Chiller Size Estimator

Building Inputs

Conditioned Floor Area (ft²)

Building Type

Climate Zone

Occupancy Density

Chiller Type

Plant Sizing Margin (%) Added to the block load to set plant capacity. 10–15% typical. No diversity factor — the benchmark is already a block figure.

Chilled-Water ΔT (°F) Return minus supply. 44/56°F = 12°F; low-flow plants run 14–16°F.

Chiller Plant Results

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Block Cooling Load (tons)

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Chiller Plant Capacity (tons)

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Chiller Power at Block Load (kW)

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CHW Flow (GPM)

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ft² per Plant Ton

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Est. Supply Air (CFM)

Cooling Load Benchmarks

Building Type	ft²/Ton (typical)	Notes
Office	300–450	Modern code-built offices; internal loads dominate the core
Retail	200–400	Lighting and people density
School	225–450	Dense classrooms; OA load significant
Hospital	150–350	24/7 operation; high OA; process loads
Hotel / Multifamily	400–800	Lower density; residential-type loads
Data Center	50–200	IT load governs — size from IT kW (1 kW ≈ 0.284 ton)
Lab	100–350	100% OA and process loads
Restaurant	100–200	Dining density plus kitchen and hood makeup air

Ranges are rule-of-thumb sanity bands for the whole-building block load — the same bands the AIM Works app benchmarks a load calc against (restaurants are banded separately here). Every benchmark this tool uses stays inside its band.

About This Calculator

This chiller size estimator produces a fast block cooling load and central-plant size for early design. Enter the conditioned floor area, building type, climate zone, occupancy density, chiller type, a plant sizing margin, and the chilled-water ΔT; the tool returns block tons, plant capacity, chiller power (or heat input for absorption), chilled-water flow in GPM, the floor area per plant ton, and an estimated supply airflow.

It is a screening tool for feasibility and budgeting, not a substitute for a block load calculation. Confirm the result with an hour-by-hour analysis per the ASHRAE Handbook — Fundamentals or an energy model before selecting equipment.

Formula & Method

Block load (tons) = Area (ft²) ÷ benchmark (ft²/ton)
Plant (tons) = Block × (1 + margin)
Chiller kW = Block tons × kW/ton · CHW flow (GPM) = 24 × Block tons ÷ ΔT
Supply air (CFM) ≈ Block tons × 400

The benchmark is a whole-building (block) figure by building type and climate, scaled ×1.10 for low and ×0.90 for high occupancy density (data centers are not adjusted). Because it is already coincident, no diversity factor is applied — only the plant sizing margin. Chiller kW/ton starts from the ASHRAE 90.1-2022 Table 6.8.1-3 Path A full-load limit for the type and plant size class (air-cooled 10.1 EER = 1.188 kW/ton; water-cooled screw/scroll 0.750–0.560; centrifugal 0.610 below 300 tons, 0.560 above) and assumes a typical selection beats that floor by 10%. It is compressor power only — tower fans and pumps are extra. Absorption shows heat input at a double-effect COP of about 1.0 instead of kW. CHW flow uses Q = 500 × GPM × ΔT with 1 ton = 12,000 BTU/h. Supply air assumes about 400 CFM/ton (500 for data centers).

Frequently Asked Questions

How do I estimate chiller plant size in tons?

Divide the conditioned floor area by a square-feet-per-ton benchmark for the building type and climate to get the block (coincident) cooling load in tons, then add a sizing margin, typically 10 to 15 percent, to set the plant capacity. This tool runs that chain and also returns chiller power at ASHRAE 90.1-2022 efficiency levels, chilled-water flow at your design delta-T, and the floor area per plant ton.

What is a typical square-feet-per-ton value?

It varies widely with use. Modern offices typically land around 300 to 450 square feet per ton, retail 200 to 400, schools 225 to 450, hospitals 150 to 350, labs 100 to 350, hotels and multifamily 400 to 800, and restaurants 100 to 200. Data centers should be sized from the IT load rather than floor area. The tool stores a benchmark for each building type and climate and adjusts it for occupancy density.

Should I apply a diversity factor to a square-feet-per-ton benchmark?

No. A square-feet-per-ton benchmark is already a whole-building block figure, so the non-coincidence of zone peaks is built in. Multiplying it by a diversity factor would size the plant below its own block load. Diversity applies only when you start from a sum of individual zone peaks; this tool applies a sizing margin instead.

How is chilled water GPM calculated from tons?

Use Q = 500 × GPM × ΔT. One ton is 12,000 BTU/h, so GPM per ton = 24 ÷ ΔT: 2.4 GPM/ton at 10°F, 2.0 at 12°F (44/56°F, the tool default), and 1.5 at 16°F. The tool applies your design ΔT to the block load. A larger ΔT lowers flow and pump energy, which is why low-flow plants run 14 to 16°F.