Skip to content
RFrftools.io
General

Ohm's Law Calculator

Calculate voltage, current, resistance, and power using Ohm's Law. Enter any two values to solve for the remaining two quantities.

Loading calculator...

Formula

V=IR,P=IV=I2R=V2RV = IR, \quad P = IV = I^2R = \frac{V^2}{R}
V— Voltage (V)
I— Current (A)
R— Resistance (Ω)
P— Power (W)

How It Works

Ohm's Law calculator computes voltage, current, resistance, and power dissipation — essential for circuit design, component selection, and power budgeting. Electrical engineers, technicians, and hobbyists use this to size resistors, calculate load currents, and verify power ratings before prototyping. According to Horowitz & Hill's 'Art of Electronics' (3rd ed., p.1), V=IR is the foundational relationship governing all resistive circuits. Standard resistors have tolerances of 1% (precision) to 5% (general purpose), with temperature coefficients typically 50-100 ppm/°C per MIL-PRF-55342. Power resistors are derated to 50-70% of rated wattage in continuous operation — a 1W resistor should dissipate no more than 0.5-0.7W to maintain junction temperatures below 125°C. For high-precision applications, 0.1% tolerance thin-film resistors achieve temperature coefficients as low as 5 ppm/°C.

Worked Example

A 5V Arduino GPIO must drive an LED requiring 20mA forward current at 2.1V forward voltage. Calculate the current-limiting resistor: V_drop = 5V - 2.1V = 2.9V. R = V/I = 2.9V / 0.020A = 145 ohms. Select the nearest E24 standard value: 150 ohms. Verify power dissipation: P = I²R = (0.020)² × 150 = 0.06W — a standard 1/8W (0.125W) resistor provides 2× safety margin. This calculation follows IPC-2221B guidelines for component derating. The LED will draw I = 2.9V / 150 ohms = 19.3mA — within 4% of the target current, acceptable for indicator applications.

Practical Tips

  • ✓Use E24 (5%) or E96 (1%) standard values to ensure component availability — custom values add 4-8 weeks lead time and 3-5× cost premium
  • ✓For power resistors above 1W, verify thermal resistance (typically 20-50°C/W for TO-220 packages per JEDEC JESD51-1) and add heatsinking
  • ✓Cross-verify calculations using P=VI, P=I²R, and P=V²/R — all three must agree within component tolerance

Common Mistakes

  • ✗Using total supply voltage instead of voltage drop across the resistor — causes 40-60% current calculation errors in LED and bias circuits
  • ✗Selecting resistors at exactly rated power — thermal stress reduces MTBF by 50% per JEDEC JEP122H; always derate to 50-70%
  • ✗Ignoring temperature coefficient in precision circuits — a 100 ppm/°C resistor shifts 1% over a 100°C temperature range

Frequently Asked Questions

Voltage in volts (V), current in amperes (A), resistance in ohms (Ω), power in watts (W). Per SI standards, 1V across 1Ω produces 1A and dissipates 1W.
Ohm's Law applies to linear resistive elements. Nonlinear components (diodes, transistors) require I-V curve analysis — for example, a 1N4148 diode has ~0.7V forward drop at 10mA but varies 2mV/°C with temperature per ON Semiconductor datasheet.
Resistance changes with temperature per R(T) = R₀(1 + α×ΔT). Copper has α = +0.393%/°C (resistance increases), while carbon composition is -0.02 to -0.05%/°C. Precision resistors specify TCR in ppm/°C — 25 ppm/°C means ±0.25% drift over 100°C range.
Current-limiting resistor selection for LEDs, voltage dividers for sensor interfaces, and bias resistor calculation for transistor amplifiers. Over 80% of electronic designs use voltage dividers according to industry surveys.
Theoretical accuracy is exact; practical accuracy depends on component tolerances. A circuit with 5% resistors and 10% capacitors yields ±11% worst-case error (RSS method). Use 1% resistors and measured values for ±2-3% accuracy.
Apply I = V/R using the voltage across the resistor (not supply voltage if other components are in series). For a 470Ω resistor with 5V across it: I = 5/470 = 10.6mA. Use a multimeter in series to verify — Fluke meters have 0.5% accuracy per manufacturer specs.
V_drop = 12V - 5V = 7V. R = 7V / 0.1A = 70Ω. Use 68Ω (E24 series). Power: P = 7V × 0.1A = 0.7W — select a 2W resistor (35% loading). Note: This wastes 0.7W as heat. An LM7805 regulator has 85% efficiency at this load; a TPS563200 buck converter achieves 92% efficiency per TI datasheet.
Three equivalent forms: P = V²/R = I²R = V×I. For 100Ω with 10V: P = 100/100 = 1W. Per IPC-9592B, derate power resistors to 50% in enclosed spaces without forced airflow. Surface-mount 0805 resistors handle 0.125W; 2512 packages handle 1W.

Shop Components

As an Amazon Associate we earn from qualifying purchases.

Resistor Kit (1%, E24)

Precision 1% thin-film SMD resistor assortment, 0402 package

Amazon →

Ceramic Capacitor Kit

MLCC ceramic capacitor assortment in 0402 package

Amazon →

Solderless Breadboard

Full-size breadboard for circuit prototyping

Amazon →

Related Calculators

Power

Voltage Divider

Calculate voltage divider output voltage, current, Thévenin impedance, and power dissipation from Vin, R1, and R2. Ideal for bias networks and level shifting.

General

RC Time Constant

Calculate RC circuit time constant τ, charge time to 63.2% and 99%, and −3dB cutoff frequency. Essential for filter and timing circuit design.

Power

LED Resistor

Calculate the correct current limiting resistor for an LED. Shows exact value, nearest E24 standard, actual current, and power dissipation.

General

Color Code

Decode resistor color bands to resistance value and tolerance. Supports 4-band, 5-band, and 6-band resistors. Instant color band to ohms conversion.