Resistor Calculator
Calculate resistor values and color codes. Enter values for instant results with step-by-step formulas.
Formula
R = (Band1 ร 10 + Band2) ร Multiplier
Combine digits and multiply by the multiplier factor.
Worked Examples
Example 1: 1k Ohm
Problem:Brown, Black, Red, Gold
Solution:10 * 100
Result:1000 ฮฉ 5%
Frequently Asked Questions
How to read resistor bands?
Read from the end with bands closer together. First 2 bands are digits, 3rd is multiplier, 4th (gap) is tolerance.
How do I read a resistor color code?
Each color band represents a digit or multiplier. The first two bands are digits (0-9), the third is the multiplier (number of zeros), and the fourth is tolerance. For example, brown-black-red-gold means 1-0-x100 = 1,000 ohms (1K) with 5% tolerance. Use the mnemonic: Bad Beer Rots Our Young Guts But Vodka Goes Well.
How do I calculate the current-limiting resistor for an LED?
Use R = (V_supply - V_forward) / I_desired. Typical LED forward voltages: red 1.8-2.2V, green 2.0-3.5V, blue/white 3.0-3.5V. Standard LED current is 20mA. For a red LED on 5V: R = (5 - 2) / 0.02 = 150 ohms. Always round up to the nearest standard resistor value.
What is a pull-up resistor and when do I need one?
A pull-up resistor connects a signal line to the supply voltage (typically 10K ohms to VCC) to ensure a defined high state when no other device is driving the line. Without it, floating inputs on microcontrollers read unpredictably. Pull-down resistors do the same to ground. Many microcontrollers have built-in pull-ups that can be enabled in software.