Resistor Calculator
Color Code · Ohm's Law · Series / Parallel
“Note: This calculator provides estimated values for educational purposes and hobbyist projects. Always double-check component ratings, power dissipation, and circuit designs before applying power to physical circuits to prevent component failure.”
How to Use This Resistor Calculator or Ohm’s Law Calculator
This resistor calculator is a free online resistor value calculator in one tool. Use the Color Code Decoder to find the resistance value of any 4-band or 5-band resistor by selecting its colored bands. Use the Ohm’s Law Calculator to solve for voltage, current, or resistance when you know two of the three values. Use the Series & Parallel Calculator to find the total resistance of multiple resistors in a circuit. Everything runs instantly in your browser, with no sign-up required.
What is a Resistor?
A resistor is one of the most fundamental components in electronics. It is a passive two-terminal electrical component that limits or regulates the flow of electrical current in a circuit. Resistors are measured in ohms (Ω), named after German physicist Georg Simon Ohm. They are used in virtually every electronic device — from smartphones and computers to home appliances and cars.
Resistors serve many purposes: protecting sensitive components from too much current, dividing voltage, pulling a signal line high or low, and setting bias points in amplifier circuits. Use this resistor calculator to find the value of any resistor from its color bands in seconds.
Resistor Color Code Chart – 4-Band Resistors
Most resistors have colored bands printed on them that encode their resistance value and tolerance. Reading these color bands is an essential skill for any electronics hobbyist or engineer.
| Color | Band 1 (1st Digit) | Band 2 (2nd Digit) | Band 3 (Multiplier) | Band 4 (Tolerance) |
|---|---|---|---|---|
| Black | 0 | 0 | ×1 (Ω) | – |
| 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 | ×100M | ±0.05% |
| White | 9 | 9 | ×1G | – |
| Gold | – | – | ×0.1 | ±5% |
| Silver | – | – | ×0.01 | ±10% |
Resistor Value Calculator
The Resistor Value Calculator works like the Color Code Decoder above: choose the color of each band and it converts the bands into a resistance value in ohms (Ω). For a 4-band resistor the value is calculated as:
Value = (Band 1 digit × 10 + Band 2 digit) × multiplier
For a 5-band resistor the value is calculated as:
Value = (Band 1 digit × 100 + Band 2 digit × 10 + Band 3 digit) × multiplier
The result also shows the tolerance and the valid min/max range, so you can confirm a resistor meets its printed band rating before you build.
How to Read a 4-Band Resistor Color Code
Step-by-step example: Yellow – Violet – Red – Gold
- Band 1 (Yellow): First digit = 4
- Band 2 (Violet): Second digit = 7
- Band 3 (Red): Multiplier = ×100
- Band 4 (Gold): Tolerance = ±5%
- Result: 47 × 100 = 4,700 Ω (4.7 kΩ) ±5%
| Color Code Example | Calculation | Resistance Value | Tolerance |
|---|---|---|---|
| Brown – Black – Red – Gold | 1, 0, ×100 | 1,000 Ω (1 kΩ) | ±5% |
| Red – Red – Brown – Silver | 2, 2, ×10 | 220 Ω | ±10% |
| Orange – Orange – Orange – Gold | 3, 3, ×1K | 33,000 Ω (33 kΩ) | ±5% |
| Yellow – Violet – Yellow – Gold | 4, 7, ×10K | 470,000 Ω (470 kΩ) | ±5% |
| Green – Blue – Green – Brown | 5, 6, ×100K | 5,600,000 Ω (5.6 MΩ) | ±1% |
| Brown – Black – Black – Brown | 1, 0, ×1 | 10 Ω | ±1% |
5-Band Resistor Color Code
Precision resistors (1% tolerance and better) use a 5-band color code for greater accuracy. The first three bands represent digits, the fourth is the multiplier, and the fifth is tolerance. You can decode 5-band resistors in the Color Code tab of the resistor calculator above.
| Band | 4-Band Resistor | 5-Band Resistor |
|---|---|---|
| Band 1 | 1st digit | 1st digit |
| Band 2 | 2nd digit | 2nd digit |
| Band 3 | Multiplier | 3rd digit |
| Band 4 | Tolerance | Multiplier |
| Band 5 | – | Tolerance |
Ohm’s Law Calculator
Ohm’s Law is the fundamental relationship between Voltage (V), Current (I), and Resistance (R). It states:
V = I × R
| What You Want to Find | Formula | Example |
|---|---|---|
| Voltage (V) | V = I × R | 2A × 50Ω = 100V |
| Current (I) | I = V ÷ R | 12V ÷ 100Ω = 0.12A (120mA) |
| Resistance (R) | R = V ÷ I | 9V ÷ 0.05A = 180Ω |
| Power (P) | P = V × I | 5V × 0.5A = 2.5W |
Ohm’s Law Triangle – Easy Memory Aid
💡 The Ohm’s Law Triangle: Cover the value you want to find. What remains shows the formula.
[V]
[I] × [R]
Cover V → V = I × R | Cover I → I = V/R | Cover R → R = V/I
Practical Ohm’s Law Examples
| Scenario | Known Values | Formula | Answer |
|---|---|---|---|
| LED current limiting resistor (5V, 20mA LED, 2V drop) | V=3V, I=0.02A | R = V/I | 150 Ω → use 150Ω or 220Ω |
| Arduino output pin current (5V, 470Ω) | V=5V, R=470Ω | I = V/R | 10.6 mA (safe for GPIO) |
| Car fuse rating (12V system, 6A load) | V=12V, I=6A | P = V×I | 72W — use 7.5A fuse |
| Voltage divider output (10kΩ + 10kΩ, 9V supply) | V=9V, R=20kΩ | V_out = 9V × (10k/20k) | 4.5V output |
Series Resistor Calculator
When resistors are connected end-to-end (series), the total resistance is simply the sum of all individual resistances.
