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SZ-04Sizing · HVAC

BTU and AC tonnage calculator

A room-level cooling estimate from the ENERGY STAR room air conditioner chart, with its adjustments for sun, shade, people and kitchens — drawn to scale. Sizing a whole house or a central system? That needs a Manual J load calculation.

Room cooling estimate

Example: a 25 × 20 ft kitchen and dining area used by three people. Change any number.

Cooling capacity for this room Example room

About 16,600 BTU per hour, 1.38 tons, for this room.

= 1.38 tons4.86 kW

See it drawn to scale Change the numbers

Room, chart band and capacityDrawn to scale: plan in feet, capacity in BTU/h
Room cooling estimate for the example room, drawn to scaleFloor plan 25′-0″ by 20′-0″, 500 square feet, in the ENERGY STAR band 450–550 square feet (12,000 BTU per hour). Capacity bar to scale: chart 12,000, sun or shade none, people +600, kitchen +4,000, total 16,600 BTU per hour.ARoom plan25′-0″20′-0″Floor area500 sq ft3 people · kitchen051015ftBEnergy Star room chartsq ft → BTU/h100–1505,000150–2506,000250–3007,000300–3508,000350–4009,000400–45010,000450–55012,000550–70014,000700–1,00018,0001,000–1,20021,0001,200–1,40023,0001,400–1,50024,0001,500–2,00030,0002,000–2,50034,000CCooling capacityBTU/h, same scaleThis room134020,00016,600 BTU/h= 1.38 tons (÷ 12,000)
  1. ENERGY STAR chart450–550 sq ft band12,000
  2. Sun: normalno change0
  3. People over two1 × 600+600
  4. Kitchenunit used in a kitchen+4,000
  5. Room estimate16,600 BTU/h
Chart value
12,000450–550 sq ft
Tons
1.38BTU/h ÷ 12,000
Kilowatts
4.86cooling, not input

12,000 × 1.00 + 1 × 600 + 4,000 = 16,600 BTU/h16,600 ÷ 12,000 = 1.38 tons

Already showing the example.

Change the numbers

Room size

Wall to wall. 12 ft 6 in = 12.5 ft.

The other way, in feet.

Floor area500 sq ftlength × width. The chart covers 100–2,500 sq ft.
Sun on the room

Heavily shaded −10%, very sunny +10% of the chart value.

The chart allows for two; each extra person adds 600 BTU/h.

Your estimate: 16,600 BTU/h, 1.38 tons. Back to the result

How the math works

The calculator looks the room up in the ENERGY STAR room air conditioner chart, then applies the chart’s four adjustments. Only the sun/shade step is a percentage; the others are fixed additions.

  1. Floor arealength × width (ft); a triangle is length × width ÷ 2Convert inches to feet first: 12 ft 6 in = 12.5 ft. Split odd shapes into rectangles and triangles and add them.
  2. Chart valuethe ENERGY STAR band that holds the areaThe chart’s bands share their edge numbers; an exact edge (150, 250 …) goes to the smaller band. That tie-break is our convention, because ENERGY STAR warns against oversizing.
  3. Sun or shadechart value × 0.90 (heavily shaded) or × 1.10 (very sunny)Applied first, to the chart value only. ENERGY STAR does not set the order; this is our convention.
  4. People+ 600 BTU/h for each regular occupant over two
  5. Kitchen+ 4,000 BTU/h if the unit is used in a kitchen
  6. Tons and kWBTU/h ÷ 12,000 = tons · BTU/h × 0.2930711 ÷ 1,000 = kWUnit conversions from NIST (1 ton of refrigeration = 12,000 Btu/h). They do not make the estimate more precise.

