How to use the BTU calculator
- Measure the room and add adjoining space that will share the same unit.
- Choose a mode: the ENERGY STAR chart for a window unit, mini-split or similar room AC; the 20 Btu/h rule of thumb for a quick cooling check; or furnace / heat pump for a planning heating load.
- For cooling, set occupants, sun and whether the unit serves a kitchen — those are the ENERGY STAR adjustments. For heating, pick a climate band.
The result is in Btu/h and in refrigeration tons (12,000 Btu/h = 1 ton). Insulation and spray-foam takeoffs stay on the insulation and spray-foam calculators.
The ENERGY STAR chart and the 20 Btu rule
ENERGY STAR publishes a room-area chart, long used with AHAM guidance, that steps capacity with floor area. A 250–300 sq ft room is 7,000 Btu/h; a 300–350 sq ft room is 8,000 Btu/h; 350–400 is 9,000, and so on up to 34,000 Btu/h at 2,000–2,500 sq ft. On the boundaries, this page uses the top of each published row (300 sq ft stays in the 7,000 Btu/h row).
ENERGY STAR also adjusts a cooling size for a very sunny room (+10%), a heavily shaded room (−10%), each occupant beyond two (+600 Btu/h) and a kitchen (+4,000 Btu/h). The Department of Energy’s Energy Saver pages still describe about 20 Btu/h per sq ft as a starting point only. Heating uses CalcTheJob planning factors of 25, 35, 45 or 55 Btu/h per sq ft for hot, mixed, cold and very-cold bands. The 35, 45 and 55 steps sit inside Angi’s roughly 30–60 Btu/h per sq ft furnace-sizing range; the 25 Btu/h hot-climate step is below that range. The steps are not taken from a published table and are not a Manual J.
The chart itself assumes a typical 8 ft ceiling. The 20 Btu/h and heating modes scale with ceiling height ÷ 8.
A worked example: 16 × 20 ft room
A 16 × 20 ft room is 320 sq ft. On the ENERGY STAR chart that is the 300–350 sq ft row, 8,000 Btu/h, or 0.67 tons, with two people and typical sun. If the same unit also cools a kitchen, add 4,000 Btu/h for 12,000 Btu/h (1 ton). The 20 Btu/h rule of thumb on the same room is 320 × 20 = 6,400 Btu/h. A heating check in a cold climate is 320 × 45 = 14,400 Btu/h.
Why this is not a Manual J
ACCA Manual J accounts for wall and attic insulation, window area and orientation, infiltration, duct losses and the actual design temperatures for the house. A rule of thumb cannot replace that when a permit or a contractor’s design requires it. Use this page to get into the right size class before you talk with an HVAC professional. A UA planning heat-loss check is on the heat loss calculator; airflow and duct size are on the CFM and duct size calculators.
Sources & assumptions
How this calculator gets its numbers. Full method: our methodology.
- Cooling-chart mode uses the ENERGY STAR room air conditioner sizing chart, with 300 sq ft at the top of the 7,000 Btu/h row and 300–350 sf at 8,000 Btu/h.
- ENERGY STAR cooling adjustments: sunny +10%, shaded −10%, +600 Btu/h per occupant beyond two, +4,000 Btu/h for a kitchen. The chart itself is not scaled for ceiling height.
- The 20 Btu/h per sq ft cooling rule is a DOE Energy Saver starting point only, scaled by ceiling height ÷ 8.
- Heating uses CalcTheJob planning factors of 25/35/45/55 Btu/h per sq ft for hot/mixed/cold/very-cold bands, also scaled by height. The 35–55 steps sit inside Angi’s roughly 30–60 Btu/h per sq ft furnace-sizing range; the 25 hot-climate step is below that range. The steps are not copied from a published table. This is not a Manual J load.
Sources
- ENERGY STAR / U.S. EPA, Room Air Conditioners — product and sizing guidance. Room-area capacity chart (for example 300–350 sq ft = 8,000 Btu/h) and the sun, occupancy and kitchen adjustments.
- U.S. Department of Energy, Energy Saver: Air Conditioning. About 20 Btu/h per square foot described as a starting point only, not a substitute for a proper load calculation.
- Angi, How Much Does It Cost to Install a New Furnace? Updated Sep 8, 2026: furnace sizing discussed in a roughly 30–60 Btu/sf range. Used only as context for the width of the heating bands, not as the source of the 25/35/45/55 steps.
Last reviewed . Product figures vary by brand; the label on what you buy wins.
Frequently asked questions
How many BTUs do I need to cool a 16 × 20 room?
The ENERGY STAR chart puts a 320 sq ft room at 8,000 Btu/h, or 0.67 tons. A kitchen served by the same unit adds 4,000 Btu/h. The 20 Btu/h starting point is 6,400 Btu/h on the same room.
How many BTUs are in one ton of cooling?
12,000 Btu/h equals one refrigeration ton. The calculator reports both figures.
Is this a Manual J load calculation?
No. It is a planning size from the ENERGY STAR room chart, a 20 Btu/h rule of thumb, or a climate heating factor. A Manual J is still required where a permit or an HVAC design calls for one.
Does better insulation change the BTU number here?
Not automatically. Insulation and spray foam are estimated on their own pages. A tighter, better-insulated room often needs less capacity, which is one reason a Manual J can come in below a rule of thumb.