Loading Calculator...
Please wait a moment
Please wait a moment
Enter at least 2 resistor values
Use the interactive calculator on the left to decode resistor color bands. Here is a quick reference:
| Color | Digit | 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% |
| Gray | 8 | - | 0.05% |
| White | 9 | - | - |
| Gold | - | 0.1 | 5% |
| Silver | - | 0.01 | 10% |
Electrical resistance is the opposition a material offers to the flow of electric current, measured in ohms (Ω). Defined by Ohm's law as R = V / I (resistance equals voltage divided by current), it is one of the three fundamental quantities in circuit analysis alongside voltage and current. Every material has some resistance -- conductors like copper have very low resistance (about 1.7 × 10⁻⁸ Ω·m resistivity), while insulators like rubber have extremely high resistance (over 10¹³ Ω·m). Resistors are components specifically designed to provide a precise, controlled amount of resistance for voltage division, current limiting, biasing, and signal attenuation in electronic circuits.
Measure the voltage across a component and the current flowing through it. Divide voltage by current to find resistance. For example, 12 V across a component drawing 0.5 A gives R = 12 / 0.5 = 24 Ω. This method works for any resistive element in a DC circuit.
For through-hole resistors, orient the component with the tolerance band (gold or silver) on the right. Read the first two color bands as digits, the third band as the multiplier (number of zeros to add), and the fourth band as tolerance. For 5-band resistors, the first three bands are digits.
For resistors in series, simply add their values: Rtotal = R1 + R2 + R3 + ... The total resistance always increases. Series circuits share the same current but divide voltage proportionally across each resistor.
For resistors in parallel, use: 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ... The total resistance is always less than the smallest individual resistor. For two resistors, the shortcut is Rtotal = (R1 × R2) / (R1 + R2).
Resistors protect sensitive components by limiting current flow. An LED rated for 20 mA on a 5 V supply needs a series resistor of (5 - 2) / 0.02 = 150 Ω to prevent burnout. Without proper resistance calculations, components fail from overcurrent, leading to costly repairs and potential safety hazards.
Voltage dividers using two resistors convert a higher voltage to a lower one for sensor interfaces, ADC inputs, and reference voltages. The output is Vout = Vin × R2 / (R1 + R2). Accurate resistance selection ensures precise voltage levels critical for proper circuit operation.
In audio, RF, and communication systems, matching source and load resistance maximizes power transfer and minimizes signal reflections. A 50 Ω transmission line must be terminated with a 50 Ω load. Mismatched impedance causes signal loss, distortion, and standing waves that degrade system performance.
| Color | 1st Band (Digit) | 2nd Band (Digit) | 3rd Band (Multiplier) | 4th Band (Tolerance) |
|---|---|---|---|---|
| Black | 0 | 0 | ×1 | - |
| Brown | 1 | 1 | ×10 | ±1% |
| Red | 2 | 2 | ×100 | ±2% |
| Orange | 3 | 3 | ×1K | - |
| Yellow | 4 | 4 | ×10K | - |
| Green | 5 | 5 | ×100K | ±0.5% |
| Blue | 6 | 6 | ×1M | ±0.25% |
| Violet | 7 | 7 | ×10M | ±0.1% |
| Gray | 8 | 8 | - | ±0.05% |
| White | 9 | 9 | - | - |
| Gold | - | - | ×0.1 | ±5% |
| Silver | - | - | ×0.01 | ±10% |
Example: Brown-Black-Red-Gold = 10 × 100 = 1,000 Ω (1 kΩ) ±5%
The E-series defines preferred resistor values available commercially. E12 has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) with 10% tolerance. E24 doubles this to 24 values with 5% tolerance. E96 provides 96 values for 1% precision. Each value is spaced so that tolerance ranges overlap, ensuring any resistance can be closely approximated.
Most conductors increase in resistance as temperature rises (positive temperature coefficient). Copper wire resistance increases about 0.39% per degree Celsius. Carbon composition resistors have a negative coefficient. Metal film and wirewound resistors offer the best temperature stability. For precision applications, use resistors with a low temperature coefficient, specified in ppm/°C.
Four-band resistors use two digits, one multiplier, and one tolerance band, providing values in the E12/E24 series. Five-band resistors add a third digit for greater precision, supporting E48/E96 series values at 1% or better tolerance. Five-band resistors are standard in modern precision electronics. Six-band resistors add a temperature coefficient band.
SMD (surface mount) resistors are smaller, cheaper for mass production, and work well with automated assembly. Through-hole resistors are easier for prototyping and hand soldering, better for high-power applications, and necessary when mechanical strength matters. Most modern PCB designs use SMD; through-hole is preferred for hobbyist projects and high-power circuits.
Parallel resistors provide multiple paths for current flow. Two equal resistors in parallel carry twice the total current for the same voltage, which by Ohm's law means the effective resistance is halved. Generally, N identical resistors of value R in parallel give R/N total resistance. This principle is also used to increase power handling -- two 100 Ω 0.25 W resistors in parallel give 50 Ω at 0.5 W.
Calculate voltage, current, resistance, and power using the fundamental relationships of Ohm's law and the power equation.
Quickly calculate total resistance, voltage drops, and power dissipation for resistors connected in series.
Find equivalent resistance for two or more parallel resistors and determine current distribution through each branch.