BTU Required
10,500 BTU/hr
Tons (AC)
0.9
Unit Size
12k BTU
Est. Monthly
$19
10,500
0.9
12k BTU
$19
Inputs
Result
Base = 300 × 25 = 7,500 BTU. Insulation factor 1.0, climate 1.0, sun 1.0 = 7,500 BTU. Add 3 windows × 1,000 = 3,000. Total = 10,500 BTU/hr. Recommended: 12,000 BTU unit (1 ton).
Inputs
Result
Base = 500 × 25 = 12,500. Ceiling adj (+10%) = 13,750. ×1.3 insulation ×1.3 climate ×1.1 sun = 25,559. +1,200 occupant bonus + 6,000 windows = 32,759 BTU. ~2.7 tons, recommend 30k unit.
For cooling, plan on 25 BTU per square foot as a baseline. For heating, use 30 BTU per square foot. Adjust higher for poor insulation, hot climates, or many windows, and lower for well-insulated, shaded rooms.
| Room Type | Cooling (BTU/ft²) | Heating (BTU/ft²) | Notes |
|---|---|---|---|
| Well-insulated interior | 20 | 25 | Shaded, few windows |
| Average room | 25 | 30 | Standard baseline |
| Sunny / poor insulation | 30 | 35 | South-facing, old walls |
| Kitchen / server room | 35–40 | 30 | Extra heat sources |
Divide BTU by 12,000 to get tons. One ton of cooling equals 12,000 BTU/hr. A 2-ton AC unit provides 24,000 BTU/hr of cooling capacity, suitable for about 800-1,000 sq ft in moderate climates.
| AC Tons | BTU/hr | Approx. Area (moderate climate) |
|---|---|---|
| 1.0 ton | 12,000 | 400–600 ft² |
| 1.5 tons | 18,000 | 600–900 ft² |
| 2.0 tons | 24,000 | 800–1,200 ft² |
| 2.5 tons | 30,000 | 1,200–1,500 ft² |
| 3.0 tons | 36,000 | 1,500–2,000 ft² |
| 5.0 tons | 60,000 | 2,500–3,500 ft² |
Insulation dramatically affects BTU needs. Poor insulation can increase requirements by 30%, while excellent insulation (R-30+) can reduce them by 25%. Upgrading insulation is often more cost-effective than buying a larger HVAC unit.
| Insulation Level | BTU Multiplier | Wall R-Value | Impact |
|---|---|---|---|
| Poor / none | 1.30 (×) | R-4 or less | +30% more BTU needed |
| Average | 1.00 (×) | R-11 to R-13 | Baseline calculation |
| Good | 0.85 (×) | R-15 to R-19 | –15% fewer BTU |
| Excellent | 0.75 (×) | R-21 to R-30+ | –25% fewer BTU |
An oversized AC short-cycles, turning on and off frequently. This wastes energy, increases humidity (the unit shuts off before dehumidifying), causes uneven temperatures, and wears out the compressor faster. Proper sizing is critical.
Each window adds roughly 1,000 BTU to cooling load due to solar heat gain. South and west-facing windows contribute more heat. Double-pane or Low-E windows reduce this impact significantly compared to single-pane.
| Window Type | R-Value | BTU Added per Window | Solar Heat Gain |
|---|---|---|---|
| Single-pane clear | R-1 | ~1,200 | High |
| Double-pane clear | R-2 | ~1,000 | Medium-high |
| Double-pane Low-E | R-3 | ~700 | Medium |
| Triple-pane Low-E | R-5+ | ~400 | Low |
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Last Updated: Mar 9, 2026
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