How many BTU do I need per square foot?

As a rough guide, you need about 20 BTU per square foot for standard rooms. Kitchens need 30-40% more (heat from cooking). Sunny rooms need 10-20% more. Adjust for ceiling height (taller = more), insulation (worse = more), and occupants (each additional person = ~600 BTU more).

What happens if my AC is too small?

An undersized AC will run constantly trying to reach the setpoint temperature, struggle to cool the space adequately on hot days, use excess energy due to constant operation, wear out faster, and provide poor humidity control. It will also be loud as the compressor runs without rest.

What happens if my AC is too big?

An oversized AC will cool too quickly and cycle on and off frequently (short-cycling). This causes: poor humidity removal (humidity is removed during longer run cycles), wasted energy from frequent startup, uneven room temperatures, premature compressor wear, and reduced comfort despite the lower temperature.

How do I calculate central AC for my whole home?

Rough estimate: 30-60 BTU per sq ft of conditioned space. 2000 sq ft home: 60,000-120,000 BTU = 5-10 tons. For accurate sizing, get a professional Manual J load calculation — it accounts for climate, insulation, windows, and orientation. DIY estimates are often 20%+ off; Manual J is within 5-10%.

What's the difference between window AC and mini-split?

Window AC: cheaper, easier install, requires window opening, single-room only. Mini-split (ductless): more expensive but much more efficient (SEER 20+ common), requires outdoor compressor, quieter operation, can heat and cool. For long-term use in one room: mini-split. For seasonal/temporary use: window AC.

What SEER rating should I get?

Minimum: SEER2 13 (or 14 in hot climates) per federal standards. Recommended: 16+ for energy savings and ENERGY STAR. Premium: 20+ for hot climates with high electricity costs. Higher SEER costs more upfront but saves on electricity bills. Payback typically 3-8 years in hot climates.

How long does an AC unit last?

Window AC: 8-12 years typical. Portable AC: 5-10 years. Mini-split: 15-20 years. Central AC: 10-15 years. Heat pumps: 12-15 years. Lifespan depends on maintenance, climate, and usage. Annual professional servicing extends lifespan by 30-50%.

AC BTU Calculator

Determine the correct air conditioner capacity in BTU for your space based on room size, ceiling height, insulation quality, sun exposure, and occupancy. Properly sizing your AC improves efficiency and comfort.

Air conditioning capacity is measured in BTU/hour — a unit of cooling power. Sizing the AC unit correctly is critical: too small and the room won't cool adequately; too large and the unit short-cycles (cools fast but doesn't dehumidify, wastes energy, wears out parts). The rule of thumb is roughly 20 BTU per square foot of room area, but this needs adjustment for ceiling height, sun exposure, insulation quality, and number of occupants.

This calculator returns a recommended BTU rating accounting for all those factors. A 12×15 ft room (180 sq ft) typically needs 5000-8000 BTU; a 20×20 ft (400 sq ft) needs 10000-14000 BTU. Window AC units come in standard sizes: 5000, 6000, 8000, 10000, 12000, 14000, 18000, 24000 BTU. Pick the next size up from your calculated need.

Properly sized AC saves money in two ways: lower electricity bills (right-sized is efficient at producing the cooling load needed), and longer unit life (right-sized doesn't constantly short-cycle). Get this right and you save hundreds of dollars per year vs. an oversized unit.

Inputs

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Sun Exposure (0=Shaded, 1=Average, 2=Sunny)

Insulation (0=Poor, 1=Average, 2=Good)

