How Roomtherm calculates

Cooling: the ENERGY STAR room chart

The cooling number starts from the chart on ENERGY STAR's room air conditioner page (EPA, retrieved 2026-11-09; the same rows appear in a 2013 copy). It lists 14 bands of floor area, «Area To be Cooled (square feet)», each with a «Capacity Needed (BTUs per hour)»:

The page then gives four adjustments, which the calculator applies as written: If the room is heavily shaded, reduce capacity by 10 percent. If the room is very sunny, increase capacity by 10 percent. If more than two people regularly occupy the room, add 600 BTUs for each additional person. If the unit is used in a kitchen, increase capacity by 4,000 BTUs. Floor area is length × width; ENERGY STAR's advice is Always measure in feet, converting the inches to fractions of a foot.

Our assumption: band edges

The chart says «100 up to 150», then «150 up to 250», and does not say which row 150 itself is in. We put an edge in the lower row: the first row covers 100 to 150 sq ft inclusive and every other row runs from just above its first number up to its second. A 10 × 15 ft room (150 sq ft) gets 5,000 BTU/h; at 151 sq ft it is 6,000. The other reading would give 6,000 at 150 sq ft. Below 100 sq ft we use the smallest row, 5,000 BTU/h, and say so on the result; over 2,500 sq ft the calculator shows no number.

Our assumption: adjustment order

The page does not say whether the percentages apply before or after the people and kitchen additions. We apply them to the room only, in the order of the sentences: chart value, then ceiling, then sun, then + 600 per extra person, then + 4,000 for a kitchen. For a 20 × 25 ft very sunny room with four people that gives 14,400 BTU/h; adding the people first and the 10 percent last would give 14,520. For a shaded 150 sq ft kitchen: 8,500 our way, 8,100 the other way.

Our assumption: tall ceilings

ENERGY STAR notes The capacities are calculated based on an 8-foot ceiling, and Carrier says Standard calculations assume 8-foot ceilings. Neither gives a number for a taller room. We scale by volume: the chart value is multiplied by the ceiling height ÷ 8, with the height rounded to 0.1 ft first. Lower ceilings are not reduced. A metric 2.44 m ceiling is 8.0 ft and changes nothing; 2.7 m is 8.9 ft and turns a 6,000 BTU/h bedroom into 6,675.

Our assumption: next common size

The result is rounded up to a whole BTU/h and then to the smallest common size that is at least as large. The lists come from ENERGY STAR's certified product data (retrieved 2026-11-09) by one rule: capacities that are a multiple of 1,000 BTU/h with at least 20 certified room air conditioner models, or at least 500 certified mini-split heat pump models. Window and portable: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 24,000 BTU/h. Mini-split systems: 9,000, 12,000, 18,000, 24,000, 30,000, 36,000, 48,000 BTU/h (the list mixes single- and multi-zone systems). Above the largest size the result says so instead of naming one. ENERGY STAR does not say to round up; King Electric does, for heaters: If wattage recommendation falls between two heaters, select the heater with higher wattage.

Our assumption: reverse chart

The converter and the AC size guide ask the opposite question: what is the largest room a given size covers? We try every chart row with the same adjustments and keep the largest row whose need is not more than the size. With average sun and two people, 12,000 BTU/h covers 550 sq ft (51.1 m²).

Heating: watts per square foot

King Electric's heater sizing sheet (manufacturer instructions, retrieved 2026-11-09): King Electric recommends approximately 6 watts per square foot for newer homes and 10 watts per square foot for older homes. Newer means Mostly 1980’s to present, R19 walls, R30 floors and ceilings, standard windows thermal pane, 8 ft ceilings; older, Prior to 1980, R11 walls, R19 ceilings, single pane windows, 9 ft ceilings. It adds 2 W per sq ft for homes located in higher than average elevations or extreme cold climates, and for Rooms with greater than average window space, increase wattage by 50%. The sheet's own conversion is (BTU = WATTS X 3.412); we use the exact NIST factor below.

Sources disagree on the rate. Dimplex US uses 10 watts of heat per square foot of space in a standard insulated room, which is King's rate for an older home. For 100 sq ft that is 1,000 W against King's 600 W for a newer home. We follow King's two levels because they say what each level means.

Our assumptions here: King's rates are written for US houses, and using them for UK and European rooms is ours; so is extending the per-square-foot rate past the sheet's table, which stops at 400 sq ft, up to 2,500 sq ft. The heating rates already assume the ceiling heights above, so no ceiling factor is applied.

