Resistors Complete Guide: Color Codes, Types, Power Ratings, and How to Choose

What Is a Resistor?

A resistor is a passive electronic component that limits or controls the flow of electric current in a circuit. It converts electrical energy into heat energy, dropping voltage in the process according to Ohm's Law: V = I × R (Voltage = Current × Resistance). Resistors are the most commonly used component in electronics — they set bias points, divide voltages, limit current to LEDs, pull up or pull down signals, filter frequencies, and sense current.

Ohm's Law and the Power Formula

These two equations govern everything resistors do:

FormulaSolves ForExample V = I × RVoltage across resistor10mA through 1kΩ = 10V I = V ÷ RCurrent through resistor5V across 470Ω = 10.6mA R = V ÷ IResistance neededLimit LED to 20mA from 5V: R = (5V-2V)÷0.02A = 150Ω P = V × IPower dissipated10V × 0.01A = 0.1W (use 1/4W resistor) P = V² ÷ RPower (when V and R known)(12V)² ÷ 100Ω = 1.44W (use 2W resistor) P = I² × RPower (when I and R known)(0.5A)² × 10Ω = 2.5W (use 3W or 5W)

The 2× rule: Always choose a resistor with a power rating at least double the expected dissipation. A 0.1W actual dissipation needs a 1/4W (0.25W) resistor minimum.

Resistor Color Code (4-Band)

Most through-hole axial resistors use colored bands to indicate value and tolerance. Read from the band closest to one end — or skip the mental math with the Resistor Color Code tool, which works both directions (bands to value, or value to bands).

Color Values

ColorDigitMultiplierTolerance Black0×10⁰ (1)— Brown1×10¹ (10)±1% Red2×10² (100)±2% Orange3×10³ (1k)— Yellow4×10⁴ (10k)— Green5×10⁵ (100k)±0.5% Blue6×10⁶ (1M)±0.25% Violet7×10⁷ (10M)±0.1% Gray8×10⁸ (100M)±0.05% White9×10⁹ (1G)— Gold—×10⁻¹ (0.1)±5% Silver—×10⁻² (0.01)±10% None——±20%

How to Read 4-Band Resistors

Bands 1-2: First two significant digits
Band 3: Multiplier (number of zeros)
Band 4: Tolerance

Examples

Colors (4-band)Value Brown-Black-Red-Gold1 0 × 100 = 1kΩ ±5% Yellow-Violet-Brown-Gold4 7 × 10 = 470Ω ±5% Orange-Orange-Orange-Gold3 3 × 1k = 33kΩ ±5% Brown-Black-Orange-Gold1 0 × 1k = 10kΩ ±5% Red-Red-Red-Gold2 2 × 100 = 2.2kΩ ±5% Green-Blue-Black-Gold5 6 × 1 = 56Ω ±5%

5-Band Precision Resistors

Higher precision resistors (1% tolerance) use 5 bands: first 3 digits, then multiplier, then tolerance.

Colors (5-band)Value Brown-Black-Black-Red-Brown1 0 0 × 100 = 10kΩ ±1% Yellow-Violet-Black-Orange-Brown4 7 0 × 1k = 470kΩ ±1%

Memorization Trick

Bad Beer Rots Our Young Guts But Vodka Goes Well — Get Some Now Black Brown Red Orange Yellow Green Blue Violet Gray White Gold Silver

SMD Resistor Codes

3-Digit Code (5% tolerance)

First two digits + number of zeros (multiplier)

CodeValue 10210 × 100 = 1kΩ 47047 × 1 = 47Ω 47347 × 1k = 47kΩ 10010 × 1 = 10Ω 10110 × 10 = 100Ω 10410 × 10k = 100kΩ 22022 × 1 = 22Ω 22122 × 10 = 220Ω

R notation for values under 10Ω: "4R7" = 4.7Ω, "0R1" = 0.1Ω, "R10" = 0.10Ω

4-Digit Code (1% tolerance)

First three digits + multiplier

CodeValue 1001100 × 10 = 1kΩ 4702470 × 100 = 47kΩ

EIA-96 Code (1%, 0603 and smaller)

Uses a 2-digit code for value + 1 letter for multiplier

CodeValueCodeValue 0110025178 1012448301 2015475619

Multiplier letters: Z=0.001, Y=0.01, R=0.1, X=A=1, H/B=10, C=100, D=1k, E=10k, F=100k

Example: "47C" = 301 × 100 = 30.1kΩ

Resistor Types and When to Use Them

Carbon Film (most common, cheapest)