Number of Occupants

Results

Recommended BTU

3,600 BTU

AC Tonnage

0.3 tons

Suggested Size

Small Window Unit

Room Area

180 sq ft

Last updated: May 29, 2026

Formula

**Base BTU calculation:** Base BTU = Room area × 20 For 12 × 15 ft room (180 sq ft): 180 × 20 = 3600 BTU base. **Adjustments:** | Factor | Multiplier | |---|---| | Ceiling 8 ft (standard) | × 1.0 | | Ceiling 10 ft | × 1.25 | | Ceiling 12 ft | × 1.50 | | Sun exposure: shaded | × 0.9 | | Sun exposure: average | × 1.0 | | Sun exposure: sunny | × 1.10 | | Insulation: poor | × 1.10 | | Insulation: average | × 1.0 | | Insulation: good | × 0.95 | | Per additional occupant (over 2) | + 600 BTU each | **Worked example: 12 × 15 ft room, 8 ft ceiling, average sun, average insulation, 4 occupants** Base: 180 × 20 = 3600 BTU Sun: × 1.0 = 3600 Insulation: × 1.0 = 3600 Occupants (+2 additional): + 1200 = 4800 BTU Round to next standard size: **5000 BTU** **Standard AC sizes:** | BTU | Cools (sq ft) | |---|---| | 5,000 | 100-150 | | 6,000 | 150-200 | | 8,000 | 200-300 | | 10,000 | 300-400 | | 12,000 | 400-500 | | 14,000 | 500-700 | | 18,000 | 700-1000 | | 24,000 | 1000-1500 | For central AC: typical home is 30-60 BTU per sq ft of conditioned area. **Room characteristics affecting BTU needs:** **Kitchens**: +4000 BTU (heat from cooking). **Sun-facing rooms**: +10-15% from base. **High ceilings (10-12+ ft)**: +10-25% per foot over 8 ft. **Poor insulation**: +10%. **Many windows**: +5-10%. **Server room / electronics-heavy**: +1500-3000 BTU. **Each additional occupant over 2**: +600 BTU. **Climate zone considerations:** - **Hot, humid climates (Zone 1-2)**: oversized 10-15% for dehumidification. - **Hot, arid climates (Zone 3)**: standard sizing. - **Mixed climates (Zone 4)**: standard sizing. - **Cold climates (Zone 5-7)**: undersized 10% — less cooling needed. **AC types and applications:** | Type | BTU range | Best for | |---|---|---| | Window AC | 5,000-25,000 | Single room | | Portable AC | 8,000-14,000 | Where window install not allowed | | Mini-split (ductless) | 9,000-36,000 | Single room, energy efficient | | Central AC | 24,000-60,000+ | Whole house | | Heat pump (combined) | varies | Whole house, year-round | **Energy efficiency ratings (SEER2 / EER):** - **SEER2 13-14**: minimum standard, basic efficiency. - **SEER2 15-16**: mid-range, good value. - **SEER2 17-19**: high efficiency, ENERGY STAR. - **SEER2 20+**: premium, fast payback in hot climates. Higher SEER costs more upfront but saves on electricity. Payback typically 3-8 years in hot climates. **Operating cost estimate:** Operating cost = (BTU/hr / EER) × kWh price × hours used / 1000 For 12,000 BTU at SEER2 15: - Power = 12,000 / 15 × 0.293 = ~237 watts/hr - 8 hours daily for 3 months = 237 × 8 × 90 = 170,640 watt-hours = 171 kWh - At $0.13/kWh: ~$22/month **Common sizing errors:** | Mistake | Result | |---|---| | Too small | Constantly running, can't cool to setpoint | | Too large | Short-cycling, high humidity, wasted energy | | Wrong type | Inefficient operation, wrong installation | | Bad insulation | Unit underperforms regardless of size | | Old/dirty filter | Reduced capacity, higher costs |

How to use this calculator

Measure room length and width in feet.
Enter ceiling height (8 ft is standard).
Set sun exposure (0 for shaded, 1 for average, 2 for sunny).
Set insulation quality (0 for poor old homes, 1 for average, 2 for good new homes).
Enter number of regular occupants.
Calculator returns recommended BTU rating; round up to next standard size.

Worked examples

Standard bedroom

**Scenario:** 12 × 15 ft bedroom (180 sq ft), 8 ft ceiling, north-facing (shaded), average insulation, 2 occupants. **Calculation:** Base: 180 × 20 = 3600 BTU. Sun adjustment (shaded): × 0.9 = 3240. Insulation (average): × 1.0 = 3240. Occupants (no additional): 3240 BTU. **Result:** Round up to nearest standard size: **5000 BTU**. A 5000 BTU window unit is the typical choice for a small bedroom. Energy-efficient inverter mini-splits offer better comfort at slightly higher upfront cost.

Large family room with sun exposure

**Scenario:** 18 × 24 ft family room (432 sq ft), 9 ft cathedral ceiling, west-facing (sunny), average insulation, 4 occupants. **Calculation:** Base: 432 × 20 = 8640 BTU. Ceiling (9 ft): × 1.125 = 9720. Sun (sunny): × 1.10 = 10692. Insulation: × 1.0 = 10692. Occupants (+2 over 2): +1200 = 11892 BTU. **Result:** Round up: **12000 BTU**. The 12000 BTU rating fits a large family room with afternoon sun. Consider mini-split (better humidity control) or central AC if entire home needs cooling.

