Resistors are marked with coloured bands rather than printed values. Pick the four colours on your resistor to get its resistance in Ω, kΩ or MΩ, plus its tolerance range.
Hold the resistor with the gold or silver band on the right.
Select the four colours from left to right.
Read the resistance and tolerance.
Resistance: R = (10 × digit₁ + digit₂) × multiplier
Most through-hole resistors are too small to print a value on, so they carry coloured bands instead. On a standard four-band resistor, the first two bands are digits, the third is a power-of-ten multiplier and the fourth is the tolerance. This calculator decodes the four colours you select into the resistance in ohms, kilohms or megohms, the tolerance, and the minimum and maximum values the resistor could actually have.
Electronics hobbyists sorting a mixed bag of resistors, engineering and polytechnic students in lab sessions, and repair technicians replacing a burnt component all need to read these bands quickly. It is also a skill tested in Class 12 physics and in basic electronics practicals.
1. Hold the resistor so the tolerance band, usually gold or silver and often set slightly apart, is on the right.
2. Read band 1 and band 2 as digits: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9.
3. Form the two-digit number: 10 × first digit + second digit.
4. Multiply by band 3: black ×1, brown ×10, red ×100, orange ×1,000, yellow ×10,000, and so on; gold ×0.1 and silver ×0.01.
5. Read band 4 for tolerance: brown ±1%, red ±2%, gold ±5%, silver ±10%, among others.
6. Find the range: value × (1 − tolerance) to value × (1 + tolerance).
Resistors are made in preferred values from the IEC E-series, not in every possible number. The E12 series, used for ±10% parts, has twelve values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82. E24 adds values in between for ±5% parts, and E96 has 96 values for ±1% parts. The steps are roughly equal ratios, so that the tolerance bands of neighbouring values just touch or overlap. That is why you see 4.7 kΩ and 2.2 kΩ everywhere but never 5 kΩ in a basic kit.
A 4.7 kΩ resistor with ±1% tolerance is guaranteed by the manufacturer to measure somewhere between 4.653 kΩ and 4.747 kΩ at room temperature. Its actual value is fixed but unknown within that band. For LED current limiting or pull-up resistors, ±5% is plenty. For voltage dividers setting precise reference voltages, filters and measurement circuits, ±1% or better matters. Resistance also drifts with temperature, which some six-band resistors specify with an extra temperature-coefficient band.
Precision resistors often have five bands: three digit bands, a multiplier and a tolerance. This calculator covers the common four-band type. Reading direction matters because brown and red can appear both as digits and as tolerance colours. Clues for finding the right end are that gold and silver are never first-digit colours, the tolerance band is often separated by a wider gap, and the first band is usually closer to one end. When in doubt, check with a multimeter; the reading should fall within the calculated range.
Lakshmi, sorting components for her polytechnic electronics lab, picks up a resistor with bands yellow, violet, red and brown and wants to know its value and the range it is guaranteed to fall within.
Digits from bands 1 and 2: Yellow = 4, Violet = 7 → 47
Multiply by band 3: 47 × 100 = 4.7 kΩ
Tolerance from band 4: ±1% → 4.653 kΩ to 4.747 kΩ
Answer: Resistance 4.7 kΩ; Range 4.653 kΩ to 4.747 kΩ
Reading the resistor from the wrong end, especially when brown or red appears at both ends.
Treating the third band as a third digit on a four-band resistor, instead of as the multiplier.
Confusing similar colours under poor light: red and orange, brown and red, or violet and blue.
Forgetting that gold and silver in the third position mean ×0.1 and ×0.01, giving values below 10 Ω.
Applying the four-band rule to a five-band precision resistor.
Identifying resistors in mixed kits and salvaged boards.
Checking that the correct value has been placed before soldering.
Electronics and physics practicals in schools, ITIs and polytechnics.
Repairing appliances where a burnt resistor must be matched.
Choosing the tolerance class needed for a precision circuit.
A popular mnemonic?
'BB ROY of Great Britain had a Very Good Wife' for Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White.
What about 5-band resistors?
They have three digit bands before the multiplier; this calculator covers the common 4-band type.