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It is very basic question for many people. It is very easy to predict the resistance of a resistor with multimeter, but sometimes it comes with resistors which are slightly larger in size, such as, diameter 5mm, length 15mm and diameter 7.5mm, length 24mm.

I get confused when buying resistors as power because there are some catagories 2W or 3W or 5W. How can I measure or is there a formula or table to learn the power [dissipation] of resistor.

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    \$\begingroup\$ Sorry about that I ve tried to fix it \$\endgroup\$ – mehmet Sep 20 '19 at 11:30
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    \$\begingroup\$ I don't know how you expect to estimate the resistance of a resistor from size alone. A 1/8 watt resistor could have a value from 1 ohm to 1Mohm - and all would be the same size. \$\endgroup\$ – JRE Sep 20 '19 at 11:36
  • \$\begingroup\$ I am using multimeter the resistance of a resistor \$\endgroup\$ – mehmet Sep 20 '19 at 11:40
  • \$\begingroup\$ Power rating and Power dissipation are two things ...2W, 3W those are power ratings not dissipation in a ckt. \$\endgroup\$ – Mitu Raj Sep 20 '19 at 12:00
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You can only predict the resistance of a resistor, when you know its physical properties (dimension, material it is made of, etc). Then you could apply equations for electrical resistivity to determine the resistance.

But, then, if you know the physical properties, you can also predict its power rating using equations for heat capacity or thermal capacity.

You can easily measure the resistance of a resistor using a multimeter.
To determine the power rating, manufacturers test at which power dissipation the resistor becomes too hot. They measure the temperature and power dissipation using a "multimeter" (probably a more sophisticated)
$$ P_{dissipated} = \frac{ V_{\text{across resistor}}^2 }{ R } $$ or $$ P_{dissipated} = I_{\text{through resistor}}^2 \cdot R $$

and note the power dissipation.
Next, they likely go for safe and rate the resistor at a lower power (e.g. 95% of the measured power dissipation), but you don't know the choosen margin.
So, there is no way the measure the power rating.

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The power rating of a resistor is given by the manufacturer. Generally a larger resistor will be able to dissipate more power, but there is no such rule that a certain length equals a certain power. That is why the power rating is clearly shown in the datasheets or when you order from a distributor.

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    \$\begingroup\$ the safe power dissipation depends on cooling. That means the PCB layout and the solder-pads on the PCB matter. And if cooling air is used to hold down the PCB temperature rise, or if mounting bolts are nearby the resistor, to remove heat through the PCB bolts from the ground and power planes. If a leaded resistor, the lead-length is important, as are the issues I previously wrote about. Some resistors have large mechanical holddown lugs, ensuring a close contact between the insulated-case and the external chassis, to better remove heat from the ceramic-insulated resistive internal element. \$\endgroup\$ – analogsystemsrf Sep 20 '19 at 12:08
  • \$\begingroup\$ Yes. Agreed. There is so much that depends on the actual circuit. As with all electronic components, resistors have a thermal resistance to either mounting or ambient that lets you calculate how much they will heat up. \$\endgroup\$ – AndersG Sep 20 '19 at 12:17
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The power dissipation is not a property of the resistor, but of the circuit it's in. Power is \$P=I^2R\$; it's a function of the current (or equivalently the voltage) the resistor sees in the circuit. You can know what the power dissipation will be by solving the circuit to find the current through (or voltage across) the resistor.

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