Ohm's Law Calculator
Calculate voltage, current, resistance, and power using fundamental electrical laws.
Four-Way Calculations
Calculate any electrical parameter when given two others: V, I, R, or P.
Multiple Formulas
Uses all Ohm's law and power formulas for comprehensive electrical analysis.
AC/DC Applications
Applicable to DC circuits and AC circuits with proper considerations.
Ohm's Law Calculator
Calculate voltage, current, resistance, or power using Ohm's Law
Input Parameters
Calculation Results
Enter values to calculate
Provide any two values to calculate the others
Understanding Ohm's Law and Power Relationships
Ohm's law is the fundamental relationship governing electrical circuits, establishing the connection between voltage, current, and resistance. Combined with power formulas, these relationships form the foundation of electrical engineering and circuit analysis.
Ohm's Law Formulas
Power Formulas
Ohm's Law Wheel - The Complete Picture
The Ohm's law wheel shows all 12 possible calculations using the fundamental relationships. Any two known values allow calculation of the remaining two parameters.
If you know V and I:
- • R = V ÷ I
- • P = V × I
If you know V and R:
- • I = V ÷ R
- • P = V² ÷ R
If you know V and P:
- • I = P ÷ V
- • R = V² ÷ P
If you know I and R:
- • V = I × R
- • P = I² × R
Real-World Ohm's Law Applications
Electric Heater Element
Given Values:
1500 W heater element operating at 240 V
Known: P = 1500 W, V = 240 V
Find Current: I = P ÷ V = 1500 W ÷ 240 V = 6.25 A
Find Resistance: R = V² ÷ P = 240² ÷ 1500 = 38.4 Ω
Verification: P = I² × R = 6.25² × 38.4 = 1500 W ✓
Result: Element draws 6.25 A and has 38.4 Ω resistance
LED Current Limiting Resistor
Design Requirement:
LED (3.2 V forward voltage, 20 mA) powered from 12 V supply
Supply Voltage: 12 V
LED Voltage Drop: 3.2 V
Resistor Voltage: 12 V - 3.2 V = 8.8 V
Required Current: 20 mA = 0.02 A
Resistor Value: R = V ÷ I = 8.8 V ÷ 0.02 A = 440 Ω
Power Rating: P = I² × R = 0.02² × 440 = 0.176 W
Result: Use 470 Ω resistor (next standard value), 1/4 W rating
Motor Winding Resistance
Motor Test:
Motor winding resistance measurement with locked rotor test
Test Voltage: 24 V DC applied to winding
Measured Current: 8.5 A
Winding Resistance: R = V ÷ I = 24 V ÷ 8.5 A = 2.82 Ω
Test Power: P = V × I = 24 V × 8.5 A = 204 W
Result: Winding resistance 2.82 Ω cold
Voltage Divider Circuit
Circuit Design:
Create 5 V output from 12 V supply using voltage divider
Input Voltage: 12 V
Output Voltage: 5 V desired
Voltage Ratio: 5 V ÷ 12 V = 0.417
Using R1 = 10 kΩ (top): R2 = R1 × (Vout ÷ (Vin - Vout))
R2 Calculation: R2 = 10 kΩ × (5 V ÷ 7 V) = 7.14 kΩ
Total Current: I = 12 V ÷ 17.14 kΩ = 0.7 mA
Result: Use 10 kΩ and 6.8 kΩ resistors (standard values) for ~4.85 V output
Linear Power Supply Design
Design Requirements:
5 V regulated output, 2 A maximum current, input voltage 12 V
Input Voltage: 12 V
Output Voltage: 5 V
Maximum Current: 2 A
Voltage Drop Across Regulator: 12 V - 5 V = 7 V
Power Calculations:
Output Power: P = V × I = 5 V × 2 A = 10 W
Regulator Dissipation: P = 7 V × 2 A = 14 W
Total Input Power: 10 W + 14 W = 24 W
Efficiency: 10 W ÷ 24 W = 41.7 %
Thermal Considerations:
Heat sink required for 14 W dissipation
Thermal resistance calculation needed
Consider switching regulator for higher efficiency
Circuit Analysis Reference Tables
Series Circuit Relationships
| Parameter | Relationship |
|---|---|
| Current (I) | Same throughout circuit |
| Voltage (V) | V_total = V1 + V2 + V3... |
| Resistance (R) | R_total = R1 + R2 + R3... |
| Power (P) | P_total = P1 + P2 + P3... |
Series Example:
Three resistors: 10 Ω, 20 Ω, 30 Ω in series with 12 V supply:
• R_total = 10 + 20 + 30 = 60 Ω
• I = 12 V ÷ 60 Ω = 0.2 A
