Vout = Vin × R2 / (R1 + R2), so 12 V across 10 kohm and 5 kohm gives 4 V across the 5 kohm resistor. Work out the combined resistance first, then use this calculator for total current and power.
A resistor turns P = I2 × R into heat, and it must be rated to survive it. The practical rule: choose a rating at least twice the calculated dissipation. A resistor burning 0.06 W is comfortable as a standard 1/4 W part; one burning 0.4 W needs a 1 W part, since running near the limit makes it hot and short-lived.
LEDs show this in action. An LED needs a series resistor to limit current: R = (supply voltage - LED forward voltage) / current. A red LED (about 2.0 V forward) on 5 V at 20 mA needs (5 - 2) / 0.02 = 150 ohm, dissipating 3 V × 0.02 A = 0.06 W. A white or blue LED (about 3.2 V) needs about 90 ohm, so use the next standard value up, 100 ohm. Always check the LED datasheet for its forward voltage and rated current.
A 12 volt supply is connected across a 6 ohm resistor. Current: I = V / R = 12 / 6 = 2 A. Power: P = V × I = 12 × 2 = 24 W, or equivalently P = V2 / R = 144 / 6 = 24 W.
Flip it: a 60 W lamp on 120 V draws I = P / V = 60 / 120 = 0.5 A, and its hot resistance is R = V2 / P = 14,400 / 60 = 240 ohm. And a 120 V, 1,440 W heater has R = 10 ohm and draws I = V / R = 12 A. One pattern, every scenario.
The classic errors: mixing up mA and A, entering values measured in different circuit states as if they belonged together, ignoring the resistor's power rating, and measuring resistance in a live circuit (wrong readings, and it can damage the meter).
Ohm's law in its simple form applies to linear, resistive components. Lamp filaments have much lower resistance cold than hot (bulbs often fail at switch-on for this reason). Diodes and LEDs are non-linear, so you cannot apply V = IR across them directly. Batteries have internal resistance, so terminal voltage sags under load. For AC, the law works with RMS values on resistive loads like heaters, but motors, capacitors, and inductors need impedance and power factor instead.
It states that the current through a conductor between two points is directly proportional to the voltage across those points: V = I × R. It lets you find any one of voltage, current, or resistance when you know the other two.
Divide voltage by resistance: I = V / R. 12 V across 6 ohm gives 2 A.
Divide voltage by current: R = V / I. 12 V with 2 A flowing means 6 ohm.
Multiply them: P = V × I. 12 V at 2 A is 24 W. If you know a different pair, use P = I2 × R or P = V2 / R.
Divide watts by volts: I = P / V. A 2,000 W kettle draws about 16.7 A at 120 V or 8.7 A at 230 V. The same power needs about half the current at the higher voltage.
Any two of voltage, current, resistance, or power. The calculator solves the rest consistently. If you enter the wrong pair for your chosen target (for example only voltage and power when solving resistance), use the power-form R = V2 / P by hand instead.
Power describes how much heat a component dissipates. If the computed power exceeds a resistor's rating, the resistor can overheat or fail: a 9 V drop across 220 ohm dissipates 0.37 W, which would overheat a 1/4 W resistor, so a 1/2 W part is safer.
1 A is 1,000 mA, so 20 mA is 0.02 A. Choosing the wrong unit is the most common source of errors, so always convert milliamp readings to amps before calculating.
R = (supply voltage - LED forward voltage) / current. A red LED (2.0 V) on 5 V at 20 mA needs 150 ohm; a white LED (3.2 V) needs about 90 ohm, so use the next standard value up, 100 ohm.
Calculate P = I2 × R and choose a rating at least twice that. A resistor dissipating 0.06 W is fine at 1/4 W; one dissipating 0.4 W should be 1 W or more.
In series, add them: R = R1 + R2. In parallel: 1/R = 1/R1 + 1/R2, so two equal resistors in parallel give half the value.
Yes for resistive loads such as heaters and kettles, using RMS values (the figures meters and labels normally show). Circuits with motors, capacitors, or inductors have impedance rather than simple resistance, so real power is V × I × power factor.
A filament's resistance is much lower when cold than at operating temperature, so a meter reading of a cold bulb is far below the value calculated from its watts and volts. That cold low resistance is why bulbs often fail at switch-on.
No. Diodes and transistors are non-linear: their current rises sharply above the forward voltage rather than proportionally, which is exactly why LEDs need a series resistor or a constant-current driver.