Whole-home central AC sizing

**Scenario:** 2200 sq ft single-story home, average insulation, mixed climate. Replacing aging central AC. **Calculation:** Using ~45 BTU per sq ft (mid-range for mixed climate): 2200 × 45 = 99000 BTU = 99,000/12000 = 8.25 tons. Round up to 9 tons for safety margin. **Result:** ~9 tons (108,000 BTU) central AC system. Cost ~$8000-15000 installed depending on SEER2 rating and brand. Pro Manual J load calculation strongly recommended for accurate sizing — DIY estimates often vary 20%+ from professional calculations.

When to use this calculator

**Use this calculator for:**

- **Window AC purchasing**: choosing the right size for a single room. - **Portable AC selection**: rooms where windows can't accommodate window units. - **Mini-split sizing**: ductless system for one room. - **Initial estimates before professional consultation**: rough sizing before getting pro quote. - **Multi-room comparison**: which rooms need more capacity.

**For central AC sizing:**

Use this for room-level estimates but get a professional **Manual J load calculation** for whole-home central AC. Manual J accounts for: - Climate zone - Construction details - Window placement and quality - Insulation R-values - Building orientation - Internal heat gains - Air leakage rates

Manual J is more accurate (within 5-10%) vs. rough sizing (often 20%+ off). Wrong-sized central AC is expensive to retrofit.

**AC selection guide:**

| Need | Choose | |---|---| | Cooling one room only | Window AC or portable | | Cooling one room efficiently | Mini-split (much higher SEER) | | Cooling multiple rooms separately | Multi-zone mini-split | | Cooling whole house | Central AC or whole-house heat pump | | Cold climate heating + cooling | Heat pump | | Hot/humid climate | High-SEER central or mini-split |

**Energy efficiency considerations:**

- **Inverter compressors**: variable speed, much more efficient than single-speed. - **Two-stage compressors**: medium efficiency, less complex than inverters. - **High-efficiency rotary compressors**: smaller, quieter. - **SEER2 standards**: 13 minimum (most regions), 14+ recommended. - **ENERGY STAR**: 14+ SEER2 typical for compliance.

**Installation considerations:**

- **Window AC**: requires window opening, support bracket if heavy. - **Portable AC**: needs window exhaust kit; less efficient than window units. - **Mini-split**: needs outdoor compressor + indoor unit; refrigerant line connection requires HVAC pro. - **Central AC**: requires ductwork (existing or new), outdoor compressor, indoor air handler.

**Maintenance to maintain capacity:**

- **Clean filter monthly**: dirty filter reduces capacity 5-15%. - **Annual professional service**: check refrigerant, clean coils, lubricate. - **Keep outdoor unit clear**: 2 ft clearance around outdoor compressor. - **Replace aging units**: 10-15 year typical life; older units run inefficiently.

**Common sizing scenarios:**

- **Hot summer kitchen (cooking heat)**: +30% over base. - **Server/home office (electronics)**: +20% per heavy-electronics setup. - **Pet households**: minimal adjustment (pets contribute small heat). - **Many windows**: especially west-facing, +10-15%. - **Brick or masonry walls**: store heat, +5-10%.

**Lifetime cost estimate (typical 12000 BTU window unit, 5 years):**

- Purchase: $300-500 - Installation: DIY (or $100-200 pro) - Annual operating cost: $150-300 (4 months use) - Maintenance: $30-50/year - Replacement at 10 years: $300-500

Whole-home central AC: $8000-15000 install + $300-600/year operating + $100-200/year maintenance + $5000-15000 replacement.

**Common mistakes:**

- Buying biggest available "just to be sure" — short-cycles. - Skipping calculation for "small room" — undersized. - Ignoring sun exposure — significant error. - Not accounting for occupants — bodies add heat. - Outdated insulation calculations — old homes often need more cooling.

Common mistakes to avoid

Buying oversized "to be safe." Short-cycling wastes energy and reduces humidity removal.
Underestimating sun exposure. West-facing rooms need 15-25% more BTU than north-facing.
Skipping ceiling height calculation. Cathedral ceilings significantly increase needs.
Ignoring kitchen heat load. Cooking generates substantial heat; add 30% for kitchens.
Trying to cool too many rooms with one unit. Mini-split per room is more efficient.
Buying based on watts instead of BTU. They're different units; check the BTU rating.
Skipping pro Manual J for central AC. Whole-home sizing requires professional calculation.

Frequently Asked Questions

Sources & further reading

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