Resistor Color Code Calculator
Through-hole resistors are too small to print numbers on, so their value is marked with colored bands instead, a code every electronics hobbyist eventually memorizes but no one wants to puzzle out by hand at the bench. This calculator decodes a standard four-band resistor for you. The scheme is logical once you know it: the first two bands are significant digits, the third band is a power-of-ten multiplier, and the fourth band, usually gold or silver, gives the tolerance, how far the true resistance may stray from the marked value. Each color stands for a number on a fixed scale running from black at zero through to white at nine. Pick the color of each band from the dropdowns, reading from the end where the bands cluster together, and the tool returns the resistance in ohms along with the tolerance percentage and the resulting minimum and maximum values. It is the quickest way to confirm you have grabbed the right part from a drawer of look-alikes before you solder it down. The color-to-digit mapping follows the standard scheme used throughout the electronics industry. A worked example below reconciles exactly to the calculator defaults so you can check the decoding yourself.
A 4-band resistor reads as two digits times a multiplier: R = (d1 x 10 + d2) x 10^multiplier. Bands brown, black, red, gold give (1, 0) times 10^2, that is 1,000 ohms at 5% tolerance (950 to 1,050 ohms).
Resistor value formula
R = (d1 x 10 + d2) x 10^m ohms
d1 = first band digit, d2 = second band digit
m = third band multiplier exponent
tolerance band gives the plus/minus percentage
The first two bands form a two-digit number, the third band scales it by a power of ten, and the fourth band sets how far the actual resistance may vary.
Worked example
A resistor with bands brown, black, red, gold.
- Brown = 1, black = 0, so the digits give 10.
- Red multiplier = 10^2 = 100.
- R = 10 x 100 = 1,000 ohms (1 kilohm).
- Gold tolerance = 5%, so the value lies between 950 and 1,050 ohms.
These are the calculator's default bands, so the result above matches the widget exactly.
Resistor color code calculator: frequently asked questions
How does the resistor color code work?
On a standard 4-band resistor, the first two bands are significant digits, the third band is a power-of-ten multiplier, and the fourth band is the tolerance. You read the two digits as a number, multiply by ten raised to the multiplier digit, and the fourth band tells you how far the real value may drift from the marked value.
What do the colors mean?
Each color maps to a digit: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. The same scale gives the multiplier exponent. Tolerance bands are usually gold for plus or minus 5% and silver for plus or minus 10%.
Which end do I read from?
Read from the end where the bands are grouped more closely together. The tolerance band, often gold or silver, sits slightly apart at the opposite end. If you read the resistor backwards you will get a wildly different value, so orient it with the tolerance band on the right.
What does tolerance tell me?
Tolerance is how much the actual resistance may differ from the marked value. A 1,000 ohm resistor with 5% tolerance may measure anywhere from 950 to 1,050 ohms. Tighter tolerance resistors cost more and are used where precision matters. The Institute uses standardized values published in reference standards.
What is the resistor value formula?
Resistance equals (first digit times ten plus second digit) times ten raised to the multiplier band. For brown, black, red, that is (1 times 10 plus 0) times ten to the power 2, which is 10 times 100, equal to 1,000 ohms, with the tolerance band giving the spread.
Official sources
- Electrical units, standards and reference data: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.