Appendix A: Reading Resistor Codes
Identifying resistors by color bands
If you have just purchased a resistor it will generally come with some sort of label, but that doesn’t help if we find our resistor sitting unaccompanied on a table or in our parts box. Fortunately, every resistor has a set of color bands printed on its casing which tells us what the value of the resistor. While there are resistors with 6, 3 and even 1 band, the most commonly found resistors by far have 4 bands, and we are looking at that type.
Let’s take a close look at a resistor:
A resistor has two wire leads and a body with color bands on it:
Orienting the resistor
Not only does the colors of the bands matter, but also the order in which the colors appear. How do we know what each color means? The first step is to orient our resistor in the correct direction. On one side of the resistor the band color will be either silver or gold. This band should be placed on the right-hand side of the resistor.
Look for the silver or gold band on the resistor body and place this on the right-hand side.
Identifying resistors by color bands continued
Now that our resistor is oriented correctly, we can identify the other color bands on the resistor body We have labeled the bands on this resistor in order below: The colors on each band have a particular significance.
Resistor color chart
This is the standard color chart which all resistors follow. You can find other similar charts online. We’ll go over what each band means in detail.
The colors mean the same thing for all resistors
Decoding the resistor
Now that we’ve seen the color chart, let’s see how to apply it to our resistor.
The first band represents the most significant digit, also known as the first digit in the number. For example on our resistor below, the first band is red. Looking at the color chart we see that red on the first band equates to the number ‘2.’
The second band signifies the second most significant digit. On this resistor, the second band is also red. Looking at the chart, we can see that it again states the number as ‘2’ for our second red band.
The first two bands taken together give us the number “22.” The first two bands on a resistor will always represent a number between 10 and 99. (We’ll explain what these numbers mean shortly.) The third digit is a little bit different.
The third band is slightly different. Rather than representing a number, it represents a multiplier. This band multiplies the values on the first two bands by a power of ten. We can see this in the third row of the chart above. For this resistor, the band is brown, which the chart tells us means a multiplier of 10. Now that we know these three values, we can calculate the resistor’s total resistance using a simple formula: the first two digits times the multiplier equals the resistance (in Ohms).
This means that our red red brown resistor is a 220 ohm resistor. In fact, all red red brown resistors have a value of 220 ohms.
The fourth band of our resistor represents the resistor’s tolerance or possible range of accuracy. With a gold band, the accuracy is plus or minus 5%, which means that our resistor could be as high as 231 ohms (220 *1.05) or as low as 209 (220 * 0.95) ohms. (This variation is caused by imperfections in the resistor’s manufacturing process.)
Since the fourth band is always going to be gold or silver, and these are not colors any of the other bands use, we can always use the fourth band to orient our resistor correctly.
Q: Are the band colors universal, and will I have to remember what each color means?
A: All resistors use the same standard color codes we have talked about here regardless of the manufacturer. You don’t have to memorize them as; you can easily find the color info online.
Q: What if the bands are hard to see or they have been painted over or erased?
A: If your resistors are missing the color bands, you can use a multimeter to confirm the resistance value.
Q: I have noticed online and in my kit that the color of the resistor body is different. Does this mean something?
A: Sometimes different manufacturers will use alternative body colors (blue vs tan) and it can mean that they are made of different materials. For our purposes in beginning electronics, all resistor body colors are equal.
Q: How accurate do I need to be with my resistors?
A: Good question. Hobby electronics and electrical components are not super sensitive to minor variations in resistance. The difference between 209 ohms and 231 ohms is not enough to cause any issue with your LED. However, using a resistor with a much higher rating (double or more) or much smaller rating (half or less) is enough to cause issues.
Analyzing the color bands on another resistor
Let’s look at another resistor and evaluate it’s color bands to figure out its total resistance value. This one has color bands brown, black, orange, and gold.
The first step is to orient the resistor correctly. To do that, we make sure that the gold band is on the right-hand side.
Resistor color code chart
Here is the chart again. You can always look up the chart whenever you need to calculate a resistance value.
Reading the bands
The first band is brown, so we can look at the color chart on the previous page and know that the first digit is a 1.
The next color band on the resistor is black, which makes the second digit a 0.
The third band is orange, which means that value of the multiplier is 1000.
This means that our resistor value is ‘1’ ‘0’ times ‘1000.’
By doing the math, we can determine that our brown black orange resistor has 10,000 ohms of resistance.. Normally resistance values of one thousand and up are refered to by kilo-ohm value, (1,000 ohms = 1 kilo-ohm). So, we can then abbreviate ‘kilo-ohms’ as k, so this resistor has a value of 10K ohms.
The last band is gold, which gives us that same error margin of 5% we saw with our 220 ohm resistor earlier.
And here is another version of the chart.