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Duct CFM Calculator

By Muhammad Abdullah Rauf · Founder, EverydayTools.proUpdated 2026-06-28

How do I use the Duct CFM Calculator?

Required CFM ≈ AC tonnage × 400 CFM/ton. A 2.5-ton system typically needs about 1,000 CFM total supply airflow.

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.

Duct CFM Calculator methodology

Required CFM ≈ AC tonnage × 400 CFM/ton. A 2.5-ton system typically needs about 1,000 CFM total supply airflow.

Formula

See calculator inputs and results panel for step-by-step math.

Assumptions

  • Residential rule-of-thumb load factors by climate zone (zones 1–5).
  • Standard 8 ft ceiling reference with height adjustment for taller rooms.
  • 400 CFM per ton for duct airflow unless your equipment spec differs.

Limitations

  • Does not replace Manual J, Manual D, or licensed HVAC design.
  • Does not account for windows, orientation, infiltration, or duct leakage.
  • Energy cost estimates assume constant watt draw and flat $/kWh rate.

How to use Duct CFM Calculator

  1. Choose units or mode

    Select imperial/metric, rectangle vs area, tons vs BTU, or usage presets as shown.

  2. Enter measurements

    Type room dimensions, system tonnage, CFM, appliance watts, or electric rate.

  3. Set climate & insulation

    Pick climate zone 1–5 and insulation quality for load-based calculators.

  4. Read primary result

    Use BTU/hr, tons, CFM, duct diameter, or seasonal cost in the results panel.

  5. Copy or share

    Copy results or share a link with your HVAC contractor or energy auditor.

  6. Cross-check related tools

    Use Duct CFM, Duct Size, or Energy Cost calculators for a complete HVAC plan.

Duct CFM Calculator examples

300 sq ft room

Input

20×15 ft · 8 ft ceiling · zone 3

Output

≈ 7,500 BTU/hr cooling

300 sq ft × 25 BTU/sq ft × average insulation × 1.0 height factor.

2.5-ton CFM

Input

2.5 tons

Output

1,000 CFM

2.5 × 400 CFM/ton = 1,000 CFM supply airflow rule of thumb.

Who uses Duct CFM Calculator?

Common real-world scenarios where this tool saves time.

Bedroom window AC

12×15 ft room, zone 3, average insulation → ~9,000–11,000 BTU/hr cooling load.

Central AC replacement

2,000 sq ft home, zone 4 → ~3.5–4 tons cooling with average insulation.

Duct retrofit

3-ton system → 1,200 CFM target → 14" round trunk at 700 FPM velocity.

Summer window AC bill

900 W unit, 8 hr/day, 90 days at $0.16/kWh → seasonal cost estimate for budgeting.

Workflow guides

Step-by-step chains that connect related tools for common tasks.

Reference tables

CFM per ton reference

TonsCFM at 400/tonNotes
2800Small central system
31,200Typical residential
41,600Larger home

Best practices

Request Manual J for new systems

Match duct to equipment CFM

Common mistakes to avoid

Oversizing AC by square footage alone

Include climate zone, insulation, ceiling height, and sun exposure — or request Manual J.

Ignoring heating load in heat pump climates

Check heating BTU/hr and balance point, not just cooling tons.

Using nameplate watts as constant draw

Compressors cycle — actual kWh may be lower than watts × hours suggests.

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Frequently Asked Questions

How many CFM per ton of AC?

Residential rule of thumb: 400 CFM per ton — a 3-ton system needs about 1,200 CFM total supply airflow.

How do I get CFM from BTU?

BTU ÷ 12,000 = tons, then tons × 400 = CFM using the standard conversion.

Is 400 CFM per ton always correct?

Dry climates may use 350 CFM/ton; humid climates sometimes need 450 CFM/ton for moisture removal — confirm with your designer.

Supply vs return CFM — same number?

Return airflow should roughly match supply CFM for balanced pressure — this calculator estimates total supply CFM.

How does duct leakage affect CFM?

Leaky ducts waste delivered CFM at registers — seal and test ducts; you may need more fan CFM at the air handler.

What if my blower chart shows different CFM?

Use manufacturer fan tables at actual static pressure — this tool is a quick sizing starting point.

Privacy, accuracy, and trust

Privacy

Tonnage and BTU values for duct CFM estimates stay on your device — EverydayTools does not upload duct-cfm calculator inputs.

Estimates only — not professional HVAC or energy audit advice. Verify with a licensed contractor before purchasing equipment.

Part of Calculator Tools

Related Tools

More free tools for the same workflow.

Duct Size Calculator

Free duct size calculator: Find round duct diameter from required CFM and target air velocity — shows calculated size and the next standard residen Runs locally in your browser — no signup.

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Reviewed on 2026-06-28.