Duct size calculator

The equation behind the wheel

A ductulator is a slide rule for one equation. For galvanised duct the pressure drop over 100 feet is proportional to airflow to the 1.9 power and inversely proportional to diameter to the 5.02 power:

ΔP / 100 ft  =  0.109136 · Q^1.9 / D^5.02

Q  airflow, CFM        D  diameter, inches
ΔP friction loss, inches of water column

The 5.02 exponent is the whole story of duct sizing: an inch of diameter is worth far more than an inch of anything else. Going from a 6-inch to a 7-inch branch nearly doubles what it will carry at the same friction rate.

Picking a friction rate

The friction rate is not a constant of nature — it is the available static pressure divided by the total effective length of the run. Measure the blower's rated external static pressure, subtract the coil, the filter and every other fitting, and divide what is left by the longest supply-plus-return path in equivalent feet. Most houses land between 0.06 and 0.10 in. w.c. per 100 feet. If you have not measured, 0.08 is the honest default; the static pressure calculator turns your gauge readings into the number.

That is the Manual D friction-rate method as its own documentation states it: the rate falls out of the equivalent length of the system, and the equivalent length depends on the design velocity. The available pressure it divides is the design external static pressure — the figure at design airflow inclusive of the coil, a whole-house humidifier and a MERV 6 or better filter.

Sources: DOE Building America Solution Center, No Kinks or Sharp Bends in Flex Duct Installation (read 2026-09-04) · EPA ENERGY STAR, National HVAC Design Report v3.1/3.2/3.3 (Rev. 14, revised 2025-01-15), Item 5.4 and Footnote 36 (read 2026-09-04)

Flex is not rigid

Even installed properly, a 90-degree bend in flex costs 15 equivalent feet at 700 fpm. A bend tighter than one duct diameter, a kink, or flex pushed into a cavity narrower than the duct is a different animal — those are named installation defects, and none of the arithmetic above survives them.

Sources: DOE Building America Solution Center, No Kinks or Sharp Bends in Flex Duct Installation (read 2026-09-04) · EPA ENERGY STAR, National Rater Field Checklist v3.1/3.2/3.3 (Rev. 14, revised 2025-01-15), Item 6.1 and Footnote 24 (read 2026-09-04)

Worked example

A three-ton system moving 1,200 CFM, split into six branches of 200 CFM each, at 0.08 in. w.c. per 100 ft. Each branch needs 5.6 inches, so a 6-inch round: it carries 232 CFM at that rate and runs at about 1,180 fpm at full flow — fine as a short branch, loud if it feeds a bedroom register directly. The supply trunk carrying all 1,200 CFM needs 13.4 inches, so a 14-inch round or a 20 × 8 rectangular, whose equivalent round is 13.7 inches.

Figures computed from the equation above at 0.08 in. w.c. per 100 ft, smooth galvanised.