Undersized cooling is one of the most common causes of thermal shutdowns and shortened hardware life in UK server rooms. The good news: the heat you must remove is almost exactly the power your IT draws, so sizing is straightforward. This guide shows the method — and our cooling calculator does the conversion.
Power in equals heat out
Servers do not store energy — virtually 100% of the electricity they consume leaves as heat. So the cooling load in kW is essentially equal to the IT load in kW. If your UPS and PDUs deliver 8 kW, your cooling must remove about 8 kW of heat.
A tiny amount of energy leaves as fan airflow and noise, but it is negligible for sizing. This “power in = heat out” rule is the foundation of every server-room cooling calculation.
The BTU and tons maths
Cooling is often quoted in BTU/hr or tons of refrigeration. The exact conversions are: 1 kW = 3,412.142 BTU/hr, and 1 ton = 12,000 BTU/hr. So a 5 kW IT load = 17,061 BTU/hr ≈ 1.42 tons.
Our cooling calculator converts your IT load straight to BTU/hr and tons so you can match whatever units the air-conditioning supplier quotes.
Sizing with margin and redundancy
Add a design margin (15–25%) above the bare IT heat for lighting, people, solar gain through windows and future growth. Then, for any room that must stay online, provision N+1 — enough cooling units that the room stays within temperature if one fails for service or fault.
The calculator shows the IT heat, your margin and the N+1 spare as a single recommended installed capacity, so a 5 kW room ends up provisioned well above its 17,061 BTU/hr baseline.
Airflow matters as much as capacity
Total cooling capacity is only half the story — the cold air has to reach the equipment intakes. Use hot-aisle/cold-aisle layout so servers draw cold air from the front and exhaust hot air to a common return; blank unused rack U-spaces so hot and cold air do not mix.
At higher densities (above ~5 kW per rack) consider containment (aisle or rack) to stop recirculation. Even a perfectly sized CRAC unit cannot cool a rack that is breathing its own exhaust.
Common mistakes
Sizing cooling to nameplate PSU ratings (3× too big and expensive) instead of real load; forgetting redundancy (one cooler down on a hot bank holiday = shutdown); ignoring airflow; and placing UPS batteries in the hot zone, which destroys their life — see UPS runtime explained.
For a full room build, refresh or relocation, our data centre team designs power and cooling together, and Vertiv thermal kit is part of the range.