Free tool
PoE switch budget calculator: what size switch do I need?
Check a switch before you order it. Enter how many devices you have in each PoE class, add the switch budget, and see the total power the switch must reserve, what is left, and whether the headroom is comfortable. Free, and nothing leaves your browser.
PSE power required
243.2 W
Budget remaining
126.8 W
Headroom
34 %
Ports used
12
Total draw from the wall
336 W
1148 BTU/hr of rack heat
Base power is the chassis on its own with nothing powered: fans, ASIC, and the ports themselves. A 48 port switch idles far higher than an 8 port one even when both are loaded identically, so it belongs in the sum. Typical figures: 8 to 12 port desktop switch 15 to 30 W, 24 port 40 to 70 W, 48 port 80 to 150 W. The wall figure is what sizes the UPS, the circuit, and the heat you have to get out of the rack.
Comfortable. More than 20% spare, so a few extra devices will still come up.
Cable loss: roughly 15% of the PSE power is burned in the copper over a full 100 m of Cat5e or Cat6, so at 50 m expect something in the region of 8%. That loss is already inside the per-port PSE figures above, which is why a 30 W PoE+ port only guarantees 25.5 W at the device.
Planning figures based on the class maximums each standard reserves. Real devices often draw less, but a switch allocates by class, not by measured draw. Confirm against the switch datasheet before ordering.
Port count is not the limit, wattage is
A switch with 24 PoE ports will not power 24 PoE+ cameras. Its power supply has one shared budget, and every device takes a slice of it sized by the class it negotiates, not by the power it happens to draw. Add up the classes first and you find out in the design phase, rather than when half the ceiling stays dark on commissioning day.
How the sum works
- Each device reserves its class maximum at the switch: 15.4, 30, 60, or 90 W.
- Total those reservations, subtract from the switch budget, and what remains is your headroom.
- Keep at least 20 percent spare: a switch at 100 percent of its budget will drop ports.
- Roughly 15 percent of the port power is lost in 100 m of copper, less on shorter runs.
If the numbers here look tight, the fix is usually a second switch rather than a bigger one, so a single failure takes down half the devices instead of all of them. Worth reading alongside Power over Ethernet if you want the background on how the classes and standards fit together.
Common questions
What is a PoE power budget?
It is the total wattage the switch power supply can hand out across all of its PoE ports at once, and it is almost always lower than the per-port maximum multiplied by the port count. A 24 port switch that advertises PoE+ on every port rarely carries 24 × 30 W, it carries something like 370 W. The budget, not the port count, is what decides how many devices you can actually power.
Why does the calculator use the higher PSE number instead of what the device draws?
Because that is how the switch allocates. When a powered device negotiates a class, the switch reserves the full class wattage out of its budget whether the device draws it or not: 15.4 W for Class 0/3, 30 W for Class 4, 60 W for Class 6, 90 W for Class 8. A camera that idles at 6 W still takes 15.4 W off the budget if it presents as Class 3. LLDP negotiation can trim that reservation on switches that support it, but planning to the class maximum is the version that does not fail on install day.
Why is the wattage at the device lower than the wattage at the port?
The gap pays for the cable. Each standard defines a PSE figure at the switch and a lower PD figure guaranteed at the far end: 15.4 W becomes 12.95 W, 30 W becomes 25.5 W, 60 W becomes 51 W, 90 W becomes 71.3 W. The difference, around 15 percent, is the worst case loss in 100 m of Cat5e or Cat6. On a 20 m patch you lose far less, but you cannot claim that margin back because the device negotiates against the standard, not against your run length.
What happens when a switch runs out of PoE budget?
It stops powering ports, and the order is rarely the one you would pick. Most switches shed either by port priority if you configured it, or by highest port number if you did not, so the last device installed is often the first to go dark. Worse, a switch hovering at the limit can oscillate: a port powers up, pushes the total over budget, gets denied, and cycles. Leave real headroom rather than trusting the shed order.
How much headroom should I leave on a PoE switch?
Twenty percent is the working figure. It absorbs the device that turns out to be a class higher than the datasheet suggested, the pan-tilt-zoom camera that spikes when its heater runs in winter, and the two extra access points the client adds six months later. Below twenty percent you are one change order away from a second switch, and by then the rack is already built.
Should I use a bigger switch or PoE injectors?
A single high wattage device, one 802.3bt display or a heated outdoor camera, is usually cheaper to feed from a midspan injector than to buy a whole switch class upwards for. Once you have three or more, the injectors cost more in power outlets, rack space, and points of failure than the bigger switch does. Two mid-sized switches also beat one large one when you care about what stays up during a failure.
This is built into Avitly. Same maths, running on your real project instead of numbers you retype.
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