Room BTU Load Calculator

How the math works

The simplified Manual J shortcut starts from a base cooling load per square foot keyed to your climate zone, then layers on per-component loads. We multiply the base by three envelope modifiers (insulation, ceiling height, exterior-wall count), then add discrete loads for windows, doors, people, and appliances.

The DOE-derived climate zone numbers are deliberately conservative. Zone 1 (Miami) needs about 35 BTU/sqft of cooling; zone 7 (Duluth) needs 16. Heating reverses: Miami needs nearly zero, Duluth needs 65 BTU/sqft. Insulation tier modulates both directions — a Passive-House-quality envelope cuts loads by 35% vs. a typical 2000s build.

The formula

Cooling BTU/hr = room sq ft × zone cooling base × insulation × ceiling × wall factors, plus window sq ft × 90, plus exterior doors × 1,000, plus occupants × 600, plus appliance watts × 3.4

Heating BTU/hr = room sq ft × zone heating base × the same three envelope factors, plus window sq ft × 50, plus exterior doors × 600

The multipliers are Manual J shortcut values: 90 BTU/hr per square foot of glass (mixed-orientation solar average), 600 per occupant (a resting body's sensible plus latent heat), and 3.4 per watt — one watt dissipates 3.412 BTU/hr. Heating credits nothing for people or electronics: size heat for the design cold morning, when the room is empty. Tonnage is cooling divided by 12,000, rounded up to the next half ton; heating carries a 15% margin.

Worked example: a Chicago corner bedroom

A 320 sq ft Chicago corner bedroom — zone 5 (bases 20 cooling, 45 heating), post-2015 "good" insulation (0.8), 9-ft ceilings (1.05), two exterior walls (1.1), 40 sq ft of glass, one exterior door, two sleepers, 150 watts of electronics:

• Envelope factor: 0.8 × 1.05 × 1.1 = 0.924
• Base cooling: 320 × 20 × 0.924 = 5,914 BTU/hr
• Windows: 40 × 90 = 3,600 · Door: 1 × 1,000 = 1,000 · Occupants: 2 × 600 = 1,200 · Appliances: 150 × 3.4 = 510
• Cooling total: 12,224 BTU/hr — rounds up to 1.5 tons, but a 12,000-BTU/hr head is within 2% of load and the better buy
• Heating: 320 × 45 × 0.924 = 13,306, plus 2,000 for glass and 600 for the door = 15,906 BTU/hr (18,291 with margin)

Chicago's heating load runs 30% above cooling — on a heat-pump mini-split, heating selects the head.

Base loads by climate zone

The per-square-foot base loads behind the zone dropdown — DOE-derived averages, applied before envelope factors and component loads:

South of zone 3, cooling picks the equipment; zone 5 and north, heating does. Zone 4 is the crossover where one right-sized heat pump covers both.

Why room-by-room beats square-foot rules of thumb

The 20-BTU/sqft rule you hear from box-store sales staff is the right ballpark for the average US room in zone 4 with average everything. The moment you have any of: more glass than average, an exposed exterior corner, vaulted ceilings, an attic above, or a west-facing wall — the rule of thumb undersizes by 25-50%.

The oversizing trap

Going one size up "just in case" is the most common HVAC mistake. An oversized AC reaches the thermostat setpoint quickly, shuts off, and never runs long enough to remove humidity. The room ends up cold and clammy. Modern variable-speed equipment partially mitigates this but doesn't eliminate it.

Size to the load. If your peak day in the year is 95°F and the load is 8,200 BTU/hr, buy an 8,500-BTU unit, not a 12,000-BTU unit.

Common mistakes

• Sizing a heat-pump head to the cooling load in zones 5–7. Duluth's heating base is 65 BTU/sq ft vs 16 for cooling. A heat pump's nameplate is its 47°F AHRI 210/240 rating; output at 5°F can be 30–50% lower — check the NEEP cold-climate listing, not the box.
• Stacking safety margins. The half-ton round-up is already margin, and heating already carries 15%. Another 20% "to be safe" is how a 9,000-BTU bedroom gets a short-cycling 1.5-ton unit.
• Conditioning more than you measured. A head in an open plan serves every space air can reach — measure the whole open footprint.
• Guessing low on glass. At 90 BTU/hr per square foot, glass is the strongest term in the formula; one 6-ft patio slider is about 33 sq ft — nearly 3,000 BTU/hr. Measure, don't eyeball.
• Comparing to the wrong portable-AC rating. Since the DOE's 2020 test rule, a "14,000 BTU" (ASHRAE) portable is often 8,000–10,000 BTU SACC. Match your load to SACC.

When this calculator is the wrong tool

Use a full Manual J procedure for: whole-home system sizing, equipment selection for a heat pump (which has unique heating capacity curves), and any commercial application. This tool is for sizing a window AC, a single mini-split head, a portable unit, or evaluating whether an existing room's HVAC is reasonably sized.

Sources & how we keep this current

Every figure traces to a public source, and the data file records a last-verified date:

• ACCA Manual J, 8th edition — the residential load-calculation standard this shortcut adapts; IRC M1401.3 requires equipment sized per ACCA Manual S from Manual J loads.
• DOE Building America / PNNL IECC climate-zone map — the zone definitions and per-zone base loads.
• ENERGY STAR room-AC sizing guidance — cross-check for the adjustments: 600 BTU/hr per extra occupant, 4,000 BTU/hr for kitchens, plus or minus 10% for sun.
• AHRI Standard 210/240 — cooling capacity is rated at 95°F outdoors; output on a 105°F afternoon is lower.
• NEEP cold-climate heat pump list — verified heating capacity at 17°F and 5°F for zones 5–7.

When a source moves, the data file is updated and its date reset. Treat this as a single-room estimate; commission a full Manual J for anything ducted or whole-home.