• Tolerance: ±5% (some ±2%)
• Power: 1/8W to 2W
• Temp coefficient: -500 to -1000 ppm/°C (poor stability)
• Price: $0.01-0.05 each
• Use for: General purpose, LED current limiting, pull-ups, non-critical voltage dividers

Metal Film (precision, stable)

• Tolerance: ±1%, ±0.5%, ±0.1%
• Power: 1/8W to 2W
• Temp coefficient: ±50 to ±100 ppm/°C (good stability)
• Price: $0.05-0.20 each
• Use for: Precision circuits, audio, measurement, voltage references, op-amp feedback

Metal Oxide (high power)

• Tolerance: ±2%, ±5%
• Power: 1W to 10W+
• Temp coefficient: ±200 to ±300 ppm/°C
• Price: $0.20-1.00
• Use for: Power supplies, inrush current limiting, power dissipation applications

Wirewound (highest power, inductive)

• Tolerance: ±1% to ±10%
• Power: 1W to 100W+
• Temp coefficient: ±20 to ±200 ppm/°C
• Important: Inductance from coiled wire — NOT suitable for high-frequency circuits
• Price: $0.50-5.00
• Use for: Motor control, power dissipation, load banks, current sensing (when inductance doesn't matter)

Thick Film SMD (0402 to 2512)

• Tolerance: ±1% to ±5%
• Power: 1/16W (0402) to 1W (2512)
• Temp coefficient: ±100 to ±200 ppm/°C
• Use for: All surface-mount circuits — the default choice

Thin Film SMD (precision)

• Tolerance: ±0.1%, ±0.01%
• Power: 1/10W to 1/4W
• Temp coefficient: ±5 to ±25 ppm/°C (excellent stability)
• Price: $0.50-5.00
• Use for: Precision instrumentation, ADC references, matched pairs

Current Sense Resistors (low ohmic)

• Values: 0.001Ω to 1Ω
• 4-wire (Kelvin) connection for accurate measurement
• Use for: Motor current sensing, power supply monitoring, battery management

Shunt Resistors

• Very low resistance, high precision
• Designed for current measurement applications
• Often have 4 terminals for Kelvin connection
• Use for: Ammeters, power meters, motor controllers

Varistors (VDR / MOV)

• Voltage-dependent resistor — high resistance at low voltage, low resistance at high voltage
• Use for: Surge protection, transient voltage suppression

Thermistors (NTC / PTC)

• Resistance changes significantly with temperature
• NTC: resistance decreases as temperature increases
• PTC: resistance increases as temperature increases
• Use for: Temperature sensing, inrush current limiting (NTC), overcurrent protection (PTC)

Light-Dependent Resistors (LDR / Photoresistor)

• Resistance decreases with light exposure
• Typical: 1MΩ in darkness, 10kΩ in bright light
• Use for: Light sensing, automatic brightness control, simple proximity detection

Trimmer / Potentiometers

• Adjustable resistance value
• Potentiometer: user-adjustable (volume knobs, calibration)
• Trimmer PCB-mount: screwdriver-adjustable for calibration
• Use for: Variable voltage dividers, gain adjustment, offset calibration

Resistor Power Ratings (Physical Size)

PackagePower RatingTypical DimensionsApplication 0207 (axial)1/8W (0.125W)2mm × 7mmCompact circuits, low power 0207 (axial)1/4W (0.25W)2.3mm × 6mmMost common — breadboards, perfboard, general use 0207 (axial)1/2W (0.5W)3mm × 10mmHigher current circuits 0411 (axial)1W3.5mm × 12mmPower applications 0617 (axial)2W5mm × 17mmPower supplies, motor circuits 0922 (axial)3W6mm × 22mmHigher power dissipation Wirewound cement5W-10WVariesHigh power, braking resistors Aluminum housed10W-300WLarge heatsink bodySerious power applications 0402 (SMD)1/16W (0.0625W)1.0mm × 0.5mmHigh-density PCBs 0603 (SMD)1/10W (0.1W)1.6mm × 0.8mmMost common SMD size 0805 (SMD)1/8W (0.125W)2.0mm × 1.25mmGood balance of size and power 1206 (SMD)1/4W (0.25W)3.2mm × 1.6mmHigher power SMD, easy to hand solder 2512 (SMD)1W-2W6.4mm × 3.2mmCurrent sense, power applications

Standard E-Series Values (Preferred Numbers)

Resistors are manufactured in standardized value series. The E12 (5%) and E24 (1%) series cover most needs:

E12 Series (±5% and ±10% tolerance)

10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82

Each multiplied by powers of 10: 10Ω, 100Ω, 1kΩ, 10kΩ, 100kΩ, 1MΩ, etc.

