Skill system
Choosing a resistor: value, E-series, tolerance, power and colour code
By Bulan Sarkar, founder of ElectronicsInfoline (since 2000) · About a 9-minute read
Ohm's law gives you a number like 125 Ω. Nobody sells 125 Ω. Choosing a resistor means getting from that number to a part you can buy, that won't run hot, and that you can find again in the drawer.
This page takes one LED from 5 V and one from 12 V through every check, with numbers from the resistor and LED datasheets, then shows how to read the part you end up holding.
Work out the value you need, then take the next standard value up from the E24 series: for an LED, rounding up keeps the current below your target. Use 5 % carbon film for most hobby work and 1 % metal film where accuracy matters, as in dividers and timers. Check the power: I² × R should be half the rating or less. Read the colour bands with the IEC 60062 table, and SMD parts by their printed code.
Build it, buy it, or get help
Build it yourself
Any LED, pull-up, divider or timing resistor in a 5 V to 12 V hobby circuit. A few checks on paper and one meter reading tell you the part is right before you solder it.
Buy a module instead
If you need many odd values, buy an E12 or E24 assortment kit instead of single values. For an LED strip or a ready LED module, the resistors are already fitted, so there is nothing to choose.
Stop and get help
Anything on the mains side, such as a dropper resistor in a 230 V LED bulb or a bleeder across a mains capacitor. Those resistors are chosen for surge and safety ratings that this page does not cover.
The steps, in order
1. Work out the value
Have ready: Supply voltage, LED datasheet
Use Ohm's law with the worst case. For an LED, R = (supply − forward voltage) ÷ current. Kingbright's WP7113SGC green LED is 2.2 V typical and 2.5 V maximum at 20 mA. From 5 V, using the maximum: (5 − 2.5) ÷ 0.020 = 125 Ω.
Check: Value worked out from the maximum VF (125 Ω here)
Figure 1. The 5 V LED example: (5 − 2.5) V across the resistor at 20 mA gives 125 Ω exactly, which nobody sells, so it becomes 130 Ω. 2. Round to a value you can buy
Have ready: The E12/E24 table
Resistors come in standard series. E12 has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82). E24, the series for ±5 % and ±2 % parts, adds 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75 and 91. Multiply by 10, 100 or 1,000 for other decades. For an LED, round up: 125 Ω becomes 130 Ω. The current is then 19.2 mA with a 2.5 V LED and 21.5 mA with a typical 2.2 V one, both under the 25 mA the datasheet allows.
Check: Next E24 value up (130 Ω), current under the LED's maximum
Figure 2. One decade of the E12 and E24 series on a log scale. E24 adds a value between each pair of E12 values; 125 Ω falls between 120 and 130 Ω. 3. Choose the tolerance
Have ready: Carbon film and metal film in the drawer
Yageo's CFR carbon film comes in ±2 % and ±5 %, with a temperature coefficient of −500 to +350 ppm/°C. Metal film (Yageo MFR, Vishay MBA) comes in ±1 % and better, at ±50 ppm/K or less. A 4.7 kΩ ±5 % part can be anything from 4,465 to 4,935 Ω. For an LED that's fine. For a divider feeding an ADC, or a 555 timing resistor, use 1 %.
Check: 5 % for LEDs and pull-ups, 1 % for dividers and timers
Figure 3. Where a 4.7 kΩ resistor can really sit: 4,465 to 4,935 Ω at ±5 %, against a band five times narrower at ±1 %. 4. Check the power
Have ready: The current from step 2
Power = I² × R. For the 5 V LED at its typical forward voltage: 21.5 mA squared × 130 Ω = 60 mW, so 1/4 W is plenty. Now an LED from 12 V: a hyper-red WP7113SURDK (2.5 V maximum) needs (12 − 2.5) ÷ 0.020 = 475 Ω, so take 510 Ω. At the typical 1.95 V it draws 19.7 mA and the resistor dissipates 198 mW. That's 79 % of a 1/4 W rating, too close. Use a 1/2 W part (Yageo CFR-50).
