Resistor Calculator

About the Resistor Calculator

Most small through-hole resistors are far too small to print a number on, so their value is shown by a series of coloured bands. Each colour stands for a digit, and the bands together give the resistance and its tolerance. Learning to read them is a rite of passage for anyone starting in electronics — and a resistor calculator makes it quick and certain.

This resistor calculator decodes 4-band and 5-band colour codes into ohms, with the tolerance and the range of values the resistor could actually have. It also works in reverse: enter a resistance, and it gives the 4-band colours to look for, rounding to two significant figures where necessary.

How to Use the Resistor Calculator

Choose Colours to value or Value to colours.

To decode a resistor, choose 4 bands or 5 bands, then pick the colour of each band in order: the digit bands, the multiplier band and the tolerance band.

To find colours, enter the resistance, choose Ω, kΩ or MΩ, and pick a tolerance.

The Colour Code

  Colour    Digit   Multiplier     Tolerance
  black     0       × 1
  brown     1       × 10           ±1%
  red       2       × 100          ±2%
  orange    3       × 1,000
  yellow    4       × 10,000
  green     5       × 100,000      ±0.5%
  blue      6       × 1,000,000    ±0.25%
  violet    7       × 10,000,000   ±0.1%
  grey      8                      ±0.05%
  white     9
  gold              × 0.1          ±5%
  silver            × 0.01         ±10%
  (none)                           ±20%

  4-band:  digit, digit, multiplier, tolerance
  5-band:  digit, digit, digit, multiplier, tolerance

Step-by-Step Examples

Brown, black, red, gold.

  brown 1, black 0     → 10
  red multiplier       → × 100
  value                = 1,000 Ω = 1 kΩ, ±5% (950 Ω to 1.05 kΩ)

Yellow, violet, orange, gold: 47 × 1,000 = 47 kΩ ±5%.

Red, red, brown, gold: 22 × 10 = 220 Ω ±5% (209 to 231 Ω).

Five bands — brown, black, black, brown, brown: 100 × 10 = 1 kΩ ±1%.

Value to colours: 4.7 kΩ is yellow, violet, red; 4.7 Ω is yellow, violet, gold.

Remembering the Colours

The digit order follows the rainbow in the middle: black and brown for 0 and 1, then red, orange, yellow, green, blue and violet for 2 to 7, then grey and white for 8 and 9. Generations of students have learned the sequence with mnemonics built from the first letters — B, B, R, O, Y, G, B, V, G, W — and many find it easiest simply to remember that the colours after black and brown run through the spectrum.

Which End to Start From

The tolerance band, usually gold or silver on common resistors, is normally at one end and often set slightly apart from the others. Hold the resistor with that band on the right and read from the left. On some resistors the bands are evenly spaced, or the tolerance is brown or red, which can be confused with a digit. If a reading gives a value that is not in the standard series, try reading from the other end — and when in doubt, check with a multimeter, which also catches damaged or mislabelled parts.

Standard Values

Resistors are not made in every value. They follow the E-series of preferred numbers, spaced so that the tolerance ranges of neighbouring values just overlap. The E12 series, common for ±10% parts, uses 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82 in each decade; E24 adds values such as 11, 13, 16 and 20 for ±5% parts, and E96 has 96 values per decade for ±1% precision resistors. That is why values like 4.7 kΩ and 220 Ω are so common, and why the calculator rounds to two significant figures for 4-band codes.

Tolerance

Tolerance tells you how far a resistor's actual value may differ from its marked value. A 1 kΩ resistor at ±5% may measure anywhere from 950 Ω to 1,050 Ω and still be within specification. For most circuits — LED current limiting, pull-up resistors, simple dividers — 5 percent is fine. Precision circuits such as measurement instruments and filters use 1 percent or better, marked with a brown or red tolerance band on 5-band resistors.

Surface-Mount Resistors

Modern circuit boards mostly use tiny surface-mount resistors, which carry a printed number code rather than colour bands. A three-digit code such as 472 means 47 × 10² = 4.7 kΩ, and a four-digit code such as 1001 means 100 × 10¹ = 1 kΩ. The same idea — significant digits followed by a multiplier — underlies both systems.

Checking with a Multimeter

A digital multimeter set to resistance gives a definitive reading in seconds. Measure a resistor out of the circuit, because other components connected in parallel will make the reading lower than the true value. Hold the probes on the leads without touching both with your fingers, since your body's resistance can affect readings of high-value resistors. A reading within the tolerance band confirms both the colour code and the part.

Understanding Your Result

The headline is the resistance and tolerance, or the colour bands for your value.

The value line gives the resistance in ohms, or notes any rounding.

The tolerance range line gives the lowest and highest values allowed by the tolerance.

When Should You Use This Calculator?

Use it to identify resistors from a kit or an old circuit board.

Use it to find the colours of the resistor you need.

Use it for electronics coursework on components.

Common Mistakes

Reading from the wrong end. Start from the end away from the tolerance band.

Confusing brown, red and orange. Check under good light, or measure.

Mixing up 4-band and 5-band codes. Count the bands first.

Forgetting gold and silver multipliers. They divide by 10 and 100.