Resistor Calculator — Free Online Color Code Decoder

How to Use the Resistor Calculator

1. Select the calculation mode: Choose "Color Band Decoder" to read the resistance value from a physical resistor's color bands, or "Series / Parallel" to calculate the combined resistance of multiple resistors connected together in a circuit.
2. For color band decoding: First select whether your resistor has 4 or 5 color bands. Then use the dropdown menus to select each band's color in order from left to right. The first two (or three for 5-band) bands represent significant digits, followed by the multiplier band and the tolerance band.
3. For series/parallel calculation: Set the number of resistors you want to combine (2 to 5 are supported). Then enter the resistance value in ohms for each resistor. The calculator simultaneously computes both the series total and parallel total so you can compare both configurations.
4. Read the results: For color band mode, the results show the decoded resistance value in a human-readable format (ohms, kilohms, or megohms), the tolerance percentage, and the acceptable range of actual resistance values. For series/parallel mode, both totals are displayed with their respective formulas.

The visual color band display in the results panel provides a quick confirmation that you have selected the correct colors. Compare it against the actual resistor in your hand to verify accuracy.

Resistor Formulas

Variables Explained

• D1, D2, D3: Significant digit values (0-9) derived from the color bands. Each color represents a specific digit: black = 0, brown = 1, red = 2, orange = 3, yellow = 4, green = 5, blue = 6, violet = 7, grey = 8, white = 9.
• M: The multiplier exponent. The multiplier band color determines the power of 10 by which the significant digits are multiplied. For example, red (2) means multiply by 10-squared, or 100.
• R_total: The combined effective resistance of multiple resistors. In series, it equals the sum of all resistances. In parallel, it is always less than the smallest individual resistance.

Step-by-Step Example

Decode a 4-band resistor with colors: orange, orange, brown, gold:

1. First band (orange) = 3 (first significant digit)
2. Second band (orange) = 3 (second significant digit)
3. Third band (brown) = multiplier of 10^1 = 10
4. Combine: (3 x 10 + 3) x 10 = 33 x 10 = 330 ohms
5. Fourth band (gold) = plus or minus 5% tolerance
6. Range: 330 x 0.95 = 313.5 ohms to 330 x 1.05 = 346.5 ohms

This 330-ohm resistor with 5% tolerance is commonly used as a current-limiting resistor for LEDs in 5V circuits, allowing approximately 10mA of current with a standard red LED.

Practical Examples

Example 1: Alex's LED Circuit Design

Alex is building an LED indicator circuit powered by a 9V battery. The LED has a forward voltage of 2V and requires 15mA of current. He needs to calculate the required resistance using Ohm's law: R = (V_supply - V_LED) / I = (9 - 2) / 0.015 = 466.67 ohms. The nearest standard value is 470 ohms. Using the color band decoder:

• 470 ohms = yellow (4), violet (7), brown (x10) with 5% tolerance (gold)
• Tolerance range: 446.5 to 493.5 ohms
• Actual LED current range: 14.2 to 15.7 mA (safe for the LED)

Alex verifies the color code matches his resistor and confirms the current will be within the LED's safe operating range across the entire tolerance span. For a deeper understanding of the underlying voltage and current relationships, he can use our Ohm's law calculator.

Example 2: Priya's Voltage Divider

Priya needs a voltage divider to step down 12V to approximately 5V for a microcontroller reference input. Using the voltage divider formula V_out = V_in x R2 / (R1 + R2), she selects R1 = 14 kilohms and R2 = 10 kilohms. She does not have a 14k resistor, so she connects two standard resistors in series:

• R1 = 10k + 3.9k = 13.9 kilohms in series
• R2 = 10 kilohms
• V_out = 12 x 10,000 / (13,900 + 10,000) = 5.02V

The series combination gets Priya close to her target of 5V with standard components. She decodes the color bands on each resistor to confirm: 10k = brown-black-orange-gold and 3.9k = orange-white-red-gold.

Example 3: Carlos's Speaker Impedance Matching

Carlos is connecting four 8-ohm speakers to an amplifier rated for 4 ohms. He considers two configurations using the parallel calculator:

• All 4 in parallel: 1/R = 1/8 + 1/8 + 1/8 + 1/8 = 4/8, R = 2 ohms (too low)
• Two pairs in series-parallel: Each pair is 8 + 8 = 16 ohms in series, then two 16-ohm pairs in parallel = 8 ohms (too high)
• Two speakers in parallel (4 ohms each pair), then those pairs in series: 4 + 4 = 8 ohms

Carlos realizes none of the simple combinations give exactly 4 ohms with four 8-ohm speakers. He decides to use two speakers in parallel for a 4-ohm load, which perfectly matches his amplifier's optimal impedance.

