Right size your AC with a practical calculator that models envelope solar people infiltration and equipment loads Compare scenarios save presets export results draw charts and estimate airflow energy and monthly cost Get clear recommendations reduce oversizing improve comfort and plan upgrades for real rooms homes offices and studios in all climate zones worldwide
| Scenario | BTU/h | Tons | kWh/day | Est. monthly cost | Actions |
|---|
This calculator estimates required capacity in BTU per hour tons and kilowatts by summing sensible and latent loads. It separates contributions clearly so you know which changes matter.
Envelope gains use Manual J style U values. Sensible conduction equals U times area times delta T. Window solar uses glazing area SHGC orientation and sun exposure.
Infiltration adds sensible and latent load. The tool estimates airflow from air changes per hour. Sensible equals one point zero eight times CFM times delta T. Latent equals zero point six eight times CFM times the grains difference. People add about two hundred thirty sensible and two hundred latent BTU per hour each.
Plug loads and lighting are mainly sensible. Enter watts and the calculator multiplies by three point four one two to get BTU per hour. An altitude factor accounts for thinner air. Airflow target is roughly four hundred CFM per ton.
Energy and cost derive from efficiency. Divide capacity by SEER and by three thousand four hundred twelve to get kilowatts. Multiply by daily hours for kilowatt hours. The line chart shows cost versus SEER. Compare scenarios by toggling blinds occupants or insulation.
| Factor | Rule of thumb | Effect |
|---|---|---|
| Airflow target | ~400 CFM per ton | Basis for registers filters and duct sizing |
| People load | ~230 BTU/h sensible and 200 BTU/h latent | Increase for activity or crowds |
| Window solar | Higher with west and south glass | Exterior shading makes the biggest dent |
| Kitchen appliances | Account for ovens and ranges | Heat spikes raise peak capacity |
| Condition | Practical adjustment |
|---|---|
| Poor insulation plus direct sun | Choose the top of the sizing band and plan shading |
| Good insulation with shaded windows | Choose the bottom of the sizing band for quieter operation |
| Very humid climate | Favor equipment with strong dehumidification or add a dedicated unit |
| High altitude location | Expect a modest capacity increase after correction |
Right sizing prevents short cycling and sticky air. Oversized systems cool quickly yet dehumidify poorly. Undersized systems run constantly but lag on design days. Use this tool to choose a starting size and consult a professional for ducts lines and controls.
How to use Measure length width and ceiling height or enter room areas in multi room mode. Select insulation and infiltration presets that match building age. Choose orientation window fraction and glazing type. Enter people and watts for typical use. Set your indoor temperature and humidity target. Pick climate zone. Press calculate then save scenarios and compare results.
Interpreting results The total capacity is split into sensible and latent with a recommended sensible heat ratio. A higher latent portion appears in hot humid climates or crowded rooms. If the calculator flags solar risk prioritize shading or low E glass. If it flags air leakage invest in sealing and weatherstripping. For final design ductwork sizing and code compliance hire a licensed professional.
Revisit inputs after upgrades to verify savings and maintain comfort during extreme weather and humidity control.
BTU (British Thermal Unit) measures how much heat your AC can remove each hour. Too small and it runs constantly without catching up; too large and it short‑cycles—cooling the air quickly but failing to dehumidify, which feels clammy and wastes energy.
Whole‑home systems deserve a pro Manual J load calculation. For a single room (window, portable, or 1‑zone mini‑split), a chart plus realistic modifiers is accurate enough to buy with confidence—especially if you choose a variable‑speed (inverter) model that can modulate output.
| Room size (ft²) | Base BTU (window AC / mini‑split) |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1,000 | 18,000 |
| 1,000–1,200 | 21,000 |
| 1,200–1,400 | 23,000 |
| 1,400–1,500 | 24,000 |
| 1,500–2,000 | 30,000–36,000 (mini‑split) |
Smaller rooms often need more BTU per square foot (30–35 BTU/ft²) due to fixed loads (windows, electronics). As room size increases, BTU/ft² trends down (18–25 BTU/ft²).
| Ceiling height | Multiplier vs 8 ft |
|---|---|
| 7 ft | × 0.88 |
| 8 ft | × 1.00 (baseline) |
| 9 ft | × 1.13 |
| 10 ft | × 1.25 |
| 12 ft | × 1.50 |
Multiply the base BTU by (ceiling height ÷ 8) to correct for room volume.
| Climate (cooling) | Multiplier |
|---|---|
| Hot‑Humid (tropics, Gulf Coast) | +10% to +20% (× 1.10–1.20) |
| Hot‑Dry (deserts) | +5% to +10% (× 1.05–1.10) |
| Mixed/Marine (temperate) | Baseline (× 1.00) |
| Cool/Cold (northern, high altitude) | −5% to −10% (× 0.95–0.90) |
| Factor | Adjustment |
|---|---|
| Insulation: Excellent (tight) | −15% |
| Insulation: Good | −10% |
| Insulation: Poor | +20% to +30% |
| Sun exposure: Shaded | −10% |
| Sun exposure: Very sunny / west‑facing | +10% to +20% |
| Occupancy | +600 BTU per person beyond two |
| Kitchen / heavy appliances | +4,000 BTU |
| Home office (PC/monitors) | +1,000 to +2,000 BTU |
| Large west/south glazing | +10% |
| Poor ducting (if ducted air handler) | +10% |
| Scenario | Typical BTU/ft² |
|---|---|
| Small rooms (≤150 ft²), average | 30–35 |
| 150–300 ft², average | 25–30 |
| 300–500 ft², average | 20–25 |
| Well‑insulated & shaded | 15–20 |
| Hot‑humid, sunny, high glass | 22–30 |
| Open‑plan living, tall ceilings | 20–28 |
| Room (ft) | Area (ft²) | Base BTU |
|---|---|---|
| 10 × 10 | 100 | 5,000 |
| 10 × 12 | 120 | 5,000 |
| 11 × 12 | 132 | 5,000 |
| 12 × 12 | 144 | 5,000 |
| 12 × 14 | 168 | 6,000 |
| 12 × 15 | 180 | 6,000 |
| 13 × 16 | 208 | 6,000 |
| 14 × 16 | 224 | 6,000 |
| 15 × 15 | 225 | 6,000–7,000 |
| 15 × 20 | 300 | 7,000–8,000 |
| 16 × 20 | 320 | 8,000 |
| 18 × 20 | 360 | 9,000 |
| 20 × 20 | 400 | 9,000–10,000 |
| 22 × 20 | 440 | 10,000–12,000 |
| 25 × 22 | 550 | 12,000–14,000 |
| Room size (m²) | Base BTU |
|---|---|
| 9–14 | 5,000 |
| 14–23 | 6,000 |
| 23–28 | 7,000 |
| 28–33 | 8,000 |
| 33–37 | 9,000 |
| 37–42 | 10,000 |
| 42–51 | 12,000 |
| 51–65 | 14,000 |
| 65–93 | 18,000 |
| 93–112 | 21,000 |
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.