E24 Series (±1% and ±2% tolerance)

10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91

E96 Series (±1% precision)

96 values per decade — check a reference table or use an online calculator.

Common Resistor Circuits

Voltage Divider

Two resistors in series divide an input voltage:

Vout = Vin × (R2 ÷ (R1 + R2))

Example: 10kΩ and 10kΩ dividing 5V gives 2.5V out.

Key point: The divider is only accurate if the load current is much smaller than the divider current. For precision reference voltages, use a buffer op-amp after the divider.

Current Limiting for LEDs

R = (Vsupply - Vled) ÷ Iled

Example: 5V supply, red LED (2V drop), target 20mA: R = (5-2)÷0.02 = 150Ω

Nearest standard E12 value: 150Ω. Power: P = 3V × 0.02A = 0.06W → 1/4W resistor is fine.

Pull-Up and Pull-Down Resistors

• Pull-up: Resistor from signal line to VCC. Keeps the line HIGH when nothing else is driving it. Typical values: 4.7kΩ to 10kΩ for I2C, 10kΩ to 100kΩ for general logic.
• Pull-down: Resistor from signal line to GND. Keeps the line LOW by default.
• I2C bus: Requires pull-up resistors (typically 4.7kΩ for 5V, 2.2kΩ-3.3kΩ for 3.3V systems)

Current Sensing

A low-value resistor in series with the load produces a voltage proportional to current:

Vsense = Iload × Rsense

Example: 0.1Ω resistor with 2A current produces 0.2V. Feed this to an op-amp or ADC to measure current.

RC Timing Circuits

A resistor and capacitor together create time delays:

Time constant (τ) = R × C

The capacitor charges to ~63% of VCC in one time constant, ~99% in 5 time constants.

Example: 10kΩ + 100μF: τ = 10,000 × 0.0001 = 1 second.

RC Low-Pass Filter

Cutoff frequency: fc = 1 ÷ (2πRC)

Passes frequencies below fc, attenuates frequencies above. Example: 1kΩ + 0.1μF: fc = 1÷(2π×1000×0.0000001) ≈ 1.6kHz

RC High-Pass Filter

Swap R and C positions. Passes frequencies above fc.

How to Choose a Resistor

Step-by-Step Selection Process

1. Calculate required resistance: Use Ohm's Law based on voltage, current, or timing needs
2. Choose standard value: Pick the nearest E12 or E24 value
3. Calculate power dissipation: P = V²/R or I²R or V×I
4. Select power rating: Minimum 2× calculated power
5. Choose tolerance: 5% for general use, 1% for precision, 0.1% for critical references
6. Select type: Carbon film (cheap), metal film (precision), metal oxide/ wirewound (power)
7. Choose package: Axial for breadboard/through-hole, SMD for PCB (0603 or 0805 most common)
8. Check temperature: Calculate temperature rise — power resistors may need heatsinking

Reading Resistor Datasheets

ParameterWhat It Means Resistance (nominal)The stated resistance value at 25°C ToleranceMaximum deviation from nominal (±1%, ±5%) Temperature Coefficient (TCR)How much resistance changes with temperature (ppm/°C) Power RatingMaximum continuous power at specified ambient temperature Maximum Working VoltageMaximum voltage across resistor regardless of power Maximum Overload VoltageBrief overload voltage the resistor can survive Insulation ResistanceResistance between resistor body and mounting surface Operating Temperature RangeAmbient temperatures where resistor meets spec Derating CurveHow power rating decreases at higher ambient temperatures

Troubleshooting

SymptomPossible Cause Resistor visibly burned/discoloredOverpowered — resistance value was correct but wattage too low Resistor measures open circuit (∞Ω)Blown from overcurrent — check for short circuits elsewhere Resistor measures wrong valueDamaged from overvoltage; incorrect part installed; multimeter loading effect on high values Circuit works until warmed up, then failsTemperature coefficient too high — replace with metal film Noisy audio circuitCarbon composition resistors are noisy — switch to metal film Voltage divider drifts over timePoor tolerance resistors aging — use 1% metal film

Where to Buy

• Assortment kits: 400-piece or 1000-piece kits with common values ($8-15 on Amazon) — essential for any workbench
• Digi-Key / Mouser: Massive selection, datasheets, fast shipping — best for specific precision resistors
• JLCPCB Basic Parts Library: Extremely cheap SMD resistors when ordering PCBs assembled ($0.001-0.005 each)
• eBay/AliExpress: Budget option for through-hole assortments; verify values with multimeter