Reading resistor colour bands and checking the power with Ohm's law
A resistor's value is printed on it as coloured bands, and this article explains how to read them. It covers four, five and six-band parts, the tolerance range they allow, and the Ohm's law section that tells you whether the part will survive in your circuit.
How the colour code works
Every through-hole resistor carries its value as a set of coloured bands painted around the body. The system goes back to the 1920s, when printing tiny numbers on a cylinder was expensive and unreliable. Colours were easier to apply and easier to read from any angle.
The bands fall into groups. The first two or three are digits. After those comes a multiplier band, which tells you what power of ten to multiply the digits by. Last is the tolerance band, which says how far the actual resistance is allowed to stray from the printed value. A four-band resistor has two digits, a multiplier and a tolerance. A five-band part squeezes in a third digit for extra precision.
The calculator lets you set each band's colour from a dropdown. As you change them, it decodes the value and redraws a picture of the part so you can compare it with the one in your hand.
Which end to read from
The tolerance band sits slightly apart from the others, usually at the right-hand end. It is often gold or silver, colours that never appear as a first digit. Read from the other end, where the bands are grouped closely together.
Getting this backwards gives a wildly wrong answer. A brown-black-red-gold resistor is 1, 0, times one hundred, giving one thousand ohms. Read from the wrong end it becomes gold-red-black-brown, which is not a valid sequence at all. The calculator catches invalid digit colours and warns you, but the safest check is always to orient the part correctly before you start.
Four, five and six-band resistors
The Number of bands selector switches between the three common formats. Four bands is the most widely used for general-purpose parts. Two digits, a multiplier, and a tolerance of five per cent (gold) or ten per cent (silver) cover nearly every part in a hobby bin.
Five-band resistors are precision parts. The extra band is a third digit, not a different multiplier. That gives values like 1.00 kilo-ohms instead of the rounded 1 kilo-ohms of a four-band part. Precision resistors typically carry a brown tolerance band, meaning one per cent.
Six-band resistors add a temperature coefficient band that describes how much the value drifts as the part heats up. It matters in precision analogue circuits and almost nowhere else. There is no dropdown for that band here and the drawn strip stops at five, because the coefficient never changes the resistance. Choosing 6 bands gives you the same three-digit reading a five-band part gives, which is the correct answer for the value.
What tolerance actually means
A gold tolerance band means plus or minus five per cent. A silver band means ten per cent. No band at all means twenty per cent, though those are rare today.
The range is wider than most people assume. A one-thousand-ohm resistor with five per cent tolerance is legitimately anywhere from nine hundred and fifty to one thousand and fifty ohms and still within specification. The calculator shows this range clearly: the minimum and maximum values appear beneath the decoded number.
If your circuit needs a tighter value, buy one-per-cent parts rather than hoping a five-per-cent part lands close to the target. The cost difference is small, and measuring every resistor before you solder it is a poor use of an afternoon.
Ohm's law and solving for what you need
Below the band selectors sit three fields: Voltage across it, Current through it, and Resistance. Fill in any two and the calculator works out the rest, including the power in watts.
The resistance field defaults to the value decoded from the bands above, so you can just type the voltage and get the current and power straight away. If you already know the resistance you want and are picking a colour-coded part to match, type the resistance by hand and leave the decoded value aside.
The formula is simple. Voltage equals current times resistance. Power equals voltage times current. But working these out by hand with real circuit values, converting milliamps to amps and checking the units, is where most mistakes happen. The calculator handles the conversion and shows the result in sensible units.
Power is what actually burns a resistor
The value printed on a resistor is rarely the problem. The power running through it is. A standard through-hole resistor is rated at a quarter of a watt. Push more than that through it and it drifts in value, discolours, and eventually fails. At high enough power it can crack or catch fire.
The calculator shows the power alongside the other figures and warns you if it exceeds 0.25 watts. At that point you need a higher-wattage resistor, a different circuit design, or a heatsink. The warning takes two seconds to read and can save a board.
For an LED circuit, a common beginner project, the power through the resistor is usually well below the limit. But once you move to motor drivers, power supplies or audio amplifiers, checking the wattage becomes essential.
Picking a resistor for an LED
This is the most common practical use of the calculator. An LED needs a resistor in series to limit the current, and the right value depends on three things: the supply voltage, the LED's forward voltage, and the current you want.
Subtract the forward voltage from the supply. Divide by the current. That gives the resistance. For a five-volt supply, a red LED with a two-volt drop, and twenty milliamps of current: three volts divided by 0.02 amps gives one hundred and fifty ohms.
Type one hundred and fifty into the resistance field and three into the voltage field. Three is the number that belongs there, not five: the field asks for the voltage across the resistor, and the LED has already taken two of the five volts. The calculator returns twenty milliamps and sixty milliwatts, well inside a quarter-watt part. Put five in that box instead and you get thirty-three milliamps and a hundred and sixty-seven milliwatts, which is the answer to a different question. Set the bands to match and you have your component.
When the colours lie
Colour bands fade over time, especially on parts that have been running warm. Red and orange look nearly identical under warm workshop lighting. Brown and violet can blend when the band is narrow. A multimeter reads the actual value in seconds and settles any argument.
Treat the colour code as a quick identification, not a measurement. If you are building something that matters, measure the part. If you are sorting a bag of loose resistors into bins, the colour code gets you close enough. This calculator works as a cross-check in either case.
Surface-mount resistors use a printed number code instead of colour bands. A three-digit code like 472 means 47 followed by two zeros, which is 4700 ohms. That is a different system entirely and is not covered by this tool, which is built for through-hole parts with painted bands.
Common mistakes and how to avoid them
The most frequent error is reading the bands from the wrong end. If the first band you read is gold or silver, you have it backwards. Gold and silver are only valid as the multiplier or tolerance band, never as a digit. The calculator catches this and shows an error message rather than a wrong answer.
Another common slip is confusing the multiplier with a digit. On a four-band resistor the third band is always the multiplier, not a third digit. On a five-band resistor the third band is a digit, and the fourth is the multiplier. Selecting the wrong number of bands in the calculator leads to a value that is off by a factor of ten or more.
Finally, forgetting to check the power. A circuit that works at low voltage can overheat a resistor at higher voltage, and the same value that was fine in a test rig may burn on the real board. Always fill in the voltage field and glance at the power figure before you solder.
Everything stays on your device
The entire calculation runs in your own browser. No values are sent anywhere. There is no account, no tracking of what parts you look up, and no server involved in the arithmetic.
Your settings are remembered so you do not have to start from scratch each time. That memory lives in this browser on this device only and disappears if you clear your browsing data.