DHCP Pool Capacity Calculator
Check whether a DHCP pool has enough addresses once lease overhang and growth headroom are included.
Inputs
Addresses Required
192
Addresses In The Pool
244
Pool Utilisation
78.7%
Spare Addresses
52
Lease Overhang Multiplier
4.00×
Step by step
Values used
Subnet prefix length = 24 bits; Static and infrastructure reservations = 10 addresses; Peak concurrent clients = 40 clients; Lease time = 480 min; Average client online time = 120 min; Growth headroom = 20 %
DHCP Pool Capacity
addresses required = peak clients × (lease time ÷ average online time) × (1 + growth).
Usable pool size
pool size = 2^(32 − prefix) − 2 − reservations
Addresses Required
= 192
Addresses In The Pool
= 244
Pool Utilisation
= 78.7
Spare Addresses
= 52
Lease Overhang Multiplier
= 4.00 ×
How it works
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
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.