AIComputeCalc

Worked example · September 7, 2026

GPU racks: power, cooling, or space—which runs out first?

A rack with empty slots can still be full from a power perspective. Consider eight DGX H100 systems, each modeled at 10.2 kW and 8U, with 42U racks capped at 30 kW of IT power. Space alone suggests two racks. The power limit requires four racks.

Apply both rack limits

  1. Space permits floor(42 ÷ 8) = 5 systems per rack.
  2. Power permits floor(30 ÷ 10.2) = 2 systems per rack.
  3. Use the smaller limit: 2 systems per rack.
  4. Eight systems ÷ two per rack = 4 racks, each drawing up to 20.4 kW in this model.

The unused 9.6 kW in each rack cannot accommodate another 10.2 kW system. Nor can spare power in another rack make that particular rack's limit disappear. This example excludes rack space for switches, patching, cable management, and service clearances. Reduce usable rack units or budget these separately.

Convert electrical load into planning heat rejection

The calculator assumes IT electrical input becomes heat: BTU/h = watts × 3.412142. Divide by 12,000 BTU/h per refrigeration ton to express that heat as cooling tons. For the eight systems, 81.6 kW of IT load gives approximately 278,431 BTU/h, or 23.20 tons.

This is a heat-equivalent planning estimate, not a recommendation to buy a particular chiller. A cooling ton measures heat removal rate, not the cooler's electrical input. Ambient conditions, water temperatures, airflow, liquid-cooling compatibility, redundancy, distribution losses, and manufacturer thermal specifications still govern the installed solution.

Keep facility power separate

At PUE 1.2, the facility planning load is 81.6 × 1.2 = 97.92 kW. With 15% capacity reserve, the required facility capacity is 97.92 ÷ 0.85 = 115.2 kW. The IT cooling estimate above excludes facility overhead; multiplying it by PUE and treating the result as server-room cooling would mix boundaries.

Reverse the question: what fits in a 100 kW allocation?

Assume a 100 kW facility allocation, PUE 1.2, 15% power reserve, four 42U racks at 30 kW each, and 25 cooling tons available. Usable IT power is 100 × 0.85 ÷ 1.2 = 70.83 kW. At 10.2 kW per system, power permits six systems. Cooling permits eight, and rack power/space permits eight. The tightest bound is six systems, or 48 GPUs.

Try the reverse-capacity example

Before applying a result

Check that all inputs describe the same boundary and operating condition. Nameplate, maximum, design, and average power are not interchangeable. Do not sum redundant power-supply ratings to infer normal system demand. The calculator applies reserve to power capacity, not automatically to cooling or rack space; supply already-derated limits if those need headroom too.

The system size and maximum-power reference is NVIDIA's DGX H100/H200 user guide, checked September 7, 2026. These arithmetic conversions do not replace vendor thermal data. See the methodology and electricity-cost example for related assumptions.