Check: I² × R at or under half the rating
Figure 4. Power in the two worked resistors against half of each rating: 60 mW sits well inside 1/4 W, while 198 mW needs a 1/2 W part. 5. Allow for the heat inside the box
Have ready: A guess at the temperature inside the enclosure
The rating holds only up to 70 °C. Above that, Yageo's curve falls in a straight line to zero at 155 °C. Inside a closed box at 100 °C, a 1/4 W CFR-25 is good for about 0.16 W, 65 % of its label. Two parts with the same label can differ: a miniature CFR25S and a full-size CFR-25 are both sold as 1/4 W, at 3.4 mm and 6.3 mm long.
Check: Power still under half the derated rating at box temperature
Figure 5. Yageo's carbon film derating line: full power to 70 °C, zero at 155 °C, and 65 % of the rating left in a 100 °C box. Figure 6. Three Yageo carbon film bodies drawn to scale: the 3.4 mm CFR25S and 6.3 mm CFR-25 are both 1/4 W; the 9 mm CFR-50 is 1/2 W. 6. Check the voltage on high values
Have ready: The resistor's datasheet
Each size has a maximum working voltage: 250 V for a CFR-25. Above about 250 kΩ (250² ÷ 0.25), the voltage limit binds before the power limit does. At 5 V and 12 V you won't get anywhere near it, but it matters for high-value resistors in valve amplifiers and meter dividers.
Check: Voltage across the part under its working-voltage limit
7. Read the marking, then measure
Have ready: Colour table, a multimeter
Through-hole parts carry colour bands (IEC 60062). Four bands are two digits, a multiplier and the tolerance: yellow (4), violet (7), red (×100), gold (±5 %) reads 4.7 kΩ ±5 %. Five bands are three digits, a multiplier and the tolerance: brown, black, black, brown, brown reads 100 × 10 = 1 kΩ ±1 %. SMD 5 % parts print three digits, two figures and the number of zeros, so 472 is 4,700 Ω and 240 is 24 Ω. 1 % parts of size 0805 and up print four: 4701 is 4.70 kΩ. Measure before you solder.
Check: Meter reading inside the tolerance band
Figure 7. A four-band 4.7 kΩ ±5 % carbon film resistor and a five-band 1 kΩ ±1 % metal film resistor, each band labelled by its meaning in IEC 60062. Figure 8. Three SMD codes read: 472 is 4,700 Ω and 240 is 24 Ω (5 %, three digits), and 4701 is 4.70 kΩ (1 %, four digits).
What goes wrong, and how to find it
| Symptom | Likely cause | Check and fix |
|---|---|---|
| The LED is brighter than expected, or fails early | The value was rounded down. 120 Ω instead of 130 Ω pushes a 2.2 V green LED to 23.3 mA. | Measure the voltage across the resistor and divide by its value to get the current. Swap to the next E24 value up. |
| The resistor is brown, smells, or the board under it is darkened | It runs near or above its power rating, often only once the box is closed. | Work out I² × R again and fit a part rated at least twice that, or two resistors in parallel to share the heat. |
| The meter reads a value nothing like the bands | The bands were read from the wrong end, or red was mistaken for orange under yellow light. | Start from the end away from the gold or silver band, which often has a gap before it. Trust the meter over your eyes. |
| A divider gives the wrong ADC reading | Two 5 % resistors can shift a 1:1 divider's output by 5.0 %, against 1.0 % for 1 % parts. | Use 1 % metal film for both resistors, and measure the pair before fitting. |
| A chart says grey means ×0.001 | Some datasheets print the colour table wrong. Yageo's CFR datasheet shows grey and white multipliers that do not match IEC 60062. | Use the IEC 60062 table as Vishay doc 20020 reproduces it, or our colour-code calculator, which follows it. |
Variants
Voltage divider
Use 1 % metal film for both resistors. A 5 % pair can move the output of an equal divider by up to 5.0 %; a 1 % pair by 1.0 %.
Pull-up for a button
10 kΩ, 5 %, 1/4 W. The current is tiny, so any E12 value near 10 kΩ works and the power check is a formality.
SMD board
0805 (1/8 W, 150 V in Yageo RC_L) is easy to solder by hand; 0603 (1/10 W, 75 V) is the smallest most beginners manage. Halve those ratings in the power check just as for through-hole parts.
12 V LED strip
Strips have their resistors built in, one per three LEDs. You size the adapter and the wire instead; see the 12 V LED strip system.
