CFM for 3-ton AC?
About 1,200 CFM using the 400 CFM per ton rule of thumb.
Round duct area = CFM ÷ velocity (FPM). Diameter = √(4 × area ÷ π). Residential velocity is often 600–900 FPM in trunk lines.
Use the calculator form above with your room size, system capacity, or appliance usage. Results update instantly in your browser — no data is sent to a server.
Concise answers for common searches — definitions, steps, and comparisons.
About 1,200 CFM using the 400 CFM per ton rule of thumb.
12,000 BTU/hr of cooling capacity equals one ton of refrigeration.
Round duct area = CFM ÷ velocity (FPM). Diameter = √(4 × area ÷ π). Residential velocity is often 600–900 FPM in trunk lines.
Formula
See calculator inputs and results panel for step-by-step math.Select imperial/metric, rectangle vs area, tons vs BTU, or usage presets as shown.
Type room dimensions, system tonnage, CFM, appliance watts, or electric rate.
Pick climate zone 1–5 and insulation quality for load-based calculators.
Use BTU/hr, tons, CFM, duct diameter, or seasonal cost in the results panel.
Copy results or share a link with your HVAC contractor or energy auditor.
Use Duct CFM, Duct Size, or Energy Cost calculators for a complete HVAC plan.
Input
20×15 ft · 8 ft ceiling · zone 3Output
≈ 7,500 BTU/hr cooling300 sq ft × 25 BTU/sq ft × average insulation × 1.0 height factor.
Input
2.5 tonsOutput
1,000 CFM2.5 × 400 CFM/ton = 1,000 CFM supply airflow rule of thumb.
Common real-world scenarios where this tool saves time.
12×15 ft room, zone 3, average insulation → ~9,000–11,000 BTU/hr cooling load.
2,000 sq ft home, zone 4 → ~3.5–4 tons cooling with average insulation.
3-ton system → 1,200 CFM target → 14" round trunk at 700 FPM velocity.
900 W unit, 8 hr/day, 90 days at $0.16/kWh → seasonal cost estimate for budgeting.
Step-by-step chains that connect related tools for common tasks.
From room BTU to duct airflow and diameter.
Compare portable appliance electricity spending.
| CFM | Calc. diameter | Order size |
|---|---|---|
| 600 | 11.0" | 12" |
| 1,000 | 14.2" | 16" |
| 1,400 | 16.8" | 18" |
Rule-of-thumb sizing is a starting point — ACCA Manual J is the industry standard.
Undersized ducts reduce efficiency and comfort even when tonnage is correct.
Include climate zone, insulation, ceiling height, and sun exposure — or request Manual J.
Check heating BTU/hr and balance point, not just cooling tons.
Compressors cycle — actual kWh may be lower than watts × hours suggests.
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Residential round ducts are often sized for 600–900 FPM — lower velocity is quieter; higher velocity needs smaller duct.
Area (sq ft) = CFM ÷ velocity (FPM); diameter = √(4 × area × 144 ÷ π) in inches for round duct.
Common sizes: 6, 7, 8, 9, 10, 12, 14, 16, 18, and 20 inches — order the next size up from calculated diameter.
This tool sizes round duct — rectangular equivalents use the same cross-sectional area at equal CFM and velocity.
Flex duct has higher friction — upsize one diameter or use shorter runs compared to smooth metal duct.
No — it sizes one duct segment. Whole systems need friction-rate calculations and branch balancing.
CFM and velocity inputs for duct diameter sizing run locally — EverydayTools does not store duct-size calculator measurements.
Estimates only — not professional HVAC or energy audit advice. Verify with a licensed contractor before purchasing equipment.
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Reviewed on 2026-06-28.
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Residential round duct is often sized for 600–900 FPM