BTU Calculator

About This Calculator

Estimate the BTU/hr capacity you need for an air conditioner or heater. Enter the room size, sunlight exposure, and number of regular occupants. The result is a rule-of-thumb starting point — actual equipment sizing depends on insulation, ceiling height, climate, and local HVAC codes.

How It Works

Base BTU = room area (sq ft) × 20 BTU/sq ft (ACCA rule of thumb for average insulation and ceiling height). Sunlight adjustment: +10% sunny, −10% shaded. Occupant adjustment: +600 BTU for each person above 2. AC tonnage = BTU ÷ 12,000 (1 ton = 12,000 BTU/hr). These are rough estimates with high real-world variance — always consult a licensed HVAC contractor before purchasing.

The Formula

Frequently Asked Questions

Why is the base rate 20 BTU per sq ft?

The 20 BTU/sq ft figure is the ACCA (Air Conditioning Contractors of America) rule of thumb for a room with average insulation (~8 ft ceilings, temperate climate). It is an industry starting point, not a physical constant.

What's a "ton" of AC?

One ton of cooling = 12,000 BTU/hr. A typical bedroom (150 sq ft) needs ~3,000 BTU; a large living room (500 sq ft) needs ~10,000 BTU. Most portable units are 5,000–14,000 BTU; central AC systems are measured in tons (1.5–5 tons).

Should I use this to size a mini-split or central AC?

Use this as a rough check only. For equipment purchases, always get a Manual J load calculation from a licensed HVAC contractor — it accounts for insulation value, window area, duct losses, and local climate data.