BTU Calculator

About this calculator

This calculator estimates the cooling and heating capacity a room needs, measured in British Thermal Units (BTU) per hour. Enter your floor area, ceiling height, sun exposure, number of occupants, and whether the space is a kitchen to get a recommended AC size — plus the equivalent tonnage and a heating output estimate for your climate zone.

How to read your results

The headline figure is the recommended cooling BTU/hr for your room. Below it, a companion stat shows the equivalent in refrigeration tons (1 ton = 12,000 BTU/hr), which is the unit used on most split-system and central AC specs. A second stat shows the estimated heating BTU based on your climate factor. The horizontal bar chart breaks the cooling total into its four components — base load, sun-exposure adjustment, extra-occupant load, and kitchen bonus — so you can see exactly which factors are driving the number up or down.

Worked example

A 250 sq ft bedroom with 8 ft ceilings, sunny exposure, 3 occupants, and no kitchen use. Climate factor left at the default 40 BTU per sq ft.

The base load is 5,000 BTU (250 × 20). The sunny adjustment adds 500 BTU (+10%). The third occupant adds 600 BTU. Total cooling: 6,100 BTU/hr, or 0.51 tons. Estimated heating: 10,000 BTU/hr.

Frequently asked questions

What does "BTU" mean in the context of air conditioners?

BTU stands for British Thermal Unit — the amount of energy needed to raise one pound of water by 1 °F. For air conditioners, the rating is BTU per hour (BTU/hr), which describes how much heat the unit can remove from a room in an hour. A higher number means a larger capacity.

Why does sun exposure change the BTU requirement?

A sunny room absorbs more solar heat through windows and walls, raising the cooling load. The Department of Energy and ENERGY STAR both recommend increasing the base estimate by 10% for very sunny rooms and reducing it by 10% for heavily shaded rooms.

Why does the kitchen add 4,000 BTU?

Cooking appliances — ranges, ovens, and microwaves — generate significant heat. ENERGY STAR sizing guidance adds a flat 4,000 BTU/hr to the cooling load for any space used as a kitchen to account for this extra heat source.

What is a "refrigeration ton" and how does it relate to BTU?

One ton of refrigeration equals 12,000 BTU/hr — originally the amount of heat needed to melt one ton of ice in 24 hours. Central air conditioners and mini-splits are typically sold in whole or half tons (1.5, 2, 2.5 tons), so the tonnage figure makes it easy to match the calculated load to a real product.

What should I use for the heating climate factor?

The climate factor (BTU per sq ft) reflects how hard your heating system needs to work. A range of 30–35 suits mild climates, 40–45 is typical for temperate zones, and 50–60 applies to cold northern regions. Your local HVAC contractor can provide a more precise Manual J value.

How it's calculated

Cooling BTU follows the U.S. Department of Energy rule of 20 BTU per square foot as the starting point (source: energy.gov/energysaver/room-air-conditioners). That base is multiplied by a sun-exposure factor: 0.9 for shaded rooms, 1.0 for normal, and 1.1 for sunny rooms. ENERGY STAR sizing guidance (energystar.gov) adds 600 BTU/hr for each occupant beyond the first two, and 4,000 BTU/hr if the space is used as a kitchen. The sum is then scaled by the ratio of the actual ceiling height to a standard 8 ft ceiling, and rounded to the nearest 100 BTU. Refrigeration tons are derived as BTU ÷ 12,000. Heating BTU is a separate, simpler estimate: floor area multiplied by a climate factor (default 40 BTU/sq ft) that you can adjust for your region.