Electronics convert essentially all input power to heat: BTU/hr = watts × 3.412. Sum the realistic draw (not nameplate maximums — use UPS output readings where available), add lighting and the room's small envelope load, and that's the requirement. A closet with two 3 kW racks: ~6 kW × 3.412 ≈ 20,500 BTU/hr ≈ 1.7 tons.
This load runs 24/7/365 — comfort ACs cycling continuously die young. Practice favors N+1 redundancy (two units, each able to carry the load, alternating), dedicated mini-splits rated for low ambient operation (winter cooling is still needed), and condensate management. Humidity control matters less than in comfort cooling but static-dry air below ~20% RH is its own risk.
The commercial calculator accepts an equipment count for offices; for dense IT rooms, work from measured watts with the formula above and round up to the next standard size, then duplicate it for redundancy.
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About 0.285 tons per kW (1 kW = 3,412 BTU/hr ÷ 12,000). A 10 kW room needs ~2.85 tons of continuous cooling, before redundancy.
Small rooms often do, but pick units rated for continuous duty and low-ambient operation, plan N+1, and never rely on a single unit for equipment you care about.
Put the numbers to work: AC tonnage calculator · bill calculator · model number decoder.