HVAC Calculations

How to Calculate Duct Yield, Coil Capacity, Fan Motor Size, and Leakage

A step-by-step walkthrough of the four formulas that drive duct and air-handling fabrication: sheet yield, coil capacity, fan motor sizing, and leakage rate.

Start with sheet yield because material is the first cost you commit. A rectangular duct section unfolds to a flat blank of width equal to the perimeter plus seam and lock allowance. For a 20 by 12 inch duct, girth is 2 times (20 plus 12) equals 64 inches, add 1.5 inches for a Pittsburgh lock and 0.5 inch for a drive edge, giving 66 inches of blank width per running length. From a 48 by 120 inch sheet you get 120 divided by (66 divided by 12) equals about 21.8 linear feet of blank. The Duct Sheet Yield calculator tracks the nesting waste that turns that theoretical figure into the 82 to 90 percent you actually realize.

Convert yield to gauge weight so you can order steel. Galvanized 24 gauge runs 1.156 lb per square foot, 22 gauge 1.406, 20 gauge 1.656. A 4 by 10 foot sheet of 24 gauge weighs 40 square feet times 1.156 equals 46.2 lb. If a job needs 640 square feet of finished 24 gauge duct at 88 percent yield, buy 640 divided by 0.88 equals 727 square feet, which is 727 times 1.156 equals 841 lb of coil stock. Rounding gauge weight down is the most common way estimators under-order and stall a run mid-shift.

Coil capacity is the sizing check for the air handler. Sensible heat transfer follows Q equals 1.08 times CFM times delta-T, where 1.08 bundles air density and specific heat at sea level. Move 2,000 CFM across a coil dropping air from 75 to 55 degrees F and Q equals 1.08 times 2,000 times 20 equals 43,200 BTU per hour, or 3.6 tons. For total capacity including latent load, switch the constant to 4.5 times CFM times the enthalpy change in BTU per lb. The Air Handler Coil Capacity calculator carries both the sensible and total paths so you can confirm rows and fin spacing.

Fan power ties airflow to the motor you specify. Brake horsepower equals (CFM times total static pressure in inches w.g.) divided by (6,356 times fan efficiency). At 2,000 CFM against 1.5 inches static with a 62 percent efficient wheel, BHP equals (2,000 times 1.5) divided by (6,356 times 0.62) equals 0.76 hp. Add belt-drive losses of roughly 5 percent and a service factor, and you land on a 1 hp motor. The Fan Motor Sizing calculator applies the fan laws too: airflow scales with RPM, pressure with RPM squared, and power with RPM cubed, so a 10 percent speed increase costs 33 percent more horsepower.

Leakage rate proves the assembly holds air, and code writes it as a rate, not a raw reading. CL equals leakage CFM divided by duct surface area in 100 square foot units, all at a reference static of 1 inch w.g. Test a 900 square foot low-pressure system and measure 27 CFM of loss: CL equals 27 divided by (900 divided by 100) equals 3.0. SMACNA Seal Class A targets a leakage class of 3 or lower for round and 6 or lower for rectangular. The Leakage Test Rate calculator normalizes readings taken at other pressures using the 0.65 flow exponent so field data compares to the spec.

Filter and rack pressure drop feeds directly back into the fan static you just used. A clean MERV 8 pleated filter starts near 0.25 inch w.g. at 500 FPM face velocity and is typically changed at a final drop of 1.0 inch. Face velocity equals CFM divided by net free area, so 2,000 CFM through a 4 square foot rack is 500 FPM. Push velocity to 625 FPM and pressure drop climbs roughly with the square, near 0.39 inch clean. The Filter Rack Pressure Drop calculator sums filter, rack, and transition losses so your total external static is honest before it reaches Fan Motor Sizing.

Sealant and insulation quantities close out the takeoff. Water-based duct sealant covers about 300 linear feet of joint per gallon at a 1/8 inch bead, so a job with 1,100 feet of transverse and longitudinal seams needs 1,100 divided by 300 equals 3.7 gallons, round to 4. Wrap insulation is priced by square foot of duct surface: that 20 by 12 duct has a perimeter of 5.33 feet, so 40 feet of run is 213 square feet before the 10 to 15 percent lap and overlap allowance. The Sealant Usage calculator handles bead size and the Insulation Cost calculator carries R-value and facing.

Chain the numbers in the order the shop consumes them: pull surface area from the drawing, run Duct Sheet Yield to get blank feet and gauge weight, verify airside performance with Air Handler Coil Capacity and Fan Motor Sizing, then confirm the build with Leakage Test Rate before it ships. A quick sanity ratio: a typical low-pressure commercial job runs about 4.3 lb of galvanized steel and 0.9 square foot of wrap per finished square foot of duct. If your takeoff drifts more than 10 percent off those, re-check girth, seam allowance, and yield before you cut a single sheet.

Published 2026-07-01.