IN THIS GUIDE ▾
"The equipment quote was easy. The ductwork killed my profit."
It's a familiar story. Ductwork seems straightforward until you're actually estimating it; then you realize how many variables affect cost and complexity.
This guide covers everything you need to estimate ductwork accurately: CFM calculations based on proper load calculations, sizing formulas, material quantities, and labor cost estimation. To check a single run, use the free CFM calculator and duct size calculator.
Good ductwork design is invisible: it just works. Bad ductwork design is expensive: you pay to fix it twice.
Understanding CFM for ductwork estimation. 400 CFM per ton is where every duct run starts.
CFM is the volume of air your system needs to move to heat or cool the space effectively.
Basic CFM formula
CFM = Tons × 400
3-ton system = 3 × 400 = 1,200 CFM
4-ton system = 4 × 400 = 1,600 CFM
CFM = BTU Output ÷ (Temperature Rise × 1.08)
80,000 BTU furnace with 60°F temperature rise: CFM = 80,000 ÷ (60 × 1.08) = 1,235 CFM
Why CFM matters for ductwork
HVAC duct sizing methods. Rule of thumb, velocity, equal friction.
Method 1: Rule of thumb (quick estimates)
Use case: quick residential estimates, small simple layouts. Limitations: doesn't account for duct length, turns, static pressure.
Method 2: Velocity method
Duct Size (sq inches) = (CFM × 144) ÷ Velocity (FPM)
Example: need 400 CFM at 800 FPM: (400 × 144) ÷ 800 = 72 square inches. Round duct: 10" diameter. Rectangular: 12"×6".
Use case: residential and light commercial.
Method 3: Equal friction method
Use case: commercial work, complex residential systems.
Approximate sizes for typical residential systems. Adjust based on layout complexity. The duct size calculator does the velocity math for any CFM.
Room-by-room CFM calculations. 1 CFM per square foot, then adjust for the room.
Each room needs appropriate CFM based on size and function.
CFM per room = Square Footage × 1.0
Ductwork material quantity estimation. Trunk, branches, fittings, return, plus waste.
Once you know duct sizes, estimate material quantities.
Trunk line materials
Measure the distance from the air handler to the furthest supply, then add 20% for fittings and waste. Example: 40 feet of straight run + 20% (8 feet) = 48 feet total. Need: 48 feet of 14" round duct.
Branch run materials
For each room: measure from trunk to register, add for vertical drops, and add 15% for fittings. Example bedroom branch: 20 feet horizontal run + 8 feet vertical drop; 28 feet × 1.15 = 32 feet of 7" round duct.
Complete material list: 3-ton system
Ductwork labor hour estimation. Highly variable. Access sets the multiplier.
Ductwork labor is highly variable based on accessibility and complexity. For the full method, see the HVAC labor cost calculator guide.
Base labor hours
Labor multipliers
Common ductwork estimation mistakes. Six profit leaks, what each costs, and the fix.
These are some of the most common estimating mistakes specific to ductwork:
Ductwork modifications ALWAYS take longer than you think. What looks like "just adding a branch" turns into dealing with joists, wiring, plumbing, and insulation. Add a 50% buffer to your time estimates for modifications.
Simplified ductwork estimation: per-ton pricing. Speed over precision, for most homes.
For quick residential estimates, use per-ton pricing:
Example: 3-ton system, standard layout: 3 × $1,500 = $4,500 ductwork budget.
This method trades precision for speed but works well for most residential estimates.
The standard cooling rule is 400 CFM per ton: a 3-ton system needs about 1,200 CFM and a 4-ton system about 1,600 CFM. For heating, CFM = BTU output ÷ (temperature rise × 1.08).
Using the velocity method: duct size in square inches = (CFM × 144) ÷ velocity in FPM. For 400 CFM at 800 FPM that is 72 square inches, about a 10" round or 12"×6" rectangular duct. Typical target velocities are 900-1,200 FPM for supply trunks, 600-900 for branches and 500-700 for returns.
As a quick residential estimate, a complete ductwork system typically runs $800-1,200 per ton for a simple layout, $1,200-1,800 for a standard layout and $1,800-2,500 for a complex layout, including materials, labor, fittings and a 10% contingency.
For a 2-tech crew, a new complete system is typically 12-16 hours for a home under 1,500 sq ft, 16-24 hours for 1,500-2,500 sq ft and 24-40 hours above 2,500 sq ft. Apply multipliers for attic (1.40x), crawl space (1.50x), finished basement (1.30x) and two-story (1.25x) work.
Mastering HVAC ductwork estimation. The profit is in the details contractors skip.
Ductwork estimation requires attention to detail contractors often skip. The result? Lost profit, callbacks, and frustrated customers.
The shortcut: use per-ton pricing ($1,200-1,800/ton) for typical residential work, then adjust based on complexity. Get ductwork estimates right and you'll avoid one of the most common profit killers in HVAC work.
Combine ductwork estimates with equipment and labor for complete job pricing: The Complete HVAC Estimating Guide for Contractors.