Example 4: Mia's 5-Band Precision Resistor

Mia is building a precision measurement circuit and picks up a 5-band resistor with colors: blue, grey, black, brown, brown. She uses the 5-band decoder:

• Blue (6), Grey (8), Black (0) = significant digits 680
• Brown = multiplier x10
• Brown = 1% tolerance
• Result: 680 x 10 = 6,800 ohms (6.8 kilohms) at plus or minus 1%
• Range: 6,732 to 6,868 ohms

The 1% tolerance gives Mia a much tighter range than a standard 5% resistor, which would span 6,460 to 7,140 ohms. For her measurement circuit, this precision ensures consistent and reliable readings.

Resistor Color Code Reference Table

Tips and Complete Guide

Power Ratings and Derating

Every resistor has a maximum power rating, typically ranging from 1/8 watt for small surface-mount components to 5 watts or more for wirewound power resistors. The power dissipated by a resistor equals P = I-squared x R, or equivalently P = V-squared / R. Always select a resistor with a power rating at least twice the expected dissipation to provide adequate safety margin. In enclosed spaces with limited airflow, derate the power rating by 50% or more. Operating a resistor near its maximum power rating causes it to run hot, which shifts its resistance value, accelerates aging, and can eventually cause the component to fail catastrophically or catch fire.

Surface Mount Resistor Codes

Surface mount (SMD) resistors use a numeric code printed on the component rather than color bands. A three-digit code works like the first three bands of a 4-band resistor: the first two digits are significant, and the third is the multiplier. For example, "472" means 4,700 ohms (4.7 kilohms). A four-digit code adds an extra significant digit for precision: "4702" means 47,000 ohms (47 kilohms). The letter "R" indicates a decimal point, so "4R7" means 4.7 ohms. As circuits become smaller and more automated, SMD resistors have largely replaced through-hole color-coded resistors in commercial manufacturing.

Choosing Between Series and Parallel Configurations

Use series connections when you need a resistance value higher than what is available from a single standard component, or when you want to create a precise voltage divider. Series is also useful for distributing power dissipation across multiple resistors when the total power exceeds a single component's rating. Use parallel connections when you need a lower resistance than available standard values, when you need to handle more current by splitting it among multiple paths, or when building redundant circuits where the failure of one resistor does not cause total circuit failure. In both cases, our calculator helps you verify the combined resistance before building the circuit.

Temperature Coefficient and Stability

Resistors change their resistance value with temperature, characterized by the temperature coefficient of resistance (TCR), measured in parts per million per degree Celsius (ppm/C). Carbon composition resistors have high TCR (around 1,500 ppm/C), making them unsuitable for precision work. Metal film resistors offer excellent stability (50-100 ppm/C), and wirewound resistors can achieve even lower TCR. For applications requiring extreme stability, such as calibration equipment and reference standards, specialized resistors with TCR below 5 ppm/C are available. When designing temperature-sensitive circuits, select resistors with matched temperature coefficients to ensure that resistance ratios remain constant as temperature changes.

Common Mistakes to Avoid

• Reading color bands in the wrong direction: Always start from the band closest to one end of the resistor. The tolerance band (gold, silver) is always last and is often separated by a larger gap from the other bands.
• Confusing the multiplier band with a digit band: In a 4-band resistor, the third band is the multiplier, not a digit. A red third band means multiply by 100, not add the digit 2.
• Ignoring tolerance in circuit design: A circuit designed for exactly 1,000 ohms might receive a resistor anywhere from 950 to 1,050 ohms with 5% tolerance. Always verify your circuit works across the full tolerance range.
• Exceeding power ratings: A tiny 1/4 watt resistor in a high-current path will overheat and fail. Calculate the actual power dissipation (P = I-squared x R) and ensure it is well below the resistor's rating.
• Forgetting about parasitic effects at high frequencies: At radio frequencies, resistors exhibit parasitic inductance and capacitance that can significantly alter their impedance. Use resistors designed for high-frequency applications when working above a few megahertz.

Frequently Asked Questions