If your resistor only has three bands minus the tolerance one then discount the band 3 column. Let's look at an example. If I have a resistor with the colour code bands, red, green, red and gold. We have three bands minus the tolerance which means we only use columns one and two, multiplier and tolerance. I have 25 ohms x 100 ohms = 2500 ohms with a tolerance of 5%. What if I have four bands? Let's pretend I have red, brown, black, black and silver. We are going to use all columns. I have 210 ohms x 1 ohms = 210ohm with a tolerance of 10%.
Now we know how to find out how much resistance a resistor has, but how do we find out how much resistance we need for a particular circuit. We have an equation by Mr. Ohm himself.
R = V / I
R represents our final resistance value. V is our voltage drop. For example if we have 9 volts and want to drop it to 1v we take 1 from 9 and get 8. 8 will be our V figure. Now we need I. I represents the amount of amps we need. If we have an Led that only uses 40 mila-Amps then .04 is our I figure. If we slot these in to the formula we get:
R = 8 / 0.04 = 200 ohms
P represents the power in watts, I represents current in Amps and R represents our resistance value. We need .04 amps for our circuit. .04 squared is .0016. If we multiply .0016 by 200 ohms we get .32 watts. We need a resistor that can withstand .32 watts.
There are many websites on the internet that have resistance calculators and colour band decoders for you to use for free which definitely helps speed things up than the old pen and paper way.