The design of a thick film resistor means the selection of the paste with the suitable sheet resistance, and the determination of the geometry, i.e. the width and the length of a rectangular resistor. If necessary, resistors with other shape, like top-hat and pseudo-meander, can be also used.

In determining the geometry of a thick film resistor, the required power dissipation (P) and the number of squares (l/w) should be considered. In general, specific power dissipation (p) with the recommended 50 mW/mm² and a maximum value of 200 mW/mm² is used. Overheating can cause permanent change in resistance, can overheat the surrounding areas of the substrate and damage neighboring components, and can damage plastic overcoats or encapsulations.

The dissipating area (A) - which is simply determined by the product of the length and the width of the resistor in case of rectangular shape - is calculated from the power of the resistor by the following formula:

     A = l*w = P/p .

Since the aspect ratio of the resistor is determined by the R/r ratio (l/w = R/r), from these two formulas the length and the width of the resistor can be calculated:

     l = sqrt(P*R/P*r) and

     w = sqrt(P*r/P*R)

For avoiding extremely small resistors, a minimum area of 0.5 * 0.5 mm² is used. Furthermore the actual dimensions are limited by possible resistor line resolution of about 0.25 mm for low power resistors, and an acceptable maximum size of about 10 - 15 mm for high power resistors. If a resistor is made shorter than 1 mm, the diffusion and interface reaction between the conductor and the resistor play increasingly important role, and the control of the thickness of the resistor becomes more difficult.