What we would do
We would buy a 5 % E12 carbon film kit at 1/4 W and a small bag of 1 % metal film in 1 kΩ, 4.7 kΩ and 10 kΩ. Size every LED resistor from the maximum forward voltage, round up to the next E24 value, then check I² × R against half the rating, using the derated figure if the board lives in a closed box. Keep the printed card from this page next to the breadboard.
Tips and tricks
- Buy an E12 kit, not single values. Twelve values per decade cover almost every hobby circuit, and two in series make anything in between.
- Measure before you solder. A meter settles a doubtful band faster than any chart, especially red against orange under a yellow bulb.
- For LEDs and transistor bases, round up. A slightly bigger resistor is the safe direction; a smaller one raises the current.
- Pick a power rating at least twice I² × R. The part runs cooler, and the margin covers a warm enclosure.
- Keep resistors on their tape strips, cut with the label still on. Loose resistors in a box cost you a meter reading every time.
- Two equal resistors in parallel give half the value and share the heat: two 1 kΩ make 500 Ω at twice the power.
Print the working copy
One A4 page for the bench: the parts list as a checklist, every connection with a tick box, and the tests with the readings to expect. It prints in black and white.
Source and assumptions
Vishay Dale, Standard Electronic Decade Value Tables (doc 31001). p. 1: E12 under ±10 %; E24 for ±2 % and ±5 % (11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91 added to E12); E96 for ±1 %; E192 for ±0.1 % to ±0.5 %; other decades by multiplying by powers of ten. Checked 8 October 2026.
Our own choices: 20 mA LED current, sizing from the maximum forward voltage, the 100 °C box and the "half the rating" margin. Also cited: Vishay doc 20020 p. 1 (IEC 60062 colour code: four bands for ±5 % E24, five bands for ±1 % and better); Yageo CFR product specification V.2 p. 3 (1/4 W CFR-25, 250 V, 6.3 × 2.4 mm; 1/2 W CFR-50, 350 V, 9 × 3.3 mm; CFR25S 3.4 × 1.9 mm; derating 70 to 155 °C) and p. 4 (−500 to +350 ppm/°C under 100 kΩ); Yageo MFR (±1 %, down to ±50 ppm/°C); Vishay MBA/SMA 0204 pp. 1-2 (0.25 W standard, 0.4 W power mode); Yageo RC_L V.12 p. 3 (SMD marking; 0603 1 % uses EIA-96, 0402 and smaller unmarked); Kingbright WP7113SGC and WP7113SURDK p. 2 (forward voltage and DC current). All checked 8 October 2026.
Questions people ask
What resistor do I need for an LED on 5 V? For a green LED at 20 mA with a 2.5 V maximum forward voltage: (5 − 2.5) ÷ 0.02 = 125 Ω. Use the next standard value up, 130 Ω, at 1/4 W.
What does E24 mean? A standard series of 24 values per decade (10, 11, 12, 13, 15 … 91). Vishay's decade table says ±2 % and ±5 % resistors use E24, and ±1 % parts use E96.
How do I read a four-band resistor? The first two bands are digits, the third is the multiplier and the fourth is the tolerance. Yellow, violet, red, gold is 47 × 100 = 4.7 kΩ, ±5 %.
Is a 1/4 W resistor enough? If I² × R is under about 0.125 W, yes. The 1/4 W rating applies up to 70 °C and falls to zero at 155 °C, so at 100 °C a CFR-25 manages about 0.16 W.
Tools and guides this system uses
- LED resistor calculatorSteps 1, 2 and 4 for any LED, supply and string length, rounding up to the next E-series value.
- Resistor colour code calculatorStep 7 in both directions, three to six bands, IEC 60062.
- SMD resistor code calculatorThree-digit, four-digit, EIA-96 and R codes.
- Voltage divider calculatorThe divider variant, with the tolerance spread worked out.
- Resistor combination finderStep 2 when the value is not in your drawer: the best two or three standard parts in series or parallel.
- E-series lessonWhy the values are spaced the way they are.
- Resistor power rating lessonDerating and hot spots in more depth.
- 12 V LED strip lighting systemWhen the resistors come built into the strip.
- 555 LED flasher systemTiming resistors, where 1 % pays off.
All build systems: Build systems.