Updated July 2026 · by Murugan Vellaichamy · reviewed against current standards
Sizing basics: BTUs, tons & rooms Electricity, bills & efficiency Ductwork & installation Diagnostics & load components
Multiply floor area by your climate’s BTU/sq ft (about 20 in a moderate US climate, 25–35 in hot US zones or India), add ~10% per foot of ceiling above 8 ft, then add ~600 BTU per occupant past two plus window and kitchen heat. Example: a sunny 180 sq ft bedroom ≈ 180×20×1.10 ≈ 4,000 BTU + 600 ≈ 4,600 BTU. Run it exactly with the BTU calculator.
Divide BTU/hr by 12,000. 30,000 ÷ 12,000 = 2.5 tons. Use the BTU → tons converter.
Multiply finished area by your climate BTU/sq ft, nudge up for attic ducts or poor insulation, then divide by 12,000. Example: a 2,000 sq ft Atlanta home ≈ 2,000×24×1.15 ≈ 55,000 BTU ≈ 4.5–5 tons. Use the tonnage calculator — and above ~1,500 sq ft, size room by room (Manual J).
Size the floor area normally, then add 10% per foot of ceiling above 8 ft. A 400 sq ft loft at 14 ft: 400×20 = 8,000 × 1.30 = 10,400 BTU → a 12,000 BTU (1-ton) unit. See the ceiling-height guide.
Multiply tons by 3.517 for thermal kW. 3.5 tons ≈ 12.3 kW thermal — that is heat moved, not electricity drawn (which is far lower). Use the tons → kW converter.
Size each room’s head, sum them, then apply a diversity factor of 0.85–0.90 to the condenser (rooms rarely peak together). 9k + 18k + 12k = 39k × 0.90 ≈ 35k → a 36,000 BTU (3-ton) condenser. Use the mini-split calculator.
Sunrooms gain roughly 3× the heat of an insulated room through their glass — use ~60 BTU/sq ft instead of 20. A 150 sq ft sunroom ≈ 9,000 BTU (0.75 ton). See the sunroom sizing page.
Daily kWh = tons×12,000 ÷ (SEER2×1,000) × run-hours; monthly cost = daily kWh × 30 × rate. A 3-ton 16-SEER2 unit at 8 h/day and $0.16 ≈ 18 kWh/day ≈ $86/month. Use the bill calculator (₹ or $).
Savings % = (1 − old SEER ÷ new SEER) × 100. Going SEER 10 → 18 ≈ 44% less cooling energy — a ceiling figure; real savings depend on runtime. Put your tariff on it with the bill calculator.
NEC minimum circuit ampacity = compressor RLA × 1.25 + fan FLA. An 18.2 RLA condenser: 22.75 + 1.4 = 24.2 A → a 25 A minimum circuit. Estimate with the ampere calculator, then follow the nameplate MCA/MOCP.
Payback (years) = extra upfront cost ÷ annual energy saving. A $1,600 efficiency premium saving $200/year = 8 years — weigh that against the ~15-year system life. Compare running costs in the bill calculator.
Heat-strip kW = (winter heat loss − heat-pump output at the design temperature) ÷ 3,412. A 17,000 BTU/hr deficit ≈ 5 kW of backup heat. See heat pump sizing.
Fixed-speed: size exactly or round down — oversizing short-cycles. Inverter: you can round up to the next 0.5 ton, because it throttles down (to ~30%) and won’t short-cycle. See inverter vs non-inverter.
Find the two-digit code (a multiple of 6 or 12) in the model number — it is BTU in thousands; ÷12 for tons. Goodman GSX140241K → 24 → 24,000 BTU → 2 tons. The model decoder does 22 brands.
Airflow = tons × 400 CFM; size ducts for a quiet velocity (about 6″≈100 CFM, 8″≈200, 10″≈350). A 3-ton (1,200 CFM) system uses roughly an 18″ trunk and 8″ branches. Use the duct size calculator.
Garages leak heat — use ~30 BTU/sq ft (vs 20) and add ~1,500 BTU for power tools. A 480 sq ft garage ≈ 14,400 + 1,500 ≈ 16,000 BTU → a 1.5-ton mini-split. See the garage sizing page.
Size to the winter heating load, not summer cooling: the heat pump should meet ~100% of your heat loss at the local winter design temperature, or add backup heat below its balance point. See heat pump sizing.
Match room area to a standard unit; above ~14,000 BTU many need a dedicated circuit:
| Room (sq ft) | Window AC | Circuit |
|---|---|---|
| 100–150 | 5,000 BTU | 115 V outlet |
| 150–250 | 6,000 BTU | 115 V outlet |
| 250–350 | 8,000 BTU | 115 V outlet |
| 350–450 | 10,000 BTU | 115 V outlet |
| 450–550 | 12,000 BTU | dedicated 115/230 V |
Right-size with the window AC calculator.
Codes require a 3/4″ minimum drain for residential systems up to 5 tons, sloped at least 1/4″ per foot toward the outlet. Add a cleanout tee and a float safety switch to shut the AC off if the line clogs.
U = 1 ÷ R; conductive gain q = U × wall area × (outdoor − indoor). An R-11 wall, 200 sq ft, 20°F difference ≈ 0.091 × 200 × 20 ≈ 364 BTU/hr. See the glossary for R-value vs U-factor.
ΔT = return-air temp − supply-air temp; a healthy split is about 15–20 °F (8–11 °C). Below suggests low charge or airflow; above suggests restricted airflow (dirty filter, undersized ducts). Diagnose symptoms with is my AC the right size.
Sensible Heat Ratio = sensible ÷ (sensible + latent). Home ACs run ~0.70–0.75: 70–75% of capacity lowers temperature and 25–30% removes moisture — humid climates need a lower SHR. See the humidity guide.
q = glass area × solar intensity (~120–220 BTU/hr·ft²) × SHGC. A 25 sq ft west window at SHGC 0.30 ≈ 25×120×0.30 ≈ 900 BTU/hr. Low-SHGC glass and shading cut this sharply.
Sensible infiltration q = home volume × ACH × 0.018 × ΔT. A 12,000 ft³ drafty home (ACH 1.5) at 20°F ≈ 6,480 BTU/hr; a tight home (ACH 0.5) cuts that to a third.
Four signs of an oversized unit: cooling cycles under 10 minutes; indoor humidity stays above 60% (cold but clammy); high bills from constant start/stop; and hot/cold spots between rooms. Check yours with the symptom diagnostic and the oversizing guide.
These how-tos follow ACCA Manual J/S/D, ASHRAE Fundamentals, AHRI 210/240 (SEER2), BEE ISEER and NEC Article 440 — detailed on the methodology page. Figures are planning estimates, not a substitute for a stamped load calculation or a licensed electrician.
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