Use the PCB Thermal Relief Width Calculator by entering any two of the following: (1) the spoke/trace width, (2) an approximate pad edge length available for the spoke attachments, and (3) the number of thermal spokes (connecting traces).

PCB Thermal Relief Calculator

Enter your pad current and copper weight to get the spoke width needed.

Spoke size for current
Thermal resistance
Enter a current greater than 0.
Enter a width greater than 0.
Enter a length greater than 0.
How this is calculated
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PCB Thermal Relief Width Formula

The following example problem outlines the steps and information needed to estimate a thermal relief spoke width using a simple pad-edge allocation heuristic.

TW = \frac{X\cdot f}{Y}

Variables:

  • TW is the thermal relief spoke width (trace width)
  • X is the approximate pad edge length available for spoke attachments (use the same length units as TW)
  • Y is the number of thermal connecting traces (spokes)
  • f is a unitless allocation factor (0–1). A common starting value is 0.60, meaning 60% of the chosen pad edge length is allocated to spoke attachment width in total.

To estimate the thermal relief spoke width with this heuristic, multiply the selected pad edge length (X) by the allocation factor (f), then divide by the number of spokes (Y). Note: thermal relief geometry is normally controlled by explicit design rules (spoke width/length and plane clearance), so treat this as a starting point and confirm it meets your PCB fab limits and soldering requirements.

How to Calculate PCB Thermal Relief Width?

The following steps outline how to estimate the thermal relief spoke width using the formula above.

  1. First, determine the approximate pad edge length available for spoke attachments (X).
  2. Next, determine the number of thermal connecting traces/spokes (Y).
  3. Next, choose an allocation factor f (0–1) and use the formula TW = (X·f)/Y.
  4. Finally, calculate the thermal relief spoke width (TW).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge (using the default f = 0.60).

pad edge length available for spoke attachments, X = 4

number of thermal connecting traces/spokes, Y = 3

TW = (X·0.60)/Y = (4·0.60)/3 = 0.8 (in the same length units as X)