Cooling Capacity Calculator
Calculate cooling requirements from IT load, PUE, and airflow parameters for data center HVAC design.
About this calculator
Every watt a data center draws eventually turns into heat, which is why this calculator's first move is to multiply IT load by PUE (Power Usage Effectiveness) to get total facility power — PUE captures the overhead of cooling, lighting, and UPS losses on top of the servers themselves, so the gap between total facility power and IT load is that overhead. Total heat output converts from kW to BTU/hr using the standard 3,412.14 BTU/hr per kW factor, and dividing by 12,000 BTU/hr per ton of refrigeration gives base cooling tons; a safety/growth factor percentage (defaulting to 20%) inflates that to a design figure that leaves headroom for future load growth and peak conditions rather than sizing exactly to today's draw. Required airflow uses the standard sensible-cooling equation, CFM = BTU/hr ÷ (1.08 × ΔT), where ΔT is the gap between return and supply air temperatures and the heat load is first scaled down by the sensible heat ratio (since some cooling load is latent — humidity removal — rather than sensible temperature drop; data centers run high, typically 0.9-1.0, because server rooms produce almost no moisture).
CRAH/CRAC unit count assumes a typical 25-ton unit size, and chilled water flow uses GPM = BTU/hr ÷ (500 × ΔT) at a standard 12°F water-side temperature delta. Because PUE, sensible heat ratio, and unit sizing are all facility-specific in reality, treat this as a first-pass HVAC sizing tool for budgeting and equipment-count estimates — a full mechanical design still needs your actual measured PUE and site-specific ASHRAE psychrometric analysis.
Inputs
Results
Design cooling capacity (tons)
54.59
How to Use This Calculator
- Enter the total IT load in kilowatts and the facility PUE.
- Set the safety and growth factor percentage.
- Input supply and return air temperatures for airflow calculations.
- Review design cooling capacity in tons and number of CRAH units required.
- Use chilled water GPM and total facility power to size the chiller plant.
How the result changes with IT load (kW)
| IT load (kW) | Design cooling capacity (tons) |
|---|---|
| 50 | 27.3 |
| 75 | 40.95 |
| 150 | 81.89 |
| 250 | 136.49 |
What each input means
- IT load (kW)
- Total IT equipment electrical load in kilowatts.
- Power Usage Effectiveness (PUE)
- Facility PUE. Industry average ≈ 1.58, efficient ≈ 1.2-1.4.
- Safety / growth factor (%)
- Design margin for future growth and peak conditions.
- Supply air temp (°F)
- Cold aisle supply air temperature. ASHRAE recommended: 64-80°F.
- Return air temp (°F)
- Hot aisle return air temperature.
- Sensible heat ratio
- Fraction of heat that is sensible (vs. latent). Data centers are typically 0.9-1.0.
What each result means
- Design cooling capacity (tons)
- Required cooling capacity including safety factor, in refrigeration tons.
- Base cooling (tons)
- Minimum cooling capacity without safety margin.
- Total heat load (BTU/hr)
- Total heat rejected by all facility equipment.
- Required airflow (CFM)
- Cubic feet per minute of conditioned air needed.
- CRAH units needed
- Number of 25-ton CRAH/CRAC units required.
- Chilled water flow (GPM)
- Chilled water flow rate at 12°F ΔT.
- Overhead power (kW)
- Non-IT power consumption (cooling, UPS losses, lighting, etc.).
- Total facility power (kW)
- IT load × PUE = total power draw.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersIT load (kW) = 100, Power Usage Effectiveness (PUE) = 1.6, Safety / growth factor (%) = 20, Supply air temp (°F) = 55 = 6 input(s) provided
- Calculate Design cooling capacityDesign cooling capacity = baseCoolingTons * safetyMultiplier54.59 = 54.59
- Calculate Base coolingBase cooling = totalHeatBtuHr / 1200045.5 = 45.5
- Calculate Total heat loadTotal heat load = totalFacilityKw * 3412.14545942 = 545942
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator multiply IT load by PUE before converting to cooling tons instead of using IT load directly?
Every watt of total facility power eventually becomes heat that has to be removed, not just the watts going into servers. Multiplying itLoadKw by PUE gives totalFacilityKw, which captures the overhead of cooling equipment, lighting, and UPS losses running alongside the IT gear — using itLoadKw alone would undersize the cooling system by exactly the overhead amount (overheadKw = totalFacilityKw - itLoadKw).
What does the sensible heat ratio input actually change in the calculation?
It only affects the airflow (CFM) calculation, not the tonnage figures. The engine multiplies totalHeatBtuHr by sensibleHeatRatio to get sensibleHeatBtu before plugging it into CFM = BTU/hr ÷ (1.08 × ΔT), because some cooling load is latent (removing humidity) rather than sensible (dropping temperature), and only sensible heat determines how much air volume you need to move. Data centers default to 0.95 here since server rooms produce almost no moisture, so this ratio rarely drags airflow down much from the un-scaled figure.
Why might the CRAH units needed number seem like an overestimate for my facility?
crahUnitsNeeded divides designCoolingTons (which already includes your safety/growth factor markup) by a fixed assumed unit size of 25 tons and rounds up. If your actual CRAH units are larger or smaller than 25 tons, or if you don't want the safety factor's headroom baked into the unit count, you'll need to recompute using baseCoolingTons and your real per-unit tonnage instead.
How is chilled water flow rate calculated, and what if my chiller plant uses a different temperature delta?
chilledWaterGpm uses GPM = BTU/hr ÷ (500 × ΔT) with a fixed 12°F water-side temperature delta assumption, applied to total heat load (not the safety-factored design load). If your plant is designed around a different chilled water ΔT — some systems run higher deltas to reduce flow and pump energy — you'd need to substitute your actual ΔT into that formula rather than relying on this output directly for final sizing.
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