Formula: R_total = R1 + R2 + R3 + …
| Resistors in Series | Total Resistance |
|---|---|
| 100Ω + 220Ω | 320 Ω |
| 1kΩ + 2.2kΩ + 3.3kΩ | 6.5 kΩ |
| 10Ω + 10Ω + 10Ω + 10Ω | 40 Ω |
| 4.7kΩ + 10kΩ + 22kΩ | 36.7 kΩ |
Parallel Resistors Calculator
When resistors are connected side-by-side (parallel), the total resistance is always less than the smallest individual resistor.
For 2 resistors: R_total = (R1 × R2) ÷ (R1 + R2)
For multiple: 1/R_total = 1/R1 + 1/R2 + 1/R3 + …
| Resistors in Parallel | Formula | Total Resistance |
|---|---|---|
| 100Ω ∥ 100Ω | (100×100)/(100+100) | 50 Ω |
| 1kΩ ∥ 2kΩ | (1000×2000)/(1000+2000) | 667 Ω |
| 10kΩ ∥ 10kΩ ∥ 10kΩ | 1/(3×0.0001) | 3,333 Ω (3.3kΩ) |
| 220Ω ∥ 470Ω | (220×470)/(220+470) | 150 Ω |
Standard Resistor Values – E24 Series
Resistors are manufactured in standard values. The most common series is E24 (5% tolerance). If your calculated value doesn’t exist, choose the nearest standard value.
| Decade | Standard E24 Values |
|---|---|
| 1Ω – 10Ω | 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1 |
| 10Ω – 100Ω | 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91 |
| 100Ω – 1kΩ | 100, 110, 120, 130, 150, 180, 200, 220, 240, 270, 300, 330, 360, 390, 430, 470, 510, 560, 620, 680, 750, 820, 910 |
Common Resistor Use Cases
| Application | Typical Resistor Value | Purpose |
|---|---|---|
| LED current limiter (5V) | 220Ω – 470Ω | Protect LED from excess current |
| Pull-up resistor (3.3V GPIO) | 4.7kΩ – 10kΩ | Hold signal HIGH when not driven |
| Pull-down resistor | 4.7kΩ – 10kΩ | Hold signal LOW when not driven |
| Voltage divider | 10kΩ + 10kΩ | Scale voltage down proportionally |
| I2C bus pull-up | 2.2kΩ – 4.7kΩ | Required for I2C communication |
| Potentiometer / trimmer | 10kΩ | Variable resistance for calibration |
| Audio volume control | 10kΩ – 100kΩ | Logarithmic pot for audio taper |
| Current sense resistor | 0.1Ω – 1Ω | Measure current via voltage drop |
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Frequently Asked Questions
Hold the resistor so the tolerance band (usually gold or silver) is on the right. Read the color bands left to right. For a 4-band resistor: Band 1 = first digit, Band 2 = second digit, Band 3 = multiplier (power of 10), Band 4 = tolerance. Multiply the two-digit number by the multiplier to get the resistance in ohms. For example, Red-Red-Brown-Gold = 2, 2, ×10 = 220Ω ±5%.
The gold or silver band indicates the resistor’s tolerance — how close the actual resistance is to the stated value. Gold means ±5% tolerance, silver means ±10%. A 1kΩ ±5% resistor can have an actual value between 950Ω and 1,050Ω. For most general electronics projects, 5% tolerance (gold band) is perfectly acceptable. Precision circuits may require 1% (brown band) or tighter.
Ohm’s Law (V = I × R) describes the fundamental relationship between voltage, current, and resistance in an electrical circuit. It is the most important equation in basic electronics. Understanding Ohm’s Law allows you to calculate the correct resistor value to protect components, determine how much current a circuit draws, calculate power dissipation to avoid overheating, and design voltage dividers and bias networks. Every electronics engineer and hobbyist uses it daily.
Use series resistors when you want to increase total resistance — the values simply add together. Use parallel resistors when you want to decrease total resistance or when you need to handle more power than a single resistor can dissipate. For example, if you need a 2W resistor but only have 1W resistors, connect two 1W resistors in series (if the values work out) or parallel to share the power load. Parallel combinations also give you intermediate values not available in standard resistor series.
The most widely used mnemonic in electronics is: “Better Be Right Or Your Great Big Venture Goes Wrong” — standing for Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Gray, White (0 through 9). Another popular version is “Bad Boys Rape Our Young Girls But Violet Gives Willingly.” Whatever mnemonic works best for you, the key is memorizing the color sequence 0–9: Black(0) Brown(1) Red(2) Orange(3) Yellow(4) Green(5) Blue(6) Violet(7) Gray(8) White(9).
Calculate the power dissipated using P = V² ÷ R or P = I² × R. Then choose a resistor with a wattage rating at least 2x your calculated value for safety margin. Common ratings are 1/8W, 1/4W, 1/2W, 1W, 2W, and 5W. Most signal-level resistors in logic circuits dissipate well under 1/4W. Power supply circuits and motor controls often need 1W+ resistors. Never run a resistor at its rated maximum wattage — it will run very hot and fail prematurely.
Choose the tab for what you want to solve. In the Color Code tab, select the color of each band on your resistor and the calculator returns the resistance value, tolerance, and min/max range. Use the Ohm’s Law tab to find voltage, current, or resistance from any two values, and the Series/Parallel tab to combine resistors and read the total resistance.
4-band resistors use two digit bands, a multiplier band, and a tolerance band. 5-band resistors add a third digit band, so they can express more precise values, typically with 1% tolerance or better. The comparison table above shows how each band is read for both types.