PoE Switch Sizing Calculator

Work out poe switch sizing instantly with clear inputs, formula shown and shareable results.

About PoE Switch Sizing Calculator

PoE budgets are consumed by class allocation rather than measured draw on many switches, so 36 Type 3 devices can reserve 2,160 W even if they actually use 60 percent of that. Where LLDP power negotiation is supported, allocation tracks real demand more closely — but a switch that reserves by class will refuse power at boot once the budget is committed.

Formula

PoE sizing

worst case = devices x class maximum; expected = worst case x average draw share; devices supported = floor(budget / class maximum)

Worked example

With the default values below, PoE Switch Sizing Calculator returns Expected PoE load of 1,296. Change any input above and every figure here is recalculated in your browser.

Inputs used

Powered devicesminimum 1: 36
PoE classone of 802.3af Type 1 (15.4 W port), 802.3at Type 2 / PoE+ (30 W port), 802.3bt Type 3 (60 W port), 802.3bt Type 4 (90 W port): 802.3bt Type 3 (60 W port)
Switch PoE budgetminimum 1 W: 740 W
Average draw as a share of class maximumaccepts 1–100 %: 60 %

Results

Expected PoE load: 1,296 W
Worst-case load at class maximum: 2,160 W
Devices supported at class maximum: 12
Assessment: Worst-case demand exceeds the switch budget - devices may be denied power at boot

Step by step

1Values used
2PoE sizing
3Expected PoE load= 1,296 W
4Worst-case load at class maximum= 2,160 W
5Devices supported at class maximum= 12
6Assessment= Worst-case demand exceeds the switch budget - devices may be denied power at boot

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, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to whole numbers, 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

Why does the switch reserve the full class power?

Because it must guarantee power if the device demands it. LLDP-based negotiation allows a finer allocation, but not all powered devices support it.

Do cable losses matter?

Yes. The standard accounts for up to 15 to 20 percent loss in a 100 m run, which is why port power at the switch exceeds the power available at the device.