Free tool
Rack space & heat load calculator
Check a rack before you order it. Enter the height, the units you have used, and the connected load, and get the free U left, the heat in watts and BTU/hr, the current the rack pulls, and the airflow it needs to stay cool. Free, and nothing leaves your browser.
Free rack units
10U of 24U
Heat load
720 W
2,457 BTU/hr
Current draw
3.1 A
Required airflow
223 m³/h
131 CFM
Airflow assumes sea level air and an open path in at the bottom and out at the top. Sealed or glass-door racks need active cooling regardless of the number. Estimate only.
Two questions every rack has to answer
Does it fit, and can it lose the heat. Space is the one people check, usually by counting rack units on the back of a drawing. Heat is the one that gets found later, when the amplifiers in a cupboard start dropping into protect on a warm afternoon and nobody can say why.
How it works out
- Heat in watts = connected load × duty factor, because electrical input becomes heat.
- BTU/hr = watts × 3.412, for specifying against US cooling equipment.
- Current = the same real draw divided by the supply voltage.
- Airflow in m³/h = 3.1 × watts / allowed temperature rise in kelvin, with CFM alongside.
The duty factor is what makes the answer usable. Add up nameplate ratings and you will spec cooling for a rack that never exists. Laying out the real thing? Avitly builds rack elevations in U with collision detection, so the used units you type here come straight off the drawing.
Common questions
How much heat does a rack actually produce?
Almost exactly as much as it consumes. AV and IT gear does no mechanical work, so every watt drawn from the wall leaves the enclosure as heat, minus the small fraction that goes out as sound or light. That is why this calculator does not ask for an efficiency figure: real power in equals heat out, near enough for sizing a fan tray. The only number you need to get right is the real draw, not the nameplate total.
What duty factor should I use?
Nameplate ratings are worst case: an amplifier rated 1200 W is quoting a sine wave at full output into the lowest rated impedance, which no programme material ever does. For background music amplifiers, 20 to 30 percent is realistic. Mixed AV racks with switchers, processors, and a couple of amplifiers sit around 50 to 60 percent, which is why 60 is the default here. Switches, servers, and anything with spinning storage run closer to 80 percent because they draw much the same power all day. If in doubt, measure with a clamp meter on a busy day rather than guessing.
How many rack units should I leave free?
Plan for around 20 percent spare at handover, and never finish a rack completely full. You want a free U above every amplifier and power supply so heat is not trapped against the chassis of the box above, and you want somewhere to put the extra encoder or switch that gets added six months later. Vented blanking panels in the gaps are better than open holes: open holes let cool air short circuit straight back out without passing over anything.
How do I read the airflow figure?
It is the volume of air per hour that has to pass through the rack to carry the heat away while staying within the temperature rise you allowed. The formula is airflow = 3.1 × watts / temperature rise in kelvin, which comes from the density and specific heat capacity of air at sea level. A 10 K rise is a sensible default: it means a rack in a 22 °C room runs at about 32 °C at the top, which is comfortable for everything except tape drives. Allowing a 20 K rise halves the airflow you need but pushes the top of the rack past 40 °C.
When do I need active cooling instead of vents?
Roughly above 1500 W in a single enclosure, passive convection stops keeping up in a normal room. Below that, a rack with a vented or mesh door, open top vents, and a clear path underneath will usually manage on its own. Above it, fit a thermostatic fan tray at the top pulling air out, or duct the rack into the room cooling. Sealed racks, glass doors, and racks in cupboards change the answer completely: those need forced air whatever the number says, because the air in the enclosure has nowhere to go.
Why is the current draw useful?
It tells you how many circuits the rack needs before you get to site. A 16 A circuit at 230 V gives you about 3680 W, and you should not plan past roughly 80 percent of that for continuous load. In the US at 120 V, a 20 A circuit gives 2400 W, so the same rack needs more than twice as many circuits. Work it out at design stage, because adding a second feed to a finished rack means a return visit and an electrician.
Design it once. Sell it, install it, service it.
Avitly runs the whole install off one floor plan: quote, client-signed offer, bill of materials, rack layout, then device credentials and tickets after handover. Presales to maintenance, no re-entry.
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