What is a BTU?
A BTU is a British Thermal Unit — the energy needed to raise one pound of water by one degree Fahrenheit. It's a tiny amount of energy on its own. The number that matters for air conditioning is BTU per hour (BTU/hr), which is the rate at which an AC can pull heat out of a room. A 12,000 BTU/hr window unit removes 12,000 BTU of heat every hour the compressor runs.
The same unit shows up on furnaces, water heaters, and grills, just running the other direction — adding heat instead of removing it. In AC sizing conversations, "BTU" almost always means "BTU per hour, cooling capacity," and that's how the BTU Calculator uses it.
Getting the number right is the difference between an AC that runs quietly in the background and one that runs forever without cooling the room. Quick example: a 200 sq ft bedroom with average sun and two people in it needs roughly 4,000–5,000 BTU/hr. A 5,000 BTU window unit will do the job. A 12,000 BTU unit in the same room will short-cycle, leave the air clammy, and wear itself out years early.
How to use the BTU Calculator
The calculator starts with the residential rule of thumb — 20 BTU per square foot — and adjusts for the things that actually move the load: ceiling height, sun exposure, number of occupants, and whether cooking happens in the room. The output rounds up to the smallest standard AC size that meets your need, so you can shop directly.
- Measure the room area in square feet. For irregular rooms, sum each section. A 12×16 bedroom is 192 sq ft; a 20×14 living room is 280.
- Enter ceiling height. 8 ft is the residential standard; 9–10 ft is common in newer construction. The calculator scales linearly — a 10-ft ceiling room needs 25% more cooling than an 8-ft one of the same floor area, because you're cooling more air.
- Pick sun exposure. "Heavily sunny" means several hours of direct sun through south or west windows. "Heavily shaded" means north-facing or under tree cover. Most rooms are average.
- Enter regular occupants — the people typically in the room at the same time. Two is the baseline; each additional adult adds 600 BTU.
- Check the kitchen box if cooking happens in this room. Cooktops, ovens, and dishwashers add about 4,000 BTU of heat during use.
The recommended AC size shows below. Round UP when you're between sizes — undersized AC runs forever; oversized AC short-cycles, leaving humidity high. Both are bad, but undersized is the more punishing mistake on a hot day.
The formula behind the calculator
The baseline is empirical. ASHRAE engineering tables, simplified for residential use, give a sea-level, US-climate-zone-4, 8-ft-ceiling, average-insulation, two-occupant room a load of 20 BTU per square foot of floor area.
Required BTU/hr = (sq ft × 20) × ceiling factor × sun factor + occupant adjustment + kitchen adjustment
Each piece in plain terms:
- Ceiling factor = ceiling height ÷ 8. A 10-ft ceiling gives 1.25; a 9-ft ceiling gives 1.125.
- Sun factor = 0.9 for heavily shaded, 1.0 for average, 1.1 for heavily sunny.
- Occupant adjustment = (people − 2) × 600 BTU/hr, never negative.
- Kitchen adjustment = +4,000 BTU/hr if cooking happens in the room.
Worked example. A 200 sq ft bedroom, 8-ft ceiling, average sun, 2 occupants, no kitchen:
- Base: 200 × 20 = 4,000 BTU/hr
- Ceiling: 4,000 × (8 ÷ 8) = 4,000
- Sun: 4,000 × 1.0 = 4,000
- Occupants: (2 − 2) × 600 = 0
- Kitchen: 0
- Total: 4,000 BTU/hr
That rounds up to the standard 5,000 BTU window unit — the smallest size sold in the US, and exactly what most people put in a small bedroom.
Same room with three people, west-facing windows, and a 10-ft ceiling:
- Base: 200 × 20 = 4,000
- Ceiling: 4,000 × 1.25 = 5,000
- Sun: 5,000 × 1.1 = 5,500
- Occupants: (3 − 2) × 600 = 600
- Total: 6,100 BTU/hr
That rounds up to 6,000–8,000 BTU. The same 200 sq ft of floor, three real-world adjustments, and the AC requirement jumps 50%. The rule-of-thumb number is fine as a starting point but rarely the final answer.
