❄️ Cooling Load
tons

Total cooling capacity to be delivered

🌡️ Temperature Settings
44°F

Typical: 42–46°F (ASHRAE standard 44°F)

54°F

Typical: 52–56°F · Delta-T usually 10–14°F

⚙️ System Parameters

Glycol reduces heat capacity (cp)

1.1×

1.1× is standard for chilled water

Flow will be split equally between pumps

🔄 Variable Flow System (VFD Pumps)
🏗️ Primary-Secondary Loop
— GPM
Live estimate (updates as you type)

Enter your cooling load and temperatures above

Calculating chilled water flow…

📊 Chilled Water Flow Results
—
Required Chilled Water Flow Rate
—
GPM · Flow split across all system zones
🌡️
—
°F
Delta-T
💧
—
L/s
Litres/sec
🔢
—
m³/h
Cubic m/hr
⚡
—
kW
Cooling Load
CHILLER —TR SUPPLY 44°F AHU / LOAD RETURN 54°F PUMP ▶ ▶ ◀ ◀ — GPM Total System Flow

System Overview

Chilled water leaves the chiller cold. It picks up heat at the AHU or load. Then it returns warmer. The pump keeps it moving. This calculator finds the exact flow rate you need.

System Delta-T Efficiency
Low ΔT High ΔT —

🔧 Recommended Pipe Sizes

Pipe Size (Nominal)
Velocity
Pressure Drop
Suitability

📊 Flow Parameter Breakdown

Parameter
Value
Unit
Benchmark
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🛠️ Design Recommendations

Chilled Water Flow Rate — What You Need to Know

Getting the flow rate right is one of the most important steps in any HVAC design. Too little flow and your system can’t deliver the cooling. Too much and you waste pump energy and money.

❄️

What Is Chilled Water Flow Rate?

It’s the volume of chilled water that must move through your pipes every minute (GPM) to carry away the heat load. Think of water as the vehicle. The cooling load is the cargo. The flow rate tells you how many vehicles you need.

🌡️

Delta-T Is Everything

Delta-T (ΔT) is the difference between supply and return temperature. A bigger ΔT means each gallon of water carries more heat. That means you need less flow. Lower flow means smaller pipes and less pump energy. Aim for 10–14°F for most systems.

📐

The GPM Formula

The basic formula is simple: GPM = (Load in BTU/hr) ÷ (500 × ΔT). The number 500 comes from water’s density and specific heat. For glycol systems, the factor changes — this calculator handles that automatically.

💧

Why Glycol Changes Things

Glycol antifreeze is added to protect pipes from freezing. But glycol also reduces the heat-carrying ability of water. A 40% glycol mix carries about 8% less heat per gallon. So you need slightly more flow — or a larger ΔT — to compensate.

⚡

Low ΔT Syndrome

This is a common problem in real systems. When coils foul or controls don’t work right, return temperatures drop. The delta-T shrinks. Suddenly you need 3× more flow to deliver the same cooling. Pumps overwork and energy bills spike.

🔧

Variable vs. Constant Flow

Constant flow systems always pump the same GPM regardless of load. Variable flow systems use VFD pumps to reduce flow at part loads. VFD systems can cut pump energy by 40–60%. Always choose variable flow for systems with varying loads.

📐 The Formulas We Use

GPM = Q (BTU/hr) ÷ [500 × ΔT (°F) × Cp_factor] Q (BTU/hr) = Tons × 12,000  |  Q (BTU/hr) = kW × 3,412 Velocity (fps) = GPM × 0.4085 ÷ D² (inches)

Q = Cooling load · ΔT = Return temp − Supply temp · Cp_factor = 1.0 for water, reduces for glycol
500 = 8.33 lb/gal × 60 min/hr × 1.0 BTU/lb·°F (specific heat of water)
The velocity formula uses the Hazen-Williams method adapted for pipe inner diameter.

Chilled Water System Design Reference

Common design parameters for different system types and sizes.

System TypeTypical ΔTGPM / TonSupply TempDesign Velocity
Small Commercial (ASHRAE)10°F2.4 GPM/ton44–46°F3–4 fps
Large Commercial12°F2.0 GPM/ton42–44°F3–5 fps
Hospital / Critical8–10°F2.4–3.0 GPM/ton44°F3–4 fps
Data Center14–18°F1.3–1.7 GPM/ton40–44°F4–5 fps
District Cooling16–22°F1.1–1.5 GPM/ton38–42°F4–6 fps
Glycol System (40%)10°F*2.6 GPM/ton*30–40°F3–4 fps

Frequently Asked Questions

The standard answer for pure water at 10°F delta-T is 2.4 GPM per ton. But this changes with delta-T. At 12°F ΔT you need 2.0 GPM/ton. At 8°F ΔT you need 3.0 GPM/ton. Glycol also increases the required GPM slightly. Always calculate using your actual supply and return temperatures for the most accurate result.
10–14°F is the industry sweet spot for most commercial buildings. A 10°F ΔT (44°F supply, 54°F return) is the most common design condition. Higher delta-T (12–16°F) is better — you need less flow, smaller pumps, and less pipe. District cooling and data centers often target 16–22°F ΔT for maximum efficiency.
Low delta-T happens when return water comes back almost as cold as it left. Common causes: fouled or undersized coils, oversized control valves, poor control system tuning, or parallel paths that allow bypass flow. It forces the system to pump much more water than designed. Energy and pipe costs go up significantly. Regular commissioning and coil cleaning prevent it.
It depends on your coldest expected temperature. For freeze protection to 20°F, use 25% glycol. For 10°F, use 35%. For −10°F, use 45–50%. More glycol means less heat capacity per gallon — so you need slightly more flow. Never use more glycol than you need. It increases pumping costs and reduces system efficiency.
Start with your GPM from this calculator. Then choose a pipe size that keeps velocity between 3–5 fps for main headers and 2–4 fps for branches. You can also use the pressure drop method — typically 1–4 ft of water per 100 ft of pipe for chilled water mains. Use the Pipe & Velocity tab in this calculator to check different pipe sizes against your flow rate.
Variable primary systems (one set of VFD pumps) are now the industry standard for most new systems. They’re simpler, more efficient, and cheaper to build than primary-secondary setups. Primary-secondary still makes sense for very large district cooling plants or where multiple chillers need to run at different flow rates. For most commercial buildings under 1,000 tons, go variable primary.
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