Worked examples

  • 500 sq ft kitchen and dining area, three people12,000 + 1 × 600 + 4,000 = 16,600 BTU/h = 1.38 tons
  • 400 sq ft heavily shaded den, four people9,000 × 0.90 + 2 × 600 = 9,300 BTU/h = 0.78 tons
  • 1,000 sq ft very sunny open kitchen, five people18,000 × 1.10 + 3 × 600 + 4,000 = 25,600 BTU/h = 2.13 tons · 7.50 kW

The ENERGY STAR room chart

Area to be cooled and the capacity the chart gives (ENERGY STAR, read October 4, 2026; the page shows no revision date). A band includes its upper number.
Area (sq ft)BTU per hourTons
100–1505,0000.42
150–2506,0000.50
250–3007,0000.58
300–3508,0000.67
350–4009,0000.75
400–45010,0000.83
450–55012,0001.00
550–70014,0001.17
700–1,00018,0001.50
1,000–1,20021,0001.75
1,200–1,40023,0001.92
1,400–1,50024,0002.00
1,500–2,00030,0002.50
2,000–2,50034,0002.83

Before you use the number

  • One room, one room unit. The chart is ENERGY STAR’s guide for window, through-the-wall and similar room air conditioners. It is not a size for a central air conditioner, furnace or heat pump.
  • Whole houses need a load calculation. ENERGY STAR and the Department of Energy say central equipment should be sized from an ACCA Manual J load calculation, and ENERGY STAR calls that more accurate than square-footage rules of thumb. ACCA states that codes in most places require it.
  • Bigger is not better. An oversized room unit cools the air before it removes the humidity and leaves the room damp and clammy (ENERGY STAR).
  • 8-foot ceilings are assumed. With higher ceilings ENERGY STAR says you may want a larger unit; it gives no factor, so the calculator adds none.
  • No heating estimate. We found no federal or code-body source for a square-foot heating method; the guidance points to Manual J.
  • Tons are a unit, not a model size. 1 ton = 12,000 BTU/h by definition. Matching a number to a product is the installer’s call, using the maker’s data.

More on the chart, the adjustments and Manual J: BTU per square foot: the room chart vs Manual J.

Sources

Opened and checked October 4, 2026. The worked examples are our own arithmetic.

  1. Room Air Conditioners (sizing chart and adjustments)ENERGY STAR (U.S. EPA and DOE)
  2. Clean Heating and CoolingENERGY STAR (U.S. EPA and DOE)
  3. ENERGY STAR Qualified Heating Equipment (PDF)ENERGY STAR (U.S. EPA and DOE)
  4. HVAC Proper Sizing of HVAC SystemsU.S. Department of Energy, Building Science Education Solution Center
  5. Manual J Residential Load CalculationAir Conditioning Contractors of America
  6. NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity or field of scienceNational Institute of Standards and Technology

  • 4.01Guide

    BTU Per Square Foot: The Room Chart vs Manual J

    What ENERGY STAR's room AC chart says about BTU per square foot, how its four adjustments work, and why whole houses need a Manual J load.

    9,000 × 0.90 + 2 × 600 = 9,300 BTU/h

    Length
    6 min
    Updated

  • 4.05Guide

    Voltage Drop Explained: The K Formula, 3% and 5%

    How to work voltage drop with the K-factor formula, copper vs aluminum, single vs three phase, and what the NEC's 3% and 5% notes really say.

    √3 × 12.9 × 150 × 200 ÷ 211,600 = 3.17 V

    Length
    6 min
    Updated

Tools for the job

Tool listHVAC4 itemsSpecs read

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  1. Digital manifolds and probesFieldpiece SM482V

    SM482V SMAN Wireless Refrigerant Manifold (4-Port)

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    VP87 8 CFM Vacuum Pump

    • 8 CFM two-stage rotary-vane pump with a 3/4 HP brushless DC motor; ultimate vacuum 15 microns.

    Specs: Fieldpiece product page, read

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  3. Recovery machines and scalesFieldpiece MR45

    MR45 Digital Recovery Machine

    • Oil-less twin-cylinder compressor with a 1 HP variable-speed DC motor; runs on 95 to 130 VAC.

    Specs: Fieldpiece product page, read

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  4. Flaring and swaging toolsNAVAC NEF6LM

    BreakFree Power Flaring Tool NEF6LM

    • Cordless flaring tool for copper tube in 1/4, 3/8, 1/2, 5/8 and 3/4 in. OD.

    Specs: NAVAC product page, read

    View on Amazon for NAVAC NEF6LM(paid link)

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