Room size to BTU recommendation
The table below shows the typical sizing for common room dimensions under average residential conditions — 8-ft ceiling, average sun, two occupants, not a kitchen. Use it as a sanity check against the calculator's output.
| Room size | Typical use | Recommended BTU/hr | Standard AC size |
|---|---|---|---|
| 100–150 sq ft | Small bedroom, office | 5,000 | 5,000 BTU window |
| 150–250 sq ft | Bedroom | 6,000 | 6,000 BTU window |
| 250–350 sq ft | Large bedroom, small living room | 8,000 | 8,000 BTU window/mini-split |
| 350–450 sq ft | Living room, master suite | 10,000 | 10,000 BTU window/mini-split |
| 450–550 sq ft | Large living room, studio | 12,000 | 12,000 BTU (1 ton) mini-split |
| 550–700 sq ft | Open-plan living/dining | 14,000 | 14,000 BTU portable/mini-split |
| 700–1,000 sq ft | Small apartment | 18,000 | 18,000 BTU (1.5 ton) mini-split |
| 1,000–1,200 sq ft | Apartment, small house zone | 21,000 | 21,000–24,000 BTU mini-split |
| 1,200–1,500 sq ft | Whole-house small home | 24,000 | 24,000 BTU (2 ton) central/mini-split |
| 2,000 sq ft house | Average US home | 36,000–48,000 | 3–4 ton central AC |
One ton of cooling = 12,000 BTU/hr. The naming comes from the original definition: one ton of ice melting over 24 hours produces about 12,000 BTU/hr of cooling effect. Modern AC catalogs still use both units, sometimes in the same brochure, so it pays to know they're the same thing.
Undersizing vs. oversizing — both are bad
The instinct most people have is to oversize, on the theory that bigger means better cooling. It's almost always the wrong call.
Undersized AC. The compressor runs constantly without keeping the room cool. Duty cycles disappear; the unit never gets to rest. Energy bills climb because the AC is on 100% of the day. The room stabilizes 5–10°F above the thermostat setpoint on hot afternoons. Compressors and condenser coils wear out faster from continuous load. The unit usually dies several years earlier than it should. This is the more common mistake — people pick by price, not by capacity.
Oversized AC. Counterintuitively, also bad. The unit cools the room to setpoint fast, then shuts off — known as "short cycling." Each cycle is too brief to dehumidify properly, because the cold coils need sustained operation to condense water out of the air. Result: the room feels clammy, mildew gets a foothold, and the AC's short life is shortened further because compressor starts are the highest-stress event in the cycle. Energy costs may actually be higher than a correctly-sized unit because of the inefficiency of frequent restarts. Modern variable-speed inverter ACs handle oversizing better than fixed-speed units do, but matching the size to the load is still the right approach.
The sweet spot is "just enough." A correctly sized AC runs in 15–20 minute cycles, pulls humidity down to comfortable levels (40–55% RH), and rests for a few minutes between cycles. Energy bills are predictable, the unit lasts its full 10–15 year design life, and nobody notices it because that's how good HVAC works.
What about windows, insulation, and climate?
The calculator's 20 BTU/sq ft baseline assumes typical modern residential conditions. Several real-world variables move the load up or down meaningfully:
- Windows. The baseline assumes a window-to-floor ratio of about 15%. Rooms with much more glass (sunrooms, picture windows, glass walls) can have double or triple the load — use a Manual J calculation, the professional residential cooling-load standard, for those cases. Single-pane glass leaks 4–5× more heat than double-pane.
- Insulation. Poorly insulated rooms — old construction, attic spaces, sunrooms, additions — need 30–50% more capacity. Top-floor rooms with attic above need extra capacity because hot attic air radiates down through the ceiling.
- Climate. The 20 BTU baseline is calibrated for US climate zone 4, which covers most of the country. In hot-humid climates (Florida, Gulf Coast), latent load (humidity removal) accounts for half of the total — use a slightly larger unit or one specifically rated for dehumidification. In hot-dry climates (the Southwest), evaporative cooling is dramatically cheaper than refrigeration for the same effective temperature, and worth considering as an alternative.
