Cooling load calculator
Almost all the electricity a data hall consumes leaves it as heat. This is how much.
Enter the IT load and the calculator converts it into the units cooling equipment is actually sold in — BTU per hour and tons of refrigeration — then works out the airflow that heat needs at your chosen temperature rise and how many units it takes to carry it with a spare.
The conversion itself is trivial. What it is worth doing carefully is the accounting around it: what else in the room turns into heat, what margin the design carries, and whether the temperature rise you assumed is one the cabinets can actually produce.
Worked example — a 250 kW hall at ΔT 11 °C
Eight per cent support heat, a ten per cent design margin and 100 kW units.
- IT load — 250 kW. Power drawn by IT equipment. Essentially all of it becomes heat in the room.
- Other heat in the room — 8 % of IT. Lighting, fan motor heat, UPS and PDU losses inside the room, people and envelope gain.
- Air temperature rise — 11 °C. Difference between supply and return air. Containment raises this; a mixed open room collapses it.
- Design margin — 10 %. Headroom above the calculated load for growth, measurement error and part-load behaviour.
- Capacity per cooling unit — 100 kW. Sensible capacity of one unit at the design condition, not its nameplate total capacity.
Design cooling load: 297.0 kW — 1,013,406 BTU/h — 84.5 tons of refrigeration
| Heat from IT equipment | 250.0 kW |
|---|---|
| Other heat in the room 8% of the IT load | 20.0 kW |
| Load before margin | 270.0 kW |
| BTU per hour | 1,013,406 BTU/h |
| Tons of refrigeration 1 ton = 12,000 BTU/h = 3.517 kW | 84.45 tons |
| Airflow required 80,597 m³/h — 22,388 L/s | 47,438 CFM |
| Units at N 100 kW each | 3 units |
| Units at N+1 One unit can be lost or maintained without losing capacity | 4 units |
Formula
BTU/h = kW × 3,412.14 · tons = kW ÷ 3.5169 · m³/h = kW × 3600 ÷ (1.2 × 1.005 × ΔT°C)
Airflow uses standard air at about 1.2 kg/m³ and a specific heat of 1.005 kJ/kg·K. In imperial terms the same relationship is CFM = BTU/h ÷ (1.08 × ΔT°F), which is where the familiar 1.08 constant comes from.
Conversions worth memorising
Exact to the precision shown; the rest of any cooling estimate is far less certain than these numbers.
| From | To | Multiply by |
|---|---|---|
| kW | BTU per hour | 3,412.14 |
| BTU per hour | kW | 0.000293 |
| kW | Tons of refrigeration | 0.2843 |
| Tons of refrigeration | kW | 3.5169 |
| m³/h | CFM | 0.5886 |
| CFM | L/s | 0.4719 |
Temperature rise is a design decision, not a constant
Airflow and temperature rise trade against each other for the same heat: double the rise and you need half the air. That makes ΔT the most consequential input here, and the one most often inherited from a spreadsheet nobody remembers writing.
A mixed open room typically shows a small rise, because hot exhaust finds its way back to the inlets and the return air is cooler than the equipment actually produced. Containment raises the rise by keeping the two air paths apart, which is why containment reduces fan power: the same heat is carried by less air.
Sensible capacity is the number that matters
Cooling units are often quoted with a total capacity that includes latent capacity — the ability to condense moisture. A data hall produces almost no moisture, so latent capacity is close to useless there, and a unit quoted at 120 kW total may offer 100 kW or less of sensible cooling at the conditions you actually run.
Use sensible capacity at the design supply and return conditions when counting units. Using nameplate totals is one of the more common ways a room ends up short of cooling on the hottest day of the year.
What this calculation leaves out
- Ambient conditions. Heat rejection capacity falls as outdoor temperature rises, which is exactly when the room needs it most.
- Humidity control and the energy spent on it, which is separate from sensible heat removal.
- Air distribution. Total room airflow says nothing about whether the air arrives at the cabinets that need it.
- Altitude. Thinner air carries less heat per unit volume, so airflow requirements rise with elevation.
- Partial liquid capture. Once some heat leaves by water, the air-side load is no longer the whole IT load.
Frequently asked questions
How do you convert kW to BTU per hour?
Multiply kilowatts by 3,412.14. A 250 kW IT load is about 853,000 BTU per hour. Going the other way, divide BTU per hour by 3,412.14 to get kilowatts.
How many tons of cooling do I need per kW?
One ton of refrigeration is 12,000 BTU per hour, which is 3.517 kW, so divide your load in kW by 3.517. A 250 kW load needs about 71 tons before any design margin is added.
How much airflow does a data center need?
About 1,757 CFM per kW at a temperature rise of 1 °C, so at a realistic ΔT of 11 °C it is roughly 160 CFM per kW. A 250 kW hall at that rise needs on the order of 40,000 CFM. Higher containment raises the achievable rise and lowers the airflow for the same heat.
Does all the power a server uses turn into heat?
Effectively yes. A server performs computation, not mechanical work on its surroundings, so essentially all the electrical energy it consumes ends up as heat in the room. Planning cooling as roughly equal to IT power is a sound starting point.
Why is my calculated cooling load higher than my IT load?
Because the room contains more than IT equipment. Lighting, fan motor heat, UPS and PDU losses inside the room, people and heat gain through the building envelope all add to it, and most designs then add a margin on top for growth and measurement error.
What is the 1.08 factor in the CFM formula?
It bundles the density and specific heat of standard air into imperial units, giving CFM = BTU/h ÷ (1.08 × ΔT°F). It assumes sea-level air at ordinary conditions, so at altitude or at unusual temperatures the constant drifts and the metric form is easier to keep honest.
Where this calculation stops
This is a first-pass heat balance, not a mechanical design. Selecting equipment requires the design ambient conditions, the chosen supply temperature, part-load behaviour, redundancy strategy and the hydraulics of the distribution.
Airflow here is a room total. Whether the air reaches the cabinets that need it is a question of containment, floor layout and tile placement, which arithmetic cannot answer.