DHCP Scope / Pool Size Calculator

Works out how many addresses your DHCP pool actually has left to hand out once you subtract static reservations, and whether that's enough for the number of clients you're expecting — the calculation worth doing before a scope quietly runs dry on a Monday morning with a room full of new laptops that can't get an IP.

Why "the subnet has 254 addresses" isn't the number that matters

A /24 gives you 254 usable addresses, but the DHCP scope is never the whole subnet. The gateway takes one. Printers, access points, servers and anything else with a static assignment take more, and — critically — most of those static devices sit inside the same address range as the DHCP pool rather than off to one side, so they have to be explicitly excluded or the DHCP server will eventually try to hand out an address that's already in use. What's actually left for dynamic clients is the pool size minus every one of those reservations, and that number is often smaller than people assume until they actually count it.

The utilization threshold that actually predicts problems

Once a DHCP pool sustains utilization above roughly 80%, you start seeing intermittent lease failures during peak load — not a hard outage, which is what makes it annoying to diagnose, just a steadily growing trickle of "can't get an IP address" tickets that seem to happen at random until someone actually graphs pool usage against time of day and sees the pattern. Planning to stay under that threshold, with room for growth, avoids ever finding out about it from an angry Monday morning.

Lease time is the other lever

A shorter lease time returns addresses to the pool faster after a device disconnects, which effectively stretches a tight pool further — useful on a guest network with high device turnover. A longer lease reduces DHCP renewal chatter on the network but holds addresses for devices that may have already left, which is exactly the wrong tradeoff on a pool that's already running close to capacity. If widening the actual address range isn't an option, shortening lease time is usually the next lever to pull before resorting to renumbering.

Reservations aren't just "used up" addresses — they're also documentation

Every static reservation inside a DHCP scope should be recorded somewhere a colleague can find it, not just remembered — an undocumented static IP is exactly the kind of thing that causes a conflict six months later when someone widens the scope without realizing that address was already spoken for.