Undersizing switch capacity means a second order within a year. Oversizing means paying for ports that sit empty for years. Here's a straightforward way to land in the right range.
Start with devices, not headcount
Headcount alone undercounts ports. For each person or workspace, count every device that needs a wired drop: desktop or laptop dock, desk phone, and any shared equipment nearby (printers, conference room gear). A single desk with a phone and a dock is already two ports, not one.
Add infrastructure ports
Beyond end-user devices, count wired access points, security cameras, badge readers, and any other fixed infrastructure that needs a port. These are easy to forget in a headcount-based estimate because they're not tied to a person.
Apply a growth buffer
A common rule of thumb is to size for 20-30% headroom beyond your current device count, so a reasonable amount of headcount or device growth doesn't require a switch upgrade within the first year or two. Go higher if you expect fast growth; lower if the space has a hard capacity limit (like a fixed number of desks).
Think in switch units, not just total ports
Port count tells you the minimum, but how you split it across switches matters too. A single 48-port switch serving an entire floor creates a single point of failure for everyone on it. Splitting across multiple smaller switches (by floor zone or department) limits the blast radius if one switch fails, at the cost of a bit more complexity.
PoE budget matters as much as port count
If phones, access points, or cameras are powered over Ethernet, check the switch's total PoE power budget, not just its port count — a fully populated PoE+ switch can hit its power ceiling before it hits its port ceiling, especially with higher-draw devices like PTZ cameras or Wi-Fi 6E access points.
A simple worked example
A 40-person office with desk phones and docking stations, 6 wired access points, and 4 security cameras needs roughly 80 end-user ports (40 desks x 2) plus 10 infrastructure ports — call it 90 ports before any growth buffer. With a 25% buffer, that's about 112 ports, which lands comfortably on two or three 48-port switches rather than one, giving you both headroom and some failure isolation.