ToolPile - Free Online Tools & Calculators

Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law. Enter any two values to find the other two instantly. Includes unit converter, series/parallel resistance calculator, and AWG wire reference.

Enter Any 2 Values

Type into any two fields. The other two will be calculated.

Voltage (V)

Current (I)

Resistance (R)

Power (P)

All Ohm's Law Formulas

Voltage

V = I × R

V = P / I

V = √(P × R)

Current

I = V / R

I = P / V

I = √(P / R)

Resistance

R = V / I

R = V² / P

R = P / I²

Power

P = V × I

P = I² × R

P = V² / R

Ohm's Law Wheel

VoltageCurrentResistancePower

Electrical Unit Converter

Quantity

Value

From

1,000,000,000

µV

1,000,000

1,000

0.001

0.000001

Series / Parallel Resistance Calculator

R1Ω

R2Ω

Series (R1 + R2 + ...)

300 Ω

Parallel (1 / (1/R1 + 1/R2 + ...))

66.6667 Ω

66.666667 Ω

AWG Wire Size Reference

Maximum ampacity for copper conductors (NEC Table 310.16). Ratings depend on insulation temperature.

AWG	Diameter (mm)	Area (mm²)	60°C (A)	75°C (A)	90°C (A)
0000 (4/0)	11.684	107.2	195	230	260
000 (3/0)	10.405	85.0	165	200	225
00 (2/0)	9.266	67.4	145	175	195
0 (1/0)	8.251	53.5	125	150	170
1	7.348	42.4	110	130	145
2	6.544	33.6	95	115	130
4	5.189	21.2	70	85	95
6	4.115	13.3	55	65	75
8	3.264	8.37	40	50	55
10	2.588	5.26	30	35	40
12	2.053	3.31	20	25	30
14	1.628	2.08	15	20	25
16	1.291	1.31	—	—	18
18	1.024	0.823	—	—	14

How to Use the Ohm's Law Calculator

Enter any two known electrical values in the calculator above and the other two will be computed automatically. The calculator uses the fundamental relationships V=IR, P=VI, P=I²R, and P=V²/R to derive all unknowns. Results are color-coded for easy reading: yellow for voltage, blue for current, green for resistance, and red for power.

Understanding the Ohm's Law Wheel

The Ohm's Law wheel diagram shown above organizes all twelve key electrical formulas in an easy-to-read circle. Each quadrant represents one of the four quantities (V, I, R, P) and shows the three formulas that can be used to calculate it from any combination of the other two values.

Series vs Parallel Resistors

Use the series/parallel resistance calculator to quickly find the combined resistance of multiple resistors. This is essential for circuit design, where resistor networks are used to achieve specific voltage divider ratios, current limiting values, or impedance matching.

Frequently Asked Questions

What is Ohm's Law and how does this calculator use it?+

Ohm's Law states that voltage (V) equals current (I) times resistance (R), written as V = IR. This calculator extends Ohm's Law with the power equation (P = VI) so you can enter any two of the four electrical values -- voltage, current, resistance, and power -- and it instantly calculates the remaining two using the appropriate formulas.

How do I calculate watts from volts and amps?+

Power in watts equals voltage in volts multiplied by current in amps: P = V x I. For example, a 120V circuit drawing 10A uses 1,200 watts. Simply enter the voltage and current into this calculator and it will compute the power for you automatically.

What is the difference between series and parallel resistance?+

In a series circuit, resistances add up directly: R_total = R1 + R2 + R3. In a parallel circuit, the reciprocals add: 1/R_total = 1/R1 + 1/R2 + 1/R3. Series resistance is always higher than the largest individual resistor, while parallel resistance is always lower than the smallest individual resistor.

How do I convert milliamps to amps or kilohms to ohms?+

Use the unit converter section of this tool. Select the electrical quantity (voltage, current, resistance, or power), enter a value, and see it converted across all common prefixes. For example, 1 mA = 0.001 A, and 4.7 kΩ = 4,700 Ω.

What AWG wire size do I need for my circuit?+

The AWG (American Wire Gauge) table in this tool shows maximum ampacity ratings for common wire sizes at different temperature ratings. For example, 12 AWG copper wire is rated for 20A at 60°C insulation. Always choose a wire gauge rated above your expected current draw and consult local electrical codes for your specific installation.

Can I use Ohm's Law for AC circuits?+

Ohm's Law applies directly to DC circuits. For AC circuits, the same formulas work for purely resistive loads. When inductance or capacitance is involved, you need to use impedance (Z) instead of resistance (R), where Z accounts for phase differences between voltage and current. This calculator is designed for DC and purely resistive AC calculations.