Resistor Calculator
Resistor Colour Code Calculator
Select the colours of the bands on the resistor to calculate its resistance value and tolerance.
Series & Parallel Resistance Calculator
LED Current Limiting Resistor Calculator
Calculate the resistor value needed to limit current through an LED.
Resistor Colour Code Chart
The standard colour code system allows you to identify resistor values quickly and accurately. Each colour represents a specific digit or function.
| Colour | Sample | Digit Value | Multiplier | Tolerance |
|---|---|---|---|---|
| Black | 0 | ×1 | – | |
| Brown | 1 | ×10 | ±1% | |
| Red | 2 | ×100 | ±2% | |
| Orange | 3 | ×1K | – | |
| Yellow | 4 | ×10K | – | |
| Green | 5 | ×100K | ±0.5% | |
| Blue | 6 | ×1M | ±0.25% | |
| Violet | 7 | ×10M | ±0.1% | |
| Grey | 8 | – | ±0.05% | |
| White | 9 | – | – | |
| Gold | – | ×0.1 | ±5% | |
| Silver | – | ×0.01 | ±10% |
Types of Resistors
4-Band Resistors
Standard resistors with four colour bands are the most common type. The first two bands represent significant digits, the third band is the multiplier, and the fourth band indicates tolerance. These resistors typically have a tolerance of ±5% or ±10%.
5-Band Resistors
Precision resistors feature five colour bands for greater accuracy. The first three bands represent significant digits, providing an additional digit of precision. The fourth band is the multiplier, and the fifth band shows tolerance, usually ±1% or ±2%.
SMD Resistors
Surface-mount device (SMD) resistors use numerical codes instead of colour bands. Common coding systems include 3-digit codes (e.g., 472 = 47 × 10² = 4.7kΩ) and 4-digit codes for higher precision (e.g., 4702 = 470 × 10² = 47kΩ).
Standard Resistor Values
Resistors are manufactured in standard values according to the E-series system. The E12 series contains 12 values per decade (10% tolerance), whilst the E24 series contains 24 values per decade (5% tolerance). This standardisation simplifies manufacturing and circuit design.
How to Read Resistor Colour Codes
Step 1: Identify the Band Count
Determine whether the resistor has 4, 5, or 6 colour bands. Most common resistors have 4 bands, whilst precision resistors typically have 5 bands.
Step 2: Find the Starting Point
The tolerance band (usually gold or silver) is typically separated from the other bands by a larger gap. This helps identify which end to read from – always read from the opposite end.
Step 3: Read the Significant Figures
For 4-band resistors, the first two bands represent digits. For 5-band resistors, the first three bands represent digits. Use the colour code chart to determine the numerical value of each band.
Step 4: Apply the Multiplier
The multiplier band (third band in 4-band, fourth band in 5-band) tells you how many zeros to add or the power of 10 to multiply by. Gold and silver represent decimal multipliers (0.1 and 0.01 respectively).
Step 5: Note the Tolerance
The final band indicates how much the actual resistance can vary from the calculated value. Common tolerances include ±1%, ±2%, ±5%, and ±10%.
Series and Parallel Resistor Combinations
Resistors in Series
When resistors are connected in series, the current flows through each resistor in sequence. The total resistance equals the sum of all individual resistances:
Rtotal = R₁ + R₂ + R₃ + … + Rₙ
Series connections are used when you need to increase total resistance or divide voltage across multiple components.
Resistors in Parallel
In parallel connections, resistors are connected across common points, providing multiple paths for current flow. The total resistance is less than the smallest individual resistor:
1/Rtotal = 1/R₁ + 1/R₂ + 1/R₃ + … + 1/Rₙ
Parallel connections reduce total resistance and are used when you need lower resistance values or want to maintain the same voltage across components.
Mixed Circuits
Complex circuits often combine series and parallel connections. Analyse these circuits by identifying series and parallel sections, calculating their equivalent resistances, then combining them systematically.
LED Current Limiting Resistors
Why LEDs Need Current Limiting
LEDs are current-sensitive devices that can be damaged by excessive current. Unlike incandescent bulbs, LEDs don’t naturally limit current flow, so external resistance is required to prevent burnout.
Calculating LED Resistor Values
The calculation follows Ohm’s law: R = (Vsupply – VLED) / ILED
Where Vsupply is the power supply voltage, VLED is the LED’s forward voltage drop, and ILED is the desired current through the LED.
Common LED Specifications
- Red LEDs: Forward voltage ~1.8-2.2V, typical current 20mA
- Green LEDs: Forward voltage ~2.0-2.2V, typical current 20mA
- Blue/White LEDs: Forward voltage ~3.0-3.4V, typical current 20mA
- High-power LEDs: Forward voltage varies, current 350mA-3A
Power Rating Considerations
Ensure the chosen resistor can handle the power dissipation: P = I² × R. Use a resistor with a power rating at least twice the calculated value for reliable operation.
Frequently Asked Questions
This is a common issue, especially with aged resistors. Try viewing the resistor under different lighting conditions or use a magnifying glass. Red appears more vivid, whilst brown appears more muted. If still uncertain, measure the resistance with a multimeter.
Look for the tolerance band (usually gold or silver), which is typically separated from other bands by a larger gap. Read from the opposite end. If there’s no clear gap, try both directions and see which gives a standard resistor value.
Tolerance indicates how much the actual resistance can vary from the marked value. For example, a 1kΩ resistor with ±5% tolerance can have an actual value between 950Ω and 1050Ω. Lower tolerance values indicate higher precision.
Yes, it’s always safe to use a resistor with higher power rating than required. The resistor will simply run cooler and last longer. However, higher wattage resistors are typically larger and more expensive.
Standard values follow the E-series, which provides logarithmically spaced values. This system ensures that any required resistance value is within the tolerance range of a standard value, simplifying manufacturing and inventory management.