SMD Resistor Code Calculator Guide: Read Three and Four Digit Surface Mount Codes

Surface mount resistors are the small rectangular components that crowd modern circuit boards, and they are far too small for the coloured bands used on older through-hole resistors. Instead, most carry a short printed code of three or four digits, and that code contains the resistance value in a compact form. Reading it is a useful skill for anyone repairing electronics, checking a board against a design, or sorting components. It is also easy to get wrong, because the code looks like an ordinary number and is not one: the last digit is not part of the value but an instruction about how many zeroes to add. This guide explains the system, works through examples with the arithmetic shown, and covers the special cases that catch people out.

A note on scope first. This guide is about reading codes and estimating values. Working on mains-powered or high-energy equipment, and modifying circuits, requires appropriate knowledge and safety precautions, and component values should always be confirmed against the circuit documentation where it matters. A decoded value is a starting point for identification, not a guarantee that a particular component is correct for a particular circuit.

The three-digit code

The most common code has three digits. The first two digits are the significant figures, and the third digit is the multiplier: the power of ten by which to multiply the significant figures, or equivalently the number of zeroes to append. For example, the code 472 means significant figures 47, multiplier 2, so the value is 47 followed by 2 zeroes, which is 4,700 ohms, or 4.7 kilohms. The code 103 means significant figures 10, multiplier 3, so the value is 10 followed by 3 zeroes, which is 10,000 ohms, or 10 kilohms. The code 220 means 22 followed by 0 zeroes, which is simply 22 ohms, because a multiplier of 0 means multiply by 10 to the power of 0, which is 1.

Values below 10 ohms need a different treatment, because the significant-figures-plus-multiplier scheme cannot express them without a decimal point. In three-digit codes, the letter R is used as a decimal point marker. The code 4R7 means 4.7 ohms, and R10 means 0.10 ohms, though such low values are also seen as 0R10 in some markings. The letter R simply replaces the decimal point, so 2R2 is 2.2 ohms and 0R5 is 0.5 ohms. If you read R as a digit or ignore it, the value will be wrong by orders of magnitude, which is why recognising the marker matters more than memorising any table.

The four-digit code

Precision resistors, typically with 1 percent tolerance, usually carry four digits instead of three. The rule is the same but with one more significant figure: the first three digits are the significant figures, and the fourth digit is the multiplier. The code 1002 means significant figures 100, multiplier 2, so the value is 100 followed by 2 zeroes, which is 10,000 ohms, or 10 kilohms. The code 4991 means 499 followed by 1 zero, which is 4,990 ohms, or 4.99 kilohms, a standard value in the precision series. The extra digit matters: a three-digit code can only express values to two significant figures, so 4.99 kilohms would be rounded to 5.0 kilohms as 502, and a circuit that depends on the precise value needs the four-digit marking to be identified correctly.

Four-digit codes also use R as a decimal marker for low values, in the same way: 1R00 means 1.00 ohm, and 0R01 would mean 0.01 ohms. In addition, a code of 0000, or a single 0 on a three-digit part, usually denotes a zero-ohm link, a component used as a jumper rather than as a resistor. Zero-ohm links look like resistors, are placed by the same machines, and can be used to bridge a trace or configure a board option. Treating a zero-ohm link as a resistor value of zero ohms in a calculation is technically consistent, but in circuit terms it is a connection, and its purpose is found in the schematic, not in the code.

The EIA-96 system for precision parts

Some precision resistors use a different system, EIA-96, in which a two-digit number plus a letter replaces the digit code. The two digits select a value from a table of 96 standard values, and the letter selects the multiplier. For example, in the EIA-96 system, the number 01 corresponds to the value 100, and the letter C corresponds to a multiplier of 100, so 01C means 10,000 ohms. The number 68 corresponds to 499, and the letter A corresponds to a multiplier of 1, so 68A means 499 ohms. This system is compact and unambiguous once you know it, but it is not decodable by arithmetic alone: you need the table, because the mapping from the two-digit number to the value is a standard series, not a formula. A calculator that supports EIA-96 saves looking up the table, and it is worth confirming which system a component uses before decoding, because a code that looks like an EIA-96 marking may also be readable, incorrectly, as a digit code.

Distinguishing the systems in practice is usually straightforward. If a small resistor carries two digits and a letter, it is likely EIA-96. If it carries three or four digits, it is the digit code. If it carries no marking at all, it may be a very small package, such as 0402 or smaller, where printing is impractical, or it may be a component whose value must be measured or found in documentation. Do not guess an unmarked component's value from its size or neighbours. Measure it out of circuit where possible, or consult the board's bill of materials.

Worked example with sample values

Take the three-digit code 103 as a sample and decode it step by step. Split the code into significant figures and multiplier: the first two digits are 1 and 0, giving 10, and the last digit is 3, giving the multiplier. The multiplier means 10 to the power of 3, which is 1,000. Multiply 10 by 1,000, which equals 10,000. The value is therefore 10,000 ohms. Convert to kilohms by dividing by 1,000: 10,000 divided by 1,000 equals 10, so the resistor is 10 kilohms. Check the result by the zeroes method: 10 followed by 3 zeroes is 10,000, which matches, so both methods agree.

Now decode the four-digit code 4701. Split it: the first three digits give 470, and the last digit gives the multiplier 1, meaning 10 to the power of 1, which is 10. Multiply 470 by 10, which equals 4,700 ohms. Divide by 1,000 to get kilohms: 4,700 divided by 1,000 equals 4.7, so the resistor is 4.7 kilohms. Notice that the three-digit code 472 decodes to the same value, 4,700 ohms, because 47 followed by 2 zeroes is also 4,700. The two codes describe the same resistance; the four-digit version simply comes from a precision series where the value is stated to three significant figures. If a design calls for 4.7 kilohms at 1 percent tolerance, the four-digit part is the one to look for, even though a two-significant-figure reading gives the same nominal value.

Common mistakes to avoid

The most common mistake is reading the code as a plain number: 103 is not 103 ohms, it is 10,000 ohms, and the error is a factor of about 97. The second mistake is confusing three-digit and four-digit codes, especially with codes such as 1002, which a careless reading as a three-digit code would mangle. Count the digits first, then apply the matching rule. The third mistake is misreading digits on a tiny, worn, or poorly printed component: 6 and 9 can be confused if the component's orientation is unclear, and 1 and 7 can look alike at small sizes. Check orientation against neighbouring components, and measure with a multimeter when the reading matters. The fourth mistake is assuming the code gives tolerance: it does not. Tolerance is a property of the series and the part, and a 10-kilohm resistor may be 5 percent or 1 percent depending on its series, which affects whether a measured value slightly off nominal is acceptable. The fifth mistake is measuring a resistor in circuit and trusting the reading: parallel paths on the board affect the measurement, so an in-circuit reading that disagrees with the code is not proof the code is wrong. Lift one end or consult the schematic before concluding a component has failed.

Try it next

Try the SMD Resistor Code Calculator Guide tool with your own numbers, then explore the Resistor Color Code Calculator and the Voltage Divider Calculator for related calculations. For a companion read, see /blog/resistor-color-code-calculator-guide/, which applies the same measure-carefully habit to a different problem.