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Power over Ethernet (PoE) Budget Calculator

Check a switch's PoE budget against connected devices using 802.3af, at and bt class limits, with cable loss included.

About Power over Ethernet (PoE) Budget Calculator

PoE standards define the power delivered by the sourcing equipment, and part of it is lost as heat in the cable before it reaches the device. Because most devices draw well under their class limit, the average figure sizes normal operation while the worst-case figure is what a full-load event demands. PoE budget exhaustion does not fail gracefully: the switch sheds power from the lowest-priority ports, and the cameras or access points that drop are rarely the ones you expected.

Formula

Power over Ethernet (PoE) Budget

required = devices × class limit × average draw share × (1 + cable loss); ports at full class = budget ÷ (class limit × loss factor).

class limit
802.3af 15.4 W, at 30 W, bt Type 3 60 W, Type 4 90 W at the PSE
cable loss
Power dissipated in the twisted pair, typically 10–15%

Worked example

With the default values below, Power over Ethernet (PoE) Budget Calculator returns Power Required at the Switch of 483.8. Change any input above and every figure here is recalculated in your browser.

Inputs used

Switch PoE budgetaccepts 15–20,000 W
740 W
Powered devicesaccepts 1–1,000 devices
24 devices
PoE standard per portone of 802.3af Type 1 — 15.4 W, 802.3at Type 2 (PoE+) — 30 W, 802.3bt Type 3 — 60 W, 802.3bt Type 4 — 90 W
802.3at Type 2 (PoE+) — 30 W
Average draw as a share of the class limitaccepts 5–100 %
60 %
Cable dissipation lossaccepts 0–30 %
12 %

Results

Power Required at the Switch
483.8 W
Worst-Case Power if All Ports Draw Full Class
806.4 W
PoE Budget Used
65.4%
Ports Supported at Full Class
22
Budget Status
Adequate for average draw, but oversubscribed at full class

Step by step

  1. 1Values used
  2. 2Power over Ethernet (PoE) Budget
  3. 3Power Required at the Switch= 483.8 W
  4. 4Worst-Case Power if All Ports Draw Full Class= 806.4 W
  5. 5PoE Budget Used= 65.4
  6. 6Ports Supported at Full Class= 22
  7. 7Budget Status= Adequate for average draw, but oversubscribed at full class

Assumptions and limits

  • The result is derived only from the 5 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 1 decimal place, 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 Power over Ethernet (PoE) Budget calculated?

required = devices × class limit × average draw share × (1 + cable loss); ports at full class = budget ÷ (class limit × loss factor). PoE standards define the power delivered by the sourcing equipment, and part of it is lost as heat in the cable before it reaches the device. Because most devices draw well under their class limit, the average figure sizes normal operation while the worst-case figure is what a full-load event demands.

Why does Power over Ethernet (PoE) Budget matter?

PoE budget exhaustion does not fail gracefully: the switch sheds power from the lowest-priority ports, and the cameras or access points that drop are rarely the ones you expected.

What values do I need to enter?

This calculator takes 5 inputs: Switch PoE budget, Powered devices, PoE standard per port, Average draw as a share of the class limit, Cable dissipation loss. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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