- Curtains and shades. Heavy curtains during the hottest part of the day cut sun load significantly — sometimes enough to drop you a full AC size in a west-facing room.
For most rooms in most houses, the calculator's adjustments cover it. For unusual cases, a Manual J calculation from an HVAC contractor is the right answer — they'll measure your walls, count your windows, and produce a load number that's accurate within a few hundred BTU.
Related tools
Sizing the room first usually leads to a few related measurements:
- Square Footage Calculator — quick area math for irregular rooms before they go into the BTU formula.
- Temperature Converter — Fahrenheit to Celsius and back. Useful when reading specs from European mini-split brands or any AC documentation written in metric.
- Horsepower Calculator — the other power-unit conversion you'll often see in HVAC and motor contexts, since compressor specs sometimes get written either way.
Frequently asked questions
What's a BTU?
British Thermal Unit — the energy required to raise one pound of water by 1°F. For AC sizing, BTU/hr is the rate of heat the unit can remove per hour. A 12,000 BTU/hr AC removes 12,000 BTU of heat from the room every hour. The same energy unit is also used for heating (boilers, furnaces) but with the opposite direction — adding heat instead of removing it.
What's the relationship between BTU and tons?
One ton of cooling = 12,000 BTU/hr. The name is historical: one ton of ice melting over 24 hours removes about 12,000 BTU per hour of cooling effect — the original air conditioning benchmark. Modern AC sizes: 1 ton (12k BTU) cools ~600 sq ft. 2 ton (24k) cools ~1,200 sq ft. Central AC for a typical 2,000 sq ft house is 3–4 ton (36k–48k BTU). Mini-splits and window units come in 5k–24k BTU sizes.
Why is the base 20 BTU per square foot?
An empirical rule for typical residential conditions: 8-ft ceiling, average insulation, average sun exposure, 1–2 occupants, US climate zone 4 (most of the country). The 20 BTU/sq ft figure comes from ASHRAE engineering tables simplified for residential use. For commercial spaces or extreme climates (hot/humid South, hot/dry desert), use a Manual J load calculation instead — the professional residential cooling-load standard.
What happens if I get an undersized AC?
It runs constantly without keeping the room cool. The compressor doesn't get duty cycles to cool itself; condenser coils get overworked; energy bills go up; the unit fails years earlier than it should. The room temperature typically stabilizes 5–10°F above the thermostat setpoint on hot days. Undersizing is the most common mistake — people pick by price rather than by capacity.
What about an oversized AC?
Counterintuitively, also bad. An oversized AC cools the room to setpoint fast, then shuts off — short cycling. Each cycle is too brief to dehumidify properly. The room feels clammy, mildew risk goes up, and the compressor wears out from frequent restarts. Modern variable-speed inverter ACs handle oversizing better than fixed-speed units do, but matching size to load is still the right call.
Do I add for windows directly?
Indirectly, through the sun-exposure setting. The 20 BTU/sq ft baseline assumes a typical window-to-floor ratio of around 15%. For a room with much more glass — sunrooms, picture windows, walls of glass — use a Manual J calculation; the load can easily double. Single-pane glass leaks 4–5× more heat than double-pane, so window quality matters as much as window area. Heavy curtains during the hottest part of the day reduce the load significantly.
Does insulation matter?
Yes, a lot. The calculator's 20 BTU baseline assumes average modern insulation. Poorly insulated rooms (old construction, attic spaces, sunrooms) need 30–50% more capacity. Top-floor rooms with attic above need extra capacity because hot attic air radiates down. In hot-humid climates, humidity load is half the total — use a slightly bigger unit or one rated for high dehumidification efficiency.
Is this for cooling or heating?
Cooling. AC capacity is normally specified in BTU/hr cooling. For heating, similar calculations exist but use different multipliers — heating load is dominated by air leakage and insulation R-values rather than direct sunlight. Heat pumps deliver both cooling and heating; for a heat pump, you typically size to the heating load (which is larger in cold climates) and the cooling side ends up slightly oversized as a consequence.