A resistor is the cheapest component on your workbench. And yet a surprising number of things go wrong the moment you stop thinking carefully about it.
Not because the calculation is difficult. But because choosing a resistor means making three decisions at once: the resistance value, the power it can dissipate, and how precise it needs to be. All three are connected, and overlooking one can still cause problems.
Resistance value: start with the function
The value is given in Ohms, and it determines how much current flows through the resistor at a given voltage. In practice, resistors serve one of four purposes: limiting current (for an LED), holding an input in a defined state (pull-up or pull-down), creating a voltage divider, or driving a transistor or MOSFET.
So don't start with a value — start with the question: what does this resistor need to do?
A classic example: you want to connect a red LED to 5 V. The LED has a forward voltage of around 2 V and you want 10 mA of current. The resistor needs to absorb 3 V at 10 mA — that's 300 Ω. In practice you pick the nearest standard value, like 270 Ω or 330 Ω. A slightly higher resistance means slightly less current and a slightly dimmer LED — no problem.
For a pull-up on a GPIO pin, it's not about precise current but about keeping the input in a known state. Values between 4.7 kΩ and 47 kΩ are common. A higher value draws less current; a lower value is more robust against noise and better suited for faster signals. The CRG0805F10K (10 kΩ, 1%, 0805) is a solid standard choice for pull-ups on breadboard or PCB.
Power rating: the detail everyone forgets
Every resistor has a maximum power it can dissipate as heat. The most common ratings are 0.125 W, 0.25 W, 0.5 W and higher.
The calculation: power equals voltage squared divided by resistance. For a 330 Ω resistor with 3 V across it, that's 3² / 330 = 27 mW. Well within 0.125 W — no problem at all.
But suppose you build a voltage divider from 12 V to 5 V using two 470 Ω resistors. Nearly 13 mA flows through each, dissipating around 75 mW. That technically fits within a 0.125 W resistor, but it's close to the limit. Factor in ambient temperature and poor ventilation and you're pushing it.
Rule of thumb: always choose a resistor with a power rating at least twice what you calculate. For LED driving, the CRCW0805330RFKEA (330 Ω, 1%, 0.125 W) works well. For higher power, look at the CRGH0805F10K (10 kΩ, 1%, 0.33 W).
Tolerance: when does it matter?
A 1 kΩ resistor with 5% tolerance may actually be anywhere between 950 Ω and 1050 Ω. For driving an LED or a pull-up, that's irrelevant. For a precision measurement bridge, an RC filter, or a voltage divider that provides a reference, it can matter.
In those cases, choose 1% tolerance. Metal film resistors typically offer 1% tolerance and behave more consistently across temperature than carbon film types.
For the vast majority of projects, 5% resistors are fine. Use 1% when accuracy is needed, not by default.
THT or SMD
THT resistors (through-hole, with leads) are convenient for breadboards, repairs and hand-soldered prototypes. SMD resistors are smaller and intended for PCBs.
With SMD, pay attention to the package size. A 0402 can handle around 62 mW, a 0603 slightly more, an 0805 up to 125 mW for standard types or 330 mW for higher-rated versions. If you choose an SMD resistor for an application with some power dissipation, check the datasheet — package size alone is not enough.
Where things go wrong in practice
The most common mistakes: no resistor with an LED, not calculating power and ending up with a resistor that gets hot or fails, and choosing SMD resistors that are too small for the current.
A resistor costs a few cents. The component it was supposed to protect costs considerably more. Take a moment to do the calculation.