Our assumption: cold climate with large windows

We add the 2 W to the rate and then apply the 50 %: watts = area × (rate + 2) × 1.5. A 150 sq ft newer room with both gives 1,800 W; adding 2 W per sq ft after the 50 % would give 1,650 W.

Our assumption: little or no insulation

For garages, sheds and older shops the calculator uses 50 BTU/h per sq ft. NDSU Extension's AE1620, Heating Farm Shops (2025) says that a few years ago farm shop heating systems should provide around 50 Btu per square foot per hour, and that with better insulation shops commonly need only 20. NDSU gives 50 as the former general rule, not as a rate for uninsulated buildings; using it for this level is our assumption, and no published example checks it. A 480 sq ft garage gets 24,000 BTU/h, 7,034 W. The cold-climate and window options do not apply to this level.

Radiators: the ΔT50 rating

Zehnder: In accordance with EN442 we publish our radiator outputs at ∆T=50ºC, with ΔT defined as (∆T is calculated as (Flow temp + return temp)/2 – room temp). Output at another ΔT is rating × (ΔT ÷ 50)^1.3; Zehnder's calculator assumes an n coefficient of 1.3 and Stelrad writes Average coefficient of 130 is used in the example above. The rating to look for is the room's heat divided by that factor, rounded up. We compute the factor; Stelrad prints 0.515 for ΔT30, and the formula gives 0.51475. ΔT is accepted from 5 to 75 °C.

MCS 021 converts ratings with its own factors (for example 2349 x 0.825 = 1938W), so a heat pump installer's radiator sizes may differ from ours by a few percent.

Units

NIST Special Publication 811: the International Table BTU is exactly 1.055 055 852 62 kJ, so 1 BTU/h = 0.2930711 W, and a ton of refrigeration is 12,000 BTU/h. Metric lengths are converted with 1 ft = 0.3048 m and 1 sq ft = 0.09290304 m², both exact. Watts are rounded to whole numbers, kilowatts and tons to two decimals.

Why calculators give different numbers for the same room

On 2026-11-09 we ran our own test: one room, 12 × 14 ft with average sun and two people, typed into six online sizing calculators. We do not name them here, because the point is the methods, not the sites. They answered 5,201, 6,000 (two of them), 6,229.79, 26,721 and 40,320 BTU/h. The two that gave 6,000 use the ENERGY STAR chart, as Roomtherm does. The 5,201 came from the area times 30 BTU/h per sq ft and the calculator's own percentage factors; the 40,320 came from the room's volume times an insulation factor whose default was «Poor»; the other two named no method. Roomtherm shows every step, so you can see which rule produced its number. The chart gives 6,000 BTU/h.

Climate is another source of spread. Bryant gives ranges of Cool climates: ~20–30 BTUs per sq. ft. up to Hot climates: ~40–60 BTUs per sq. ft.. These are ranges with no stated basis, so the calculator does not apply a climate factor; we show them here as context only.

Limits

• The cooling chart is ENERGY STAR's guidance for a room air conditioner in one room. Its capacities are calculated based on an 8-foot ceiling, and it does not know your climate.
• Metric ceilings: the chart assumes 8 ft, 2.44 m. A 2.5 to 2.7 m ceiling raises the number by 2.5 to 11.25 percent.
• The heating rates are rules of thumb for one room or garage. A full heat-loss calculation (walls, windows, air changes and design temperatures) can differ; the worked example in a BC plumbing textbook comes to Total heat loss: 4,280 BTUH. NDSU adds that you need heat loss calculations to properly size a farm shop heater.
• Portable air conditioners carry two ratings, ASHRAE and SACC. The US Department of Energy found their net cooling generally significantly lower than the older measurement suggests, mainly because of air infiltration, so compare the SACC figure with the result.
• Over 2,500 sq ft, whole houses, furnaces and ducted systems need a Manual J load calculation by an HVAC contractor.
• Roomtherm sizes heat output only. It does not cover wiring, circuits, gas or other fuel-burning heaters; those follow the heater's instructions and local code.

How the numbers are checked

The calculator's code is tested against 55 reference cases, each worked by hand from the sources above or read from a source's own example, including every assumption on this page. The tables in the guides are built from the same code when the site is published, and sampled cells are compared with a separate reference model.

Changelog

• 2026-11-09: v1 — launch: cooling from the ENERGY STAR room chart with ceiling, sun, people and kitchen adjustments; heating by insulation level; the ΔT50 radiator rating; the unit converter with the reverse chart.