Ohm's Law Calculator: Free Online Tool
How to Use the Ohm's Law Calculator
Our free online Ohm's Law calculator is designed for electricians, HVAC technicians, engineering students, and DIY enthusiasts. Follow these simple steps:
Step 1: Select Circuit Type
- DC — For batteries, resistors, LED circuits, and DC motors.
- AC Single-Phase — For residential 120V/240V circuits, wall outlets, and single-phase motors.
- AC Three-Phase — For commercial/industrial 208V, 480V, or 600V three-phase systems.
Step 2: Enter Known Values
You need at least 2 values. Enter any combination:
| Input Field | Symbol | Unit | Example |
|---|---|---|---|
| Voltage | V | Volts (V) | 120, 240, 480 |
| Current | I | Amperes (A) | 5, 15, 30 |
| Resistance | R | Ohms (Ω) | 10, 48, 100 |
| Power | P | Watts (W) | 100, 1500, 5000 |
| Power Factor | cos φ | — | 0.80, 0.85, 0.95 |
Step 3: View Results
The calculator instantly displays all computed values including voltage, current, resistance, power (W), apparent power (VA), and reactive power (VAR) for AC circuits. Results update in real-time as you type.
Ohm's Law Formula Reference
Behind the calculator, these are the exact formulas used:
| Calculation | DC Formula | AC Single-Phase | AC Three-Phase |
|---|---|---|---|
| Current | I = V / R | I = P / (V × cos φ) | I = P / (√3 × VLL × cos φ) |
| Voltage | V = I × R | V = P / (I × cos φ) | VLL = P / (√3 × I × cos φ) |
| Resistance | R = V / I | Z = V / I | Z = VLL / (√3 × I) |
| Real Power | P = V × I | P = V × I × cos φ | P = √3 × VLL × I × cos φ |
| Apparent Power | — | S = V × I | S = √3 × VLL × I |
| Reactive Power | — | Q = V × I × sin φ | Q = √3 × VLL × I × sin φ |
Common Applications
Our Ohm's Law calculator is used across many electrical and HVAC applications:
HVAC Technician Tasks
- Compressor winding test: Enter measured voltage and winding resistance to verify expected current draw. A 230V compressor with 3.8Ω should draw ≈60.5A at locked rotor.
- Contactor coil check: A 24V coil rated at 1.2A should have R = 24/1.2 = 20Ω. Measure resistance — if it reads 0Ω (shorted) or ∞Ω (open), replace.
- Heating element sizing: For a 5,000W heater at 240V: R = V²/P = 240²/5000 = 11.52Ω, I = P/V = 5000/240 = 20.8A.
- Voltage drop check: For a 15A load on 100 ft of 10 AWG wire (1.018 Ω/1000 ft): R = 2 × 100 × 1.018/1000 = 0.204Ω, Vdrop = 15 × 0.204 = 3.06V.
Student & Homework Uses
- Verify textbook circuit problems
- Check series and parallel resistance calculations
- Practice power factor problems for AC circuits
- Understand the relationship between V, I, R, and P
Professional Engineering Uses
- Quick wire sizing estimates based on current and voltage drop limits
- Motor starter and overload relay selection
- Panel load balancing calculations
- Transformer secondary current estimation
Understanding Power Factor in AC Calculations
Power factor (cos φ) is critical for AC circuits. It represents how efficiently electrical power is converted into useful work:
- cos φ = 1.0 — Purely resistive load (incandescent bulbs, electric heaters). All power is real.
- cos φ = 0.85–0.95 — Typical motor loads (fans, pumps, compressors). Some power is "wasted" in magnetic fields.
- cos φ = 0.5–0.7 — Lightly loaded motors or highly inductive loads. Significant reactive power.
Low power factor means higher current for the same real work, which increases wire sizes, voltage drop, and utility penalties. Power factor correction capacitors can improve cos φ to 0.95 or higher.
Unit Conversion Quick Reference
| Quantity | Common Units | Conversion |
|---|---|---|
| Voltage | V, mV, kV | 1 kV = 1,000 V; 1 V = 1,000 mV |
| Current | A, mA | 1 A = 1,000 mA |
| Resistance | Ω, kΩ, MΩ | 1 kΩ = 1,000 Ω; 1 MΩ = 1,000,000 Ω |
| Power | W, kW, MW | 1 kW = 1,000 W; 1 MW = 1,000,000 W |
Tips for Accurate Calculations
- Measure at operating temperature: Resistance changes with temperature. Copper wire resistance increases ~0.4% per °C. Always measure under load when possible.
- Account for wire resistance: The voltage at the load is less than at the source due to voltage drop in the wires. Use our Voltage Drop Calculator for precise values.
- Use true-RMS meters: For AC circuits with non-sinusoidal waveforms (VFDs, electronic ballasts), only true-RMS meters give accurate readings.
- Don't exceed rated values: Always check equipment nameplate data. Running a motor above rated current causes overheating and premature failure.
Standards Reference
- IEC 60071 — Insulation Co-ordination (voltage ratings and clearances)
- IEEE 141 — Recommended Practice for Electric Power Distribution for Industrial Plants
- NEC (NFPA 70) — National Electrical Code (conductor sizing, overcurrent protection)
- IEC 60364 — Low-Voltage Electrical Installations
Frequently Asked Questions
How does the Ohm's Law calculator work?
Enter any two known values — voltage (V), current (I), resistance (R), or power (P) — and the calculator uses Ohm's Law formulas to compute the missing values automatically. It applies V=IR, I=V/R, R=V/I, and P=VI simultaneously.
Can this calculator handle AC circuits?
Yes. For AC single-phase circuits, enter voltage, current, and power factor (cos φ) to get real power (W), apparent power (VA), and reactive power (VAR). For three-phase circuits, select 3-phase mode and enter line-to-line voltage.
What units does the Ohm's Law calculator accept?
The calculator accepts standard SI units: volts (V), amperes (A), ohms (Ω), and watts (W). You can also enter milliamps (mA), kilohms (kΩ), and kilowatts (kW) using the unit dropdown selectors.
Is the Ohm's Law calculator free?
Yes, our Ohm's Law calculator is 100% free with no registration required. It runs entirely in your browser with no data sent to any server. You can use it unlimited times for homework, engineering projects, or professional HVAC work.
What is the difference between apparent power and real power?
Real power (P, watts) is the actual energy consumed by the load. Apparent power (S, volt-amperes) is the total power supplied by the source. The ratio P/S is the power factor (cos φ). In purely resistive DC circuits, they are equal. In AC circuits with motors or capacitors, apparent power exceeds real power.