• V1 = 0.2 A × 10 Ω = 2 V
• V2 = 0.2 A × 20 Ω = 4 V
• V3 = 0.2 A × 30 Ω = 6 V
• Total: 2 V + 4 V + 6 V = 12 V ✓
Parallel Circuit Relationships
| Parameter | Relationship |
|---|---|
| Voltage (V) | Same across all branches |
| Current (I) | I_total = I1 + I2 + I3... |
| Resistance (R) | 1/R_total = 1/R1 + 1/R2 + 1/R3... |
| Power (P) | P_total = P1 + P2 + P3... |
Parallel Example:
Three resistors: 10 Ω, 20 Ω, 30 Ω in parallel with 12 V supply:
• 1/R_total = 1/10 + 1/20 + 1/30 = 0.183
• R_total = 5.45 Ω
• I1 = 12 V ÷ 10 Ω = 1.2 A
• I2 = 12 V ÷ 20 Ω = 0.6 A
• I3 = 12 V ÷ 30 Ω = 0.4 A
• I_total = 1.2 + 0.6 + 0.4 = 2.2 A
Safety and Best Practices
Electrical Safety Considerations
- •Always de-energize circuits before making resistance measurements
- •Verify meter settings and ranges before measurements
- •Consider power dissipation in components to prevent overheating
- •Account for temperature effects on component values
Measurement Accuracy
Application Limitations
- •Ohm's law applies directly to linear, resistive circuits only
- •For reactive AC circuits, use impedance (Z) instead of resistance (R)
- •Power calculations in AC circuits require power factor considerations
- •Semiconductor devices have non-linear voltage-current relationships
- •High-frequency effects may invalidate simple resistance models
Disclaimer
This calculator provides theoretical calculations based on ideal conditions. Actual circuit behavior may vary due to component tolerances, temperature effects, frequency dependencies, and parasitic elements. Always verify calculations with actual measurements and follow applicable safety standards.
Frequently Asked Questions
What is Ohm's Law and how do I use it?
Ohm's law states that voltage equals current times resistance (V = I × R). It's fundamental to electrical calculations. Given any two values, you can calculate the third: V = I × R, I = V ÷ R, or R = V ÷ I. Additionally, power relationships are P = V × I, P = I² × R, or P = V² ÷ R. These formulas apply to DC circuits and AC circuits with resistive loads.
How do I calculate power using Ohm's Law?
Power can be calculated using three formulas: P = V × I (power = voltage × current), P = I² × R (power = current squared × resistance), or P = V² ÷ R (power = voltage squared ÷ resistance). Choose the formula based on which values you know. For example, if you know voltage (120 V) and current (10 A), then P = 120 × 10 = 1200 watts.
Does Ohm's Law apply to AC circuits?
Ohm's law applies to AC circuits, but with important considerations. For purely resistive AC circuits, use RMS values for voltage and current. For reactive circuits (with inductance or capacitance), impedance (Z) replaces resistance: V = I × Z. The phase relationship between voltage and current also affects power calculations, requiring the power factor: P = V × I × cos(φ).
What's the difference between resistance and impedance?
Resistance (R) is opposition to current flow in DC circuits or AC circuits with pure resistive loads, measured in ohms. Impedance (Z) is the total opposition to current flow in AC circuits, including both resistance and reactance (from inductance and capacitance), also measured in ohms. At DC and low frequencies, R ≈ Z, but they differ significantly at higher frequencies.
How do I use Ohm's Law for series and parallel circuits?
In series circuits: total voltage = sum of individual voltages, current is the same throughout, total resistance = sum of individual resistances. In parallel circuits: voltage is the same across all branches, total current = sum of branch currents, 1/Rtotal = 1/R1 + 1/R2 + 1/R3... Apply Ohm's law to each component individually and to the total circuit.
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