Ohms Law Calculator
Calculate voltage, current, resistance, and power using fundamental electrical laws. Professional tool for electrical analysis and circuit design.
Four-Way Calculations
Calculate any electrical parameter when given two others: V, I, R, or P.
Multiple Formulas
Uses all Ohms Law and power formulas for comprehensive electrical analysis.
AC/DC Applications
Applicable to DC circuits and AC circuits with proper considerations.
Understanding Ohms Law and Power Relationships
Ohms 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.
Ohms Law Formulas
Power Formulas
Ohms Law Wheel - The Complete Picture
The Ohms 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 Ohms Law Applications
Electric Heater Element
Given Values:
1500W heater element operating at 240V
Known: P = 1500W, V = 240V
Find Current: I = P ÷ V = 1500W ÷ 240V = 6.25A
Find Resistance: R = V² ÷ P = 240² ÷ 1500 = 38.4Ω
Verification: P = I² × R = 6.25² × 38.4 = 1500W ✓
Result: Element draws 6.25A and has 38.4Ω resistance
LED Current Limiting Resistor
Design Requirement:
LED (3.2V forward voltage, 20mA) powered from 12V supply
Supply Voltage: 12V
LED Voltage Drop: 3.2V
Resistor Voltage: 12V - 3.2V = 8.8V
Required Current: 20mA = 0.02A
Resistor Value: R = V ÷ I = 8.8V ÷ 0.02A = 440Ω
Power Rating: P = I² × R = 0.02² × 440 = 0.176W
Result: Use 470Ω resistor (next standard value), 1/4W rating
Motor Winding Resistance
Motor Test:
Motor winding resistance measurement with locked rotor test
Test Voltage: 24V DC applied to winding
Measured Current: 8.5A
Winding Resistance: R = V ÷ I = 24V ÷ 8.5A = 2.82Ω
Test Power: P = V × I = 24V × 8.5A = 204W
Temperature Note: Resistance increases ~40% from cold to hot
Result: Winding resistance 2.82Ω cold, expect ~3.95Ω at operating temperature
Voltage Divider Circuit
Circuit Design:
Create 5V output from 12V supply using voltage divider
Input Voltage: 12V
Output Voltage: 5V desired
Voltage Ratio: 5V ÷ 12V = 0.417
Using R1 = 10kΩ (top): R2 = R1 × (Vout ÷ (Vin - Vout))
R2 Calculation: R2 = 10kΩ × (5V ÷ 7V) = 7.14kΩ
Total Current: I = 12V ÷ 17.14kΩ = 0.7mA
Result: Use 10kΩ and 6.8kΩ resistors (standard values) for ~4.85V output
Linear Power Supply Design
Design Requirements:
5V regulated output, 2A maximum current, input voltage 12V
Input Voltage: 12V
Output Voltage: 5V
Maximum Current: 2A
Voltage Drop Across Regulator: 12V - 5V = 7V
Power Calculations:
Output Power: P = V × I = 5V × 2A = 10W
Regulator Dissipation: P = 7V × 2A = 14W
Total Input Power: 10W + 14W = 24W
Efficiency: 10W ÷ 24W = 41.7%
Thermal Considerations:
Heat sink required for 14W 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: 10Ω, 20Ω, 30Ω in series with 12V supply:
• R_total = 10 + 20 + 30 = 60Ω
• I = 12V ÷ 60Ω = 0.2A
• V1 = 0.2A × 10Ω = 2V
• V2 = 0.2A × 20Ω = 4V
• V3 = 0.2A × 30Ω = 6V
• Total: 2V + 4V + 6V = 12V ✓
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: 10Ω, 20Ω, 30Ω in parallel with 12V supply:
• 1/R_total = 1/10 + 1/20 + 1/30 = 0.183
• R_total = 5.45Ω
• I1 = 12V ÷ 10Ω = 1.2A
• I2 = 12V ÷ 20Ω = 0.6A
• I3 = 12V ÷ 30Ω = 0.4A
• I_total = 1.2 + 0.6 + 0.4 = 2.2A
Safety and Best Practices
Electrical Safety Considerations
- •Always de-energize circuits before making resistance measurements
- •Use proper PPE and follow lockout/tagout procedures
- •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
- •Ohms 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
Professional 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 Ohms Law and how do I use it?
Ohms 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 Ohms 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 (120V) and current (10A), then P = 120 × 10 = 1200 watts.
Does Ohms Law apply to AC circuits?
Ohms 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 Ohms 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 Ohms Law to each component individually and to the total circuit.
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