The formula
How it works
Read the resistance from a resistor’s coloured bands. Pick the colour of each band and the calculator gives the resistance in ohms and the tolerance range.
FAQ
How does the colour code work?
On a common four-band resistor, the first two bands are digits, the third is a multiplier (a power of ten) and the fourth is the tolerance. So brown-black-red means 1, 0 and ×100, giving 10 × 100 = 1,000 ohms, or 1 kΩ. Each colour has a fixed meaning that is the same on every resistor.
What does tolerance mean?
Tolerance is how far the real resistance may stray from the marked value, as a percentage. A 1,000-ohm resistor with a ±5% gold band could actually measure anywhere from 950 to 1,050 ohms. Tighter tolerances (±1% or ±2%) cost more and are used where precision matters.
What about resistors with five or six bands?
A five-band resistor uses three digit bands instead of two, giving finer precision, before the multiplier and tolerance bands; six-band versions add a temperature coefficient band. This calculator covers the common four-band case; for a three-digit part, treat the first three colours as the digits and shift the remaining bands along.
Which end do I read the bands from?
Start from the band nearest one end of the resistor, reading toward the other end; the tolerance band (often gold or silver) is usually set apart with a wider gap and sits last. If a resistor has no obviously separated band, the digit bands are typically grouped closer together than the multiplier and tolerance.
Why are only certain resistance values available?
Manufacturers produce resistors in standard sets called E-series (such as E12 or E24), spaced so each value differs from its neighbours by roughly the tolerance band. That is why you see values like 220, 330 and 470 ohms rather than every round number.
What does a sixth band for temperature coefficient mean?
It shows how much the resistance shifts per degree Celsius, in parts per million, and matters mainly in precision circuits exposed to temperature swings. Everyday four- and five-band resistors omit this band because the drift is small enough to ignore in most designs.
Why might a measured resistance not exactly match the colour code?
Manufacturing tolerance means the true value can legitimately sit anywhere in the printed range, and a multimeter also has its own small measurement error on top of that. As long as the reading falls within the tolerance band shown here, the resistor is working as specified.
About the resistor calculator
This calculator decodes the colour bands printed on a resistor into its resistance value and tolerance. Resistors are too small to print numbers on, so their value is shown as a series of coloured stripes using a standard code. The calculator turns the colours you select into the resistance in ohms and the range the true value can fall within, saving you from memorising the code.
How to use it
Select the colour of each band in order: the first two digit bands, the multiplier band and the tolerance band. The calculator shows the resistance, formatted in ohms, kilohms or megohms, and the tolerance range. For example, brown-black-red-gold reads as 1 kΩ with a ±5% tolerance, meaning the true value lies between 950 and 1,050 ohms.
The formula
For a four-band resistor the value is , where and are the first two digit bands and the multiplier is a power of ten from the third band. The tolerance band then sets the range, , which the calculator shows as a minimum and maximum resistance.
Where it is used
Electronics hobbyists, students and technicians use it constantly when building or repairing circuits, since reading tiny colour bands by eye is error-prone. It helps confirm you are fitting the right resistor and understand the tolerance you are working with. Anyone learning electronics uses it as a stepping stone until the colour code becomes second nature.