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See your racks: what they draw, what's plugged where, what fails.
Free tools and 3D twins built from the spreadsheet you already keep. One for the rack in your basement, one for the data hall.
For the homelab
Homelab Twin
Your homelab rack, in 3D. Watts and cost per device, what's plugged where, and what goes dark when one thing fails. Try it free when we open the doors: no account, nothing stored.
Explore Homelab Twin →See it · walkthroughFor the data center
DC Sim
Turn one Excel file into a 3D twin of your data center. Fail any UPS, feed, switch or cooler and see what goes dark. Built to sit on top of the monitoring you run.
Explore DC Sim →Free tool · runs in your browser, no data leaves it
Rack Density Calculator
How many devices fit in your rack before space, power or cooling runs out, and which one runs out first? Works for a 10-inch mini rack in a closet or a 42U rack in a data hall.
Start from
Homelab values are typical starting points, not measurements. Change any field.
Cost = draw (kW) × 730 hours × your price. An estimate: real draw depends on load. Green needs ≥ 20 % headroom on power and cooling.
How rack density is calculated
by_space = floor(rack_U / device_U)
by_power = floor(circuit_kW × 1000 / W_per_device)
by_cooling = floor(cooling_kW × 1000 / W_per_device)
devices = min(by_space, by_power, by_cooling)
circuit_kW = V × A × PF / 1000 (single-phase)
circuit_kW = √3 × V × A × PF / 1000 (three-phase)Real rack density is the floor of three independent ceilings. Whichever is lowest is the bottleneck: usually power or heat in a homelab, cooling in a modern data center, space only in very small racks.
More calculators
- Homelab power cost: what your whole lab costs per month, device by device
- Tokens per watt: the efficiency of your home AI box
- 10-inch rack planner: will it fit, and how many U
- Idle savings and payback
- Rack power calculator: load per PDU in normal and failover against the 80 % rule
Questions
What is rack density?
Rack density is how much compute, power, or heat you pack into one rack, usually expressed as kW per rack or servers per rack. Higher density means more compute per square foot but tighter cooling and power constraints.
How many servers fit in a 42U rack?
With 1U servers, a 42U rack physically fits 42 servers, but you almost never reach that limit. Power and cooling usually cap real-world rack density at 20–35 servers depending on per-server wattage and circuit size.
What is a safe kW per rack?
Most enterprise racks operate between 5 and 15 kW. Anything above 15 kW is considered high-density and typically needs in-row cooling, rear-door heat exchangers, or liquid cooling.
How do I calculate three-phase power in kW?
Three-phase real power = √3 × line voltage × line current × power factor ÷ 1000. For an apparent-power rating in kVA: kW = kVA × power factor.
Why does cooling run out before space?
Modern 1U servers can draw 400–800 W each. A 42U rack of 1U servers would need 17–34 kW of cooling, more than most facility loops deliver per rack.
Single-phase vs three-phase for racks?
Single-phase is fine up to about 5 kW per rack. Above that, three-phase delivers more power through the same conductors and balances load; most racks above 7 kW use three-phase PDUs.
How much headroom should I leave?
Plan for 20–30 % headroom on power and cooling to survive a single PDU or cooling-unit failure (N+1). This calculator shows OK only with at least 20 % headroom on power and cooling.
How many servers can I put in a home rack?
Usually power or heat runs out before space does. A closet with no extra cooling copes with a few hundred watts. A US 15 A circuit is good for about 1.4 kW continuous (the 80 % rule); a European 16 A circuit, about 2.9 kW. Enter your rack height, device size and wattage to see which one limits you.
How much does a homelab rack cost to run per month?
Monthly cost = average draw in kW × 730 hours × your price per kWh. At 200 W and €0.30/kWh that is about €44 a month; at 200 W and $0.17/kWh, about $25.
What size rack do I need for a homelab?
Count the U your gear needs, then add about 30 % for patch panels, cable management, a UPS and growth. A 10-inch mini rack (6–12U) fits mini PCs and a small switch; a 12U wall rack fits a switch, patch panel and a couple of servers; 22U and up suits rack-mount servers.
Does a homelab rack need cooling?
Every watt your gear draws ends up as heat in the room: 1 W ≈ 3.4 BTU/h. Up to a few hundred watts, a ventilated closet or an open room is usually enough; above that, plan an exhaust fan or air conditioning, and watch the inlet temperature.