Component Sizes: Area and Diameter

Why component size matters

An electronic system needs physical space as well as the correct electrical components. A printed circuit board (PCB), for example, must fit inside its enclosure and provide enough room for the components placed on it. Its dimensions also affect how much board material is needed to make a batch.

Area measures the space inside a flat shape's boundary. Diameter measures the distance across a circular shape through its centre. These measurements help you select suitable sizes and calculate quantities, but they answer different questions: area tells you how much surface is involved, while diameter tells you how wide a circular part is.

Calculating the area of a board

For a rectangular board, multiply its length by its width:

$A = l \times w$

Suppose a PCB is 60 mm long and 40 mm wide. Its area is $60 \times 40 = 2400\text{ mm}^2$. This represents the surface within the board's outline, not the total surface area of all its faces.

For 12 identical boards, multiply the area of one board by the number required:

$A_{\text{total}} = N \times A_{\text{one}}$

The boards therefore occupy $12 \times 2400 = 28\,800\text{ mm}^2$ in total. This is the area of the finished boards; the stock needed may be greater because cutting and unused spaces consume material.

A smaller board can use less material and fit a smaller enclosure. However, reducing its size is only useful if the required components and connections still fit. Size selection balances material efficiency against the system's needs.

Using diameter to find circular area

The radius is the distance from a circle's centre to its edge. A diameter contains two radii, so:

$r = \frac{d}{2}$

The area of a circular board or part is:

$A = \pi r^2 = \frac{\pi d^2}{4}$

Suppose a circular board has a diameter of 20 mm. Its radius is 10 mm, giving an area of $\pi \times 10^2 \approx 314.16\text{ mm}^2$.

Diameter also helps you check whether a circular component will fit an opening. Compare the component's specified diameter with the available space rather than comparing their areas. Any required clearance must also be considered.

Keeping length and area units consistent

Length is measured in units such as millimetres or centimetres; area is measured in squared units. A square measuring 1 cm on each side also measures 10 mm on each side. Its area is therefore $10 \times 10 = 100\text{ mm}^2$:

$1\text{ cm}^2 = 100\text{ mm}^2$

Similarly, $1\text{ m}^2 = 1\,000\,000\text{ mm}^2$. The conversion factor is squared because both dimensions change. It is usually simplest to convert the lengths first, then calculate the area.

Checking how many parts fit

Dividing the stock area by the area of one part gives an initial quantity estimate, but the shapes must also fit within the stock's length and width.

Suppose a rectangular sheet measures 120 mm by 80 mm, and each board measures 60 mm by 40 mm. Two boards fit along the 120 mm edge and two along the 80 mm edge, making four boards in a rectangular arrangement. This assumes no cutting gap or edge margin.

For a real layout, allow for any specified gaps and margins before counting complete parts. Circular parts also leave spaces between their curved edges, so dividing the sheet area by the area of one circle cannot by itself establish the usable quantity.

Area and diameter

• Rectangle: $A = l \times w$.
• Diameter and radius: $$, so .