BTU Calculator
Sizing HVAC equipment starts with the load: how many BTUs per hour the space gains or loses. This BTU calculator multiplies your room’s square footage by a climate-based rate, corrects for ceiling height, then adjusts for insulation quality and sun exposure. You get a planning-level BTU/hr figure plus the equivalent tonnage and wattage for comparing equipment — with the reminder that final sizing belongs to a Manual J calculation.
Rule-of-thumb load per square foot for your region.
Afternoon sun through large windows raises cooling load.
Heating/cooling load
14,000BTU/hr
- Equivalent tonnage
- 1.17tons
- Approximate wattageThermal equivalent (1 BTU/hr ≈ 0.293 W).
- 4,103W
- Base load (before adjustments)Area × 35 BTU/ft² × ceiling factor 1.00.
- 14,000BTU/hr
How it was calculated
- Base: 400 ft² × 35 BTU/ft² × (8 ÷ 8) = 14000 BTU/hr
- Adjusted: 14000 × 1 (insulation) × 1 (sun) = 14000 BTU/hr
- Tonnage: 14000 ÷ 12,000 = 1.17 tons
How it works
The base load is area × a climate rate — a rule of thumb for BTU per square foot: mild 25, moderate 35, cold 50, severe 65. Each rate bundles typical construction, windows, and indoor-outdoor temperature differences into one planning number.
Ceiling height scales the result against an 8-foot baseline: a 10-foot ceiling multiplies the base by 10 ÷ 8 = 1.25, because there is more air volume to condition.
Two multipliers refine the base: insulation (poor ×1.2, average ×1.0, good ×0.85) and sun exposure (shaded ×0.9, average ×1.0, sunny ×1.1). Both are rules of thumb — a sun-baked room with single-pane windows gains and loses heat faster than the base assumes.
Tonnage = BTU/hr ÷ 12,000 (one ton of cooling equals 12,000 BTU/hr), and watts ≈ BTU/hr × 0.293, the standard thermal-to-electric conversion.
Worked examples
Bedroom in a moderate climate
A 400 ft² bedroom with 8 ft ceilings, average insulation, and average sun: 400 × 35 × 1.0 = 14,000 BTU/hr. That is 14,000 ÷ 12,000 ≈ 1.17 tons — a 1.5-ton unit would cover it with comfortable margin, drawing about 4,103 W at full load.
Sunny living area, cold climate
An 800 ft² living area with 10 ft ceilings in a cold climate, poor insulation, sunny exposure: base = 800 × 50 × 1.25 = 50,000 BTU/hr; adjusted = 50,000 × 1.2 × 1.1 = 66,000 BTU/hr (≈ 5.5 tons, ≈ 19,343 W). Poor insulation alone adds 10,000 BTU/hr versus average — a strong case for air sealing first.
How to measure
- Measure wall-to-wall inside dimensions and multiply for square footage; use the Square Footage Calculator for odd shapes.
- Count only conditioned space — exclude garages, unheated porches, and vented attics.
- Measure ceiling height from floor to finished ceiling; for vaulted areas use the average height.
- Judge insulation by attic depth and wall age: under 6 inches of attic insulation or pre-1980 walls usually means "poor".
- Rate sun exposure by afternoon sun: west-facing glass with no shade counts as "sunny".
Assumptions & limitations
Planning estimate
- Climate rates (25/35/50/65 BTU per ft²) are a rule of thumb for planning, not measured values.
- Insulation and sun multipliers are rules of thumb; windows, air leakage, occupants, and appliances are not modeled.
- Heating and cooling are approximated with the same load figure; real heating loads in severe climates run higher.
- planning estimate only — Manual J load calculation by a professional is required for equipment selection; this tool does not replace it.
- Results assume reasonably air-sealed construction; leaky ductwork or missing insulation raises real loads.
Frequently asked questions
A British Thermal Unit is the energy needed to raise one pound of water by one degree Fahrenheit. BTU/hr — what this calculator outputs — is a rate of energy flow, i.e., heating or cooling power. A 12,000 BTU/hr window unit moves twice the heat per hour of a 6,000 BTU/hr one.
One ton of cooling equals 12,000 BTU/hr, from the historical rate at which a ton of ice melts. Residential central systems are sold in half-ton steps from 1.5 to 5 tons. Divide any BTU/hr figure by 12,000 to get tons.
Conditioning is about air volume, not just floor area. A room with 10-foot ceilings holds 25% more air than the same footprint with 8-foot ceilings, so it needs roughly 25% more capacity. The calculator scales against the 8-foot baseline used by most rules of thumb.
No — oversizing is a classic mistake. An oversized AC short-cycles: it cools quickly, shuts off, and never runs long enough to dehumidify, leaving the house cold and clammy while wearing out faster. Size to the load, not above it.
No. This is a planning estimate for budgeting and comparing equipment. A Manual J accounts for window orientation, wall assemblies, infiltration, ductwork, and design temperatures room by room. Get one before buying equipment.