Ohm's Law Calculator & Converter
Calculate Voltage, Current, Resistance, and Power with automatic unit conversions
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Ohm's Law Calculator
Enter any two values to calculate the others
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Voltage (V)
0 V
Current (I)
0 A
Resistance (R)
0 Ω
Power (P)
0 W
Unit Converter
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Ohm's Law Information
What is Ohm's Law?
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance between them.
The law was named after the German physicist Georg Ohm, who first formulated it in 1827.
Basic Formulas
V = I × R (Voltage = Current × Resistance)
I = V / R (Current = Voltage / Resistance)
R = V / I (Resistance = Voltage / Current)
P = V × I (Power = Voltage × Current)
Ohm's Law Triangle
Cover the value you want to calculate to see the formula
Common Applications
- Circuit design and analysis
- Determining component values
- Troubleshooting electrical circuits
- Power supply design
- Electronic device development
Examples
Scenario: You have a circuit with a 12V battery and a 4Ω resistor. What is the current?
Solution: I = V / R = 12V / 4Ω = 3A
Scenario: A circuit has 0.5A of current flowing through a 20Ω resistor. What is the voltage?
Solution: V = I × R = 0.5A × 20Ω = 10V
Scenario: A device operates at 5V with 2A current. What is its power consumption?
Solution: P = V × I = 5V × 2A = 10W
Scenario: Convert 500mA to Amperes and 2.2kΩ to Ohms.
Solution:
- 500mA = 0.5A (since 1A = 1000mA)
- 2.2kΩ = 2200Ω (since 1kΩ = 1000Ω)
Common Mistakes
- Forgetting to convert units (e.g., mA to A, kΩ to Ω)
- Using the wrong formula for the quantity you're calculating
- Not considering the power rating of components
- Ignoring voltage drops across components
Quick Tools
Recent Calculations
Everyday Guide to Using This Ohm's Law Calculator
What This Calculator Helps With
This tool solves electrical calculations that come up in everyday situations:
- DIY Electronics: Find the right resistor for an LED circuit
- Home Projects: Determine if a power supply is adequate for your device. If you're working with different energy units, our energy converter tool can help translate between watt-hours, joules, and calories.
- Automotive: Calculate current draw for car accessories
- Renewable Energy: Size solar panel systems and battery banks. For larger electrical systems, you may also need to check the conductivity of materials used in your setup.
- Education: Learn electrical principles with instant feedback
Real-Life Examples You Can Try
Home: Calculate if your 12V car battery charger needs a fuse (enter 12V, 10A → 1.2Ω max resistance)
DIY: An LED needs 20mA at 3V. What resistor? (enter 3V, 0.02A → 150Ω resistor needed)
Gaming: Your gaming PC draws 650W from a 120V outlet. Current? (enter 650W, 120V → 5.4A). To understand the cost of running that PC, try our electrical energy cost calculator.
Travel: Convert 220V appliance to 110V? (Use unit converter: 220V to 110V = half the voltage)
Simple Unit Explanations
- Voltage (V): Electrical "pressure" - like water pressure in a hose
- Current (A): Flow rate - how much electricity is moving
- Resistance (Ω): Restriction - like a kink in the hose. For a deeper dive into material properties, check the resistivity converter (available soon).
- Power (W): Work done - how much energy is used per second
Tip: 1,000 milliamps (mA) = 1 Amp. 1,000 milliwatts (mW) = 1 Watt.
Input Guidance
- Enter any two values - the calculator finds the other two automatically
- Use decimal points for fractions (0.5 for ½, 0.25 for ¼)
- Select matching units - if measuring in mA, keep current in mA
- For small values like 0.001A, use mA instead for easier reading
Common Mistakes to Avoid
- Unit mixing: Don't mix kΩ with mA without conversion. Our resistor color code tool can help you identify precise resistance values.
- Decimal errors: 500mA = 0.5A (not 500A!)
- Real-world factors: Calculations are ideal - actual circuits have additional resistances
- Safety margin: Always use components rated higher than calculated values
- Battery voltage: Actual battery voltage drops as it discharges
Mobile & Accessibility Tips
- Touch-friendly: All buttons and inputs are sized for finger use
- Screen reader ready: Proper labels for assistive technology
- Keyboard navigation: Tab through all inputs, Space/Enter to calculate
- High contrast: Clear color differentiation for results
- Offline capable: Works without internet after first load
Quick Conversion Tips
- 1A = 1,000mA = 1,000,000μA
- 1kΩ = 1,000Ω = 0.001MΩ
- Common conversions are pre-loaded in the Examples tab
- Use the "Copy Results" button to save calculations
- Recent calculations are saved during your session
Frequently Asked Questions
The calculations are mathematically precise for ideal conditions. Real-world factors like temperature, component tolerance, and wire resistance may affect actual results.
This calculator uses basic Ohm's Law for DC circuits. For AC circuits with inductive or capacitive loads, additional factors (impedance, power factor) must be considered. You might find our impedance converter helpful for those scenarios.
Ohm's Law has four variables (V, I, R, P). Knowing any two allows calculation of the other two through mathematical relationships.
Important Safety Notes
Educational Use: This tool is for learning and planning. Actual circuit design should be done by qualified individuals.
High Voltage Warning: Never work on live circuits. Even calculations for low voltage can be dangerous if mishandled.
Component Ratings: Always use components rated for at least 20% higher than calculated values for safety margin.
Device Compatibility
✓ Works on all modern browsers (Chrome, Firefox, Safari, Edge)
✓ Mobile responsive (phones, tablets, desktops)
✓ No plugins or downloads required
Version Info
Current Version: 2.1 (November 2025)
Last Updated: Added enhanced practical guidance
Next Planned: More real-world application examples
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