DHCP Pool Size Calculator

Work out dhcp pool size instantly with clear inputs, formula shown and shareable results.

About DHCP Pool Size Calculator

A DHCP scope cannot exceed the usable addresses in its subnet minus the gateway, servers, printers and other statically assigned interfaces. Sizing then compares the remaining pool against the client count grown by a headroom factor, because guest devices, phones and short leases all inflate concurrent demand above the nominal seat count.

Formula

Pool sizing

usable = 2^(32-prefix) - 2; pool = usable - statics; needed = clients x (1 + growth); utilisation = needed / pool

Worked example

With the default values below, DHCP Pool Size Calculator returns Addresses available for the DHCP pool of 242. Change any input above and every figure here is recalculated in your browser.

Inputs used

Subnet prefix (/n)accepts 8–30 bits: 24 bits
Static / infrastructure addressesminimum 1: 12
Growth headroomaccepts 1–200 %: 20 %
Expected DHCP clientsminimum 1: 150

Results

Addresses available for the DHCP pool: 242
Addresses needed including growth: 180
Spare addresses: 62
Projected pool utilisation: 74.38%

Step by step

1Values used
2Pool sizing
3Addresses available for the DHCP pool= 242
4Addresses needed including growth= 180
5Spare addresses= 62
6Projected pool utilisation= 74.38

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What utilisation is safe?

Aim below about 80 percent. Above that, a burst of new devices or a stale lease backlog starts producing DHCP exhaustion and clients falling back to link-local addresses.

Do reservations consume pool addresses?

A reservation inside the pool range still occupies an address. Keeping statics in a block outside the pool makes the arithmetic and the audit trail cleaner.