DHCP Pool Capacity Calculator
Check whether a DHCP pool has enough addresses once lease overhang and growth headroom are included.
About DHCP Pool Capacity Calculator
A lease that lasts longer than a client's session keeps the address bound after the device leaves, so the pool has to cover the overhang as well as the live population. The ratio of lease time to session time is exactly that multiplier. This is the arithmetic behind almost every guest-Wi-Fi outage: a /24 with an eight-hour lease and two-hour sessions supports only about 60 concurrent devices, not 250.
Formulas
DHCP Pool Capacity
addresses required = peak clients × (lease time ÷ average online time) × (1 + growth).
- lease time ÷ average online time
- Overhang multiplier — a lease outliving the session keeps the address bound
- peak clients
- Devices online simultaneously at the busiest moment
- growth
- Headroom fraction for future devices
Usable pool size
pool size = 2^(32 − prefix) − 2 − reservations
- prefix
- Subnet prefix length in bits
- reservations
- Statically assigned and infrastructure addresses
Worked example
With the default values below, DHCP Pool Capacity Calculator returns Addresses Required of 192. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Subnet prefix lengthaccepts 8–30 bits
- 24 bits
- Static and infrastructure reservationsminimum 0 addresses
- 10 addresses
- Peak concurrent clientsminimum 1 clients
- 40 clients
- Lease timeminimum 1 min
- 480 min
- Average client online timeminimum 1 min
- 120 min
- Growth headroomaccepts 0–300 %
- 20 %
Results
- Addresses Required
- 192
- Addresses In The Pool
- 244
- Pool Utilisation
- 78.7%
- Spare Addresses
- 52
- Lease Overhang Multiplier
- 4.00 ×
Step by step
- 1Values used
- 2DHCP Pool Capacity
- 3Usable pool size
- 4Addresses Required= 192
- 5Addresses In The Pool= 244
- 6Pool Utilisation= 78.7
- 7Spare Addresses= 52
- 8Lease Overhang Multiplier= 4.00 ×
Assumptions and limits
- The result is derived only from the 6 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
How is DHCP Pool Capacity calculated?
addresses required = peak clients × (lease time ÷ average online time) × (1 + growth). A lease that lasts longer than a client's session keeps the address bound after the device leaves, so the pool has to cover the overhang as well as the live population. The ratio of lease time to session time is exactly that multiplier.
Why does DHCP Pool Capacity matter?
This is the arithmetic behind almost every guest-Wi-Fi outage: a /24 with an eight-hour lease and two-hour sessions supports only about 60 concurrent devices, not 250.
What values do I need to enter?
This calculator takes 6 inputs: Subnet prefix length, Static and infrastructure reservations, Peak concurrent clients, Lease time, Average client online time, Growth headroom. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.