Quick answer: Nearly all electrical power drawn by IT equipment leaves as heat, so 1 kW of IT load ≈ 1 kW of cooling load. Total heat = racks × kW per rack, add a design margin for UPS losses and room gains, then convert: TR = kW ÷ 3.517 and Airflow (m³/h) = kW × 3600 ÷ (1.2 × 1.005 × ΔT).
Why IT power equals heat
A server does no mechanical work and stores no energy. Every watt it draws is dissipated as heat into the room. That makes data centre load calculation refreshingly direct compared to a building heat load — there is no solar gain guesswork, just rack power. What you do add on top is UPS and PDU losses, lighting, envelope gain and people, which is why a design margin (typically 15–25%) is applied instead of sizing exactly to nameplate.
The formulas
IT load (kW) = racks × kW per rack
Design load (kW) = IT load × (1 + margin)
BTU/hr = kW × 3412
Tons (TR) = kW ÷ 3.517
Airflow (m³/h) = kW × 3600 / (1.2 × 1.005 × ΔT)
Airflow (CFM) = m³/h ÷ 1.699Worked example
10 racks at 5 kW each with a 20% margin and 12 °C ΔT:
- IT load = 10 × 5 = 50 kW
- Design load = 50 × 1.2 = 60 kW (204,700 BTU/hr)
- Cooling = 60 ÷ 3.517 = 17.1 TR
- Airflow = 60 × 3600 ÷ (1.2 × 1.005 × 12) ≈ 14,925 m³/h (≈ 8,785 CFM)
Airflow matters as much as tonnage
A room can have plenty of kW of cooling and still run hot if the air never reaches the rack inlets. ASHRAE TC 9.9 recommends equipment inlet temperatures of roughly 18–27 °C — and the inlet is what matters, not a room average. In practice, hot-aisle/cold-aisle containment, blanking plates in empty rack U-space and sealed floor cutouts buy more thermal headroom than extra tonnage does. Redundancy (N+1) is a separate decision: size the load first, then decide how many units deliver it.
Heat load, tonnage and airflow from rack power. Free tool.
Open the Server Rack Heat Load Calculator →Frequently Asked Questions
How do you calculate server rack heat load?
Multiply the number of racks by the power per rack in kW. Almost all IT power becomes heat, so that is your base load. Add a design margin for UPS losses, lighting and envelope gain, then divide kW by 3.517 to get tons.
How much cooling does a 5 kW server rack need?
About 5 kW of cooling, which is roughly 1.4 tons or 17,000 BTU/hr, because nearly all electrical power drawn becomes heat. Add margin for UPS and PDU losses and room gains.
How much airflow does a server rack need?
Airflow in m3/h equals kW times 3600 divided by (1.2 x 1.005 x delta-T). At a 12 degree C delta-T a 5 kW rack needs roughly 1,240 m3/h, about 730 CFM.
What temperature should a server room be?
ASHRAE TC 9.9 recommends equipment inlet temperatures of about 18-27 degrees C. Inlet temperature is what matters, not room average, which is why containment and blanking panels are so effective.