BTU Calculator

BTU Calculator
find the right heating & cooling size.

Calculate exact BTUs for any room based on size, insulation, and climate. No sign-up, instant results.

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How Many BTUs Do You Need?

BTU (British Thermal Unit) measures heating and cooling power. The industry rule of thumb is 20–25 BTU per square foot for cooling and 30–35 BTU per square foot for heating (Angi, 2026). Our calculator uses 20 for cooling and 30 for heating as baselines, then adjusts for your room's real conditions.

The BTU Formula

Cooling: Area (sq ft) Ă— 20 Ă— (Ceiling Ă· 8) Ă— Insulation Ă— Sun
Heating: Area (sq ft) Ă— 30 Ă— (Ceiling Ă· 8) Ă— Insulation Ă— Sun

  • Insulation: Poor Ă—1.20 (add 20%), Good Ă—0.90 (save 10%)
  • Sun exposure: Very sunny Ă—1.10, Shaded Ă—0.90
  • Ceiling height: scales with volume — a 10-ft ceiling needs 25% more than 8-ft

BTU to AC Tonnage

Air conditioners are sold by "tons" — 1 ton = 12,000 BTU/hour. A 24,000 BTU unit is a 2-ton system. The calculator shows tonnage automatically so you can shop with confidence.

Worked Example

A 20Ă—20 ft living room (400 sq ft), 8-ft ceilings, average insulation:
Cooling: 400 × 20 = 8,000 BTU → 0.7 tons
With two large sunny windows (+10%): 8,800 BTU — a 9,000–10,000 BTU unit is ideal.

Why Right-Sizing Matters

An undersized unit runs nonstop, never reaches the set temperature, and burns out early. An oversized unit cools too fast without removing humidity, leaving rooms clammy. A correctly sized unit (per ACCA Manual J principles) runs efficient cycles, controls humidity, and lasts longer.

Frequently Asked Questions

What is a BTU?
A British Thermal Unit is the heat needed to raise one pound of water by 1°F. In HVAC, BTU/hour rates heating and cooling power — higher BTU means a more powerful unit.
How many BTUs per square foot for cooling?
20–25 BTU per sq ft for cooling, 30–35 for heating (Angi). Poor insulation or extreme climates push you to the higher end; good insulation lets you use the lower end.
What size AC do I need for a 12Ă—12 room?
A 12×12 room is 144 sq ft → about 2,880 BTU cooling. A 5,000–6,000 BTU window unit covers it comfortably, which is the most common small-room size sold in the US.
How do I convert BTU to tons?
Divide by 12,000. 24,000 BTU Ă· 12,000 = 2 tons. Our calculator displays tonnage automatically next to the BTU result.
Does ceiling height change BTU needs?
Yes — BTU scales with air volume. A 10-ft ceiling needs 25% more capacity than an 8-ft ceiling for the same floor area. Enter your actual height for an accurate result.
Should I add extra BTUs for kitchens?
Yes — kitchens generate heat from ovens and appliances. HVAC pros typically add ~4,000 BTU for a kitchen. Our sun/insulation adjustments cover part of this; for heavy cooking areas, size up one step.