Plan a DHCP address pool scope and calculate available leases within a subnet.
A DHCP scope's usable address count is the total addresses in the start-to-end range minus any addresses reserved for static assignments or explicitly excluded from the pool: usable = (end_ip − start_ip + 1) − reserved. Comparing currently active leases against that usable count gives the lease utilization percentage, a key capacity-planning metric — pools running consistently above 80% utilization risk running out of addresses during peak demand (e.g. a Monday morning influx of devices), which causes new devices to fail to obtain an IP address entirely.
Usable DHCP addresses
usable = (end_ip − start_ip + 1) − reserved
Lease utilization
utilization% = active_leases / usable_addresses × 100
Reserved addresses (static IP reservations for servers/printers, or explicit exclusions like a gateway or existing static assignment within the range) are set aside so DHCP will never hand them out dynamically, which means they don't count toward the pool available for dynamic lease assignment.
Many administrators treat 80% sustained utilization as a warning threshold to start planning a scope expansion or lease-time reduction, and 95%+ as urgent, since pools that run out of addresses cause new or renewing devices to fail to get network connectivity entirely.
Shorter lease times free up addresses from disconnected/idle devices faster, effectively increasing a pool's turnover rate and reducing the addresses tied up by stale leases — a busy pool nearing exhaustion can sometimes be relieved by shortening lease duration rather than expanding the range.
Each subnet's DHCP scope should be calculated independently using its own start/end range and reservation count — this calculator models a single scope; a multi-subnet environment needs one calculation per scope to get an accurate picture of overall capacity.