SMD Resistor Calculator: Code to Value

This calculator decodes the three- or four-digit code printed on a surface-mount (SMD) resistor and returns its resistance in ohms, kilohms, or megohms. You enter the code as it appears on the part (digits only, or with an R for a decimal point) and the calculator gives you the resistance value and, for four-digit codes, its tolerance class. It works for the standard EIA-96 and the older two- and three-digit systems, so you can quickly identify an unknown part on a board.

Formula

The most common SMD resistor code is a three-digit number: the first two digits are the significant figures, and the third digit is the multiplier (a power of ten). The resistance in ohms is:

R = (first two digits) × 10^{third digit}

For example, a code of 472 means 47 × 10² = 4700 Ω (4.7 kΩ). A four-digit code works the same way: the first three digits are significant, and the fourth is the multiplier. So 1002 means 100 × 10² = 10,000 Ω (10 kΩ).

If the code contains the letter R, it acts as a decimal point. For instance, 4R7 is 4.7 Ω, and 0R1 is 0.1 Ω. This is common for low-value resistors where a decimal point would be easy to miss.

Worked Example

Using the default code 472:

1. Identify the first two digits: 47.
2. Identify the third digit: 2.
3. Compute the multiplier: 10² = 100.
4. Multiply: 47 × 100 = 4700 Ω.

The calculator shows 4700 Ω, which is also written as 4.7 kΩ. That matches the standard value for a 4.7 kΩ resistor.

Understanding SMD Resistor Codes

SMD resistors are tiny—often no larger than a grain of sand—so printing a full value like “4700� would be impossible to read. Instead, manufacturers use a compact code. The three-digit system covers values from 1 Ω to 99 MΩ with two significant digits. The four-digit system gives three significant digits for tighter tolerance parts (typically 1% or better).

There is also a special code called EIA-96 used for 1% tolerance resistors with three characters (two digits and a letter). The digits refer to a lookup table of values (e.g., 01 = 100, 02 = 102, … up to 96 = 976), and the letter indicates the multiplier (A = 10�, B = 10¹, C = 10², … up to H = 10�). For example, 01C means 100 × 10² = 10,000 Ω (10 kΩ). This calculator does not decode EIA-96 codes because they require a table, but you can easily find such tables online.

How to Use the Calculator

To get the resistance value, simply type the code you see on the resistor. The calculator accepts:

• Three-digit codes like 472 (47 × 10² = 4700 Ω)
• Four-digit codes like 1002 (100 × 10² = 10,000 Ω)
• Codes with an R like 4R7 (4.7 Ω)

The result is shown in ohms. If the value is large (e.g., 4700), you might prefer to read it as kilohms or megohms—just divide by 1000 or 1,000,000. For example, 4700 Ω = 4.7 kΩ, and 4,700,000 Ω = 4.7 MΩ.

Common Mistakes and Misconceptions

One frequent error is misreading the third digit as a value rather than a multiplier. For instance, 472 is not 472 Ω; it is 47 × 10² = 4700 Ω. Another mistake is ignoring the R. A code like 0R5 is 0.5 Ω, not 0.5 kΩ. Also, be careful with codes that have a leading zero, such as 010—that is 01 × 10� = 1 Ω, not 10 Ω.

Another misconception is that all SMD resistors use the three-digit code. High-precision resistors (1% or better) often use the four-digit code, and very small packages (like 0402) may use the EIA-96 code. If you see a code that doesn’t fit the pattern, check for a datasheet or use a multimeter to measure the actual resistance.

When to Use This Calculator

This tool is most useful when you are repairing a circuit board, sorting a mixed bag of components, or designing a prototype and need to confirm a part’s value. It is also handy for learning—students often encounter SMD codes in electronics courses. However, remember that the printed code is only accurate to the resistor’s tolerance. A 5% resistor marked 472 could actually measure anywhere from 4465 Ω to 4935 Ω. For critical circuits, always verify with a multimeter.

The calculator does not handle EIA-96 codes or codes with letters other than R (like K or M). Those are rare but exist on some specialty parts. For those, consult a complete SMD code chart.

Limitations of the Code System

The three-digit code can represent values from 1 Ω to 99 MΩ, but it cannot represent every possible resistance. For example, you cannot code 47.5 Ω with three digits because you only have two significant figures. That is why four-digit codes exist—they allow three significant figures, such as 4750 for 475 Ω (47.5 × 10¹). The EIA-96 system further extends precision for 1% resistors.

Also, the code does not tell you the power rating or the temperature coefficient. Those are determined by the physical size (package) and the material. A 0603 package resistor is typically rated for 0.1 W, while a 2512 package can handle 1 W or more. Always check the package size and datasheet for those specs.

Frequently Asked Questions

What does the third digit on a three-digit SMD resistor mean? It is the exponent of the multiplier. For example, in 472, the 2 means multiply 47 by 10². So 47 × 100 = 4700 Ω.

How do I read a four-digit SMD resistor code? The first three digits are the significant figures, and the fourth is the exponent. 1002 means 100 × 10² = 10,000 Ω (10 kΩ).

What does the R in a code like 4R7 mean? R is a decimal point. So 4R7 is 4.7 Ω. This is used to avoid a decimal point that might be missed or confused with a dot on the package.

Can this calculator decode EIA-96 codes? No, because EIA-96 uses a lookup table for the base value. You can find tables online, or you can measure the resistor with a multimeter.

Why does my resistor measure differently than the code suggests? Resistors have a tolerance, typically ±5% for three-digit codes and ±1% for four-digit codes. Also, temperature changes can shift the value slightly. If the measurement is far off, the resistor may be damaged.

What is the difference between a 3-digit and 4-digit code? The number of significant digits. Three-digit codes give two significant figures, which is enough for 5% tolerance parts. Four-digit codes give three significant figures, used for 1% tolerance parts.

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in code | Resistor code | - | 472 | 0
out value | Resistance | Ω | (substr(code,1,2)*10^substr(code,3,1)) | 2