Cómo funcionan las matemáticas

Sizing an air conditioner is really a heat math problem. Your room constantly gains heat from the sun, the walls, the people inside, and anything that makes heat (like a stove). The air conditioner's job is to remove that heat, and its power is measured in BTUs — British Thermal Units. One BTU is the heat needed to warm one pound of water by one degree. This calculator adds up every source of heat in your room, step by step, so the AC you buy can actually keep up.

Step 1: The 20 BTU per square foot baseline

The industry starting rule is simple: BTU = room area × 20. A 200-square-foot room starts at 4,000 BTU. Why 20? Years of field testing showed that a typical insulated room with 8-foot ceilings gains roughly 20 BTUs of heat per square foot on a warm day. It is a baseline, not the answer — the steps below adjust it for your real room.

Step 2: Ceiling height adjustment

The 20 BTU rule assumes 8-foot ceilings. Taller ceilings mean more air volume to cool, so the calculator multiplies by your ceiling height ÷ 8. A room with 10-foot ceilings gets a 1.25× multiplier. This matters because cooling is about volume, not just floor area — cathedral ceilings can quietly add 25% or more to your cooling load.

Step 3: Climate and sun exposure

A bedroom in Phoenix and an identical bedroom in Seattle do not need the same AC. The calculator applies a climate multiplier (hot climates push the number up, cold climates pull it down) and a sun multiplier (a sunny south-facing room with big windows gains far more heat than a shaded north-facing one). These two adjustments are why the same square footage can need very different units in different homes.

Step 4: People and kitchens add heat

Every person gives off roughly 400–600 BTUs of body heat per hour, so the calculator adds 600 BTU for each occupant beyond the first two. A kitchen adds a flat 4,000 BTU because ovens, stoves, and refrigerators dump serious heat into the room. Skip this step and your AC will lose the battle every time you cook dinner.

Step 5: Round up to the nearest 500

The final number is rounded up to the nearest 500 BTU, because ACs are sold in standard sizes. Rounding down would leave you short on the hottest day of the year — and that is exactly when you need the unit most.

Worked example

Take a 15 × 12 ft bedroom (180 sq ft) with 9-foot ceilings, in a hot climate, sunny exposure, no kitchen, two people. Baseline: 180 × 20 = 3,600 BTU. Ceiling factor: 9 ÷ 8 = 1.125, so 3,600 × 1.125 = 4,050. Hot climate and sun push it up (roughly 1.1 × 1.1), landing near 4,900. No extra people, no kitchen. Round up to the nearest 500: 5,000 BTU, or about half a ton (12,000 BTU = 1 ton). Come for answers. Leave with confidence.