Plenum
HVAC sizing calculators
A room estimate, a block-load tonnage, a duct diameter, and a mini-split layout. Each one says it is not a Manual J.
BTU Calculator
BTU calculator for room heating and cooling. ENERGY STAR chart, climate zone, sun, people, and kitchen. An estimate, not a Manual J.
AC Tonnage Calculator
AC tonnage calculator from floor area and climate zone. A block-load estimate. Get a Manual J before you buy equipment.
Duct Size Calculator
Duct calculator using equal friction. Round size, rectangular equivalent, and velocity from the ASHRAE friction equation.
Mini Split Calculator
Mini split calculator for head sizes and the outdoor unit. Room estimates rolled up to catalog sizes. Not a Manual J.
How these calculators work
The room BTU tool starts from the ENERGY STAR room-air-conditioner chart, which assumes an 8 ft ceiling. Shaded rooms drop 10%, sunny rooms add 10%, each person past two adds 600 BTU/h, and a kitchen adds 4,000. ENERGY STAR then rounds a window unit down to the next 1,000. A ceiling other than 8 ft, and the poor/average/good insulation multiplier, are this site’s adjustments and are labeled that way. Heating uses a planning BTU per square foot by DOE climate zone. DOE defines the zones. It does not publish those rates.
Whole-house tonnage is area times a planning cooling rate for the zone, divided by 12,000. Mini-split heads use the same room estimate and round up to a catalog size. The outdoor unit is sized from the sum of the unrounded room loads, not from the sum of the oversized heads. Duct size is the equal-friction method: Altshul–Tsal and Darcy–Weisbach at standard air, with Huebscher’s round-to-rectangular equivalent and the velocity written out. Buy equipment from a Manual J, not from these pages.