KKV Labs

Free HVAC calculator

AC tonnage calculator

This AC tonnage calculator gives a rule of thumb size from square footage: it reads the ENERGY STAR sizing chart, adjusts for sun, people and a kitchen, and divides by 12,000 BTU per hour per ton. A 1,500 sq ft space comes out at 24,000 BTU/h, or 2 tons. The size you install should come from an ACCA Manual J load calculation.

sq ft

The floor area of the rooms the system will cool.

people

Over two adds 600 BTU/h each.

Rule of thumb, not Manual J. The square feet figure comes from the ENERGY STAR room AC sizing chart. A real system size comes from an ACCA Manual J load calculation for the house.

AC size for 1,500 sq ft

2.00tons

24,000 BTU per hour. Next half ton up: 2.0 tons.

ENERGY STAR chart figure
24,000 BTU/h
Sun adjustment
0 BTU/h
Extra people
0 BTU/h
Kitchen
0 BTU/h
Estimate
24,000 BTU/h
Tons (BTU/h divided by 12,000)
2.00
Square feet per ton
750

Chart

AC size by square feet

ENERGY STAR's room AC sizing chart, with the tons added. The calculator starts from these rows.

AC size by square feet: ENERGY STAR chart in BTU per hour and tons
Area (sq ft)BTU per hourTons
100 to 1505,0000.42
150 to 2506,0000.50
250 to 3007,0000.58
300 to 3508,0000.67
350 to 4009,0000.75
400 to 45010,0000.83
450 to 55012,0001.00
550 to 70014,0001.17
700 to 1,00018,0001.50
1,000 to 1,20021,0001.75
1,200 to 1,40023,0001.92
1,400 to 1,50024,0002.00
1,500 to 2,00030,0002.50
2,000 to 2,50034,0002.83

Conversion

BTU to tons, common sizes

One ton of cooling is 12,000 BTU per hour, so tons = BTU per hour divided by 12,000.

BTU to tons for common AC sizes
TonsBTU per hour
1.5 tons18,000
2 tons24,000
2.5 tons30,000
3 tons36,000
3.5 tons42,000
4 tons48,000
5 tons60,000

How it works

What the AC tonnage calculator does, and does not

Step 1, the chart. The area you enter is matched to a row of the ENERGY STAR chart. An area on the border of two rows takes the lower row, the way the chart reads. Past 2,500 sq ft the chart stops, so the tool scales the top row by area and flags the answer as rough.

Step 2, the adjustments. ENERGY STAR takes 10% off for a heavily shaded space and adds 10% for a very sunny one, adds 600 BTU per hour for each person over two, and adds 4,000 BTU per hour where the space includes a kitchen. The tool applies the percentage to the chart figure first, then the additions.

Step 3, tons. The estimate in BTU per hour divided by 12,000 is the tonnage. The next half ton up is shown as well, since that is how sizes are usually written on a quote.

What it cannot see. The ENERGY STAR chart was written for room air conditioners. A whole house load depends on insulation, windows, air leakage, duct location and the local design temperatures. ACCA Manual J counts those, and Manual J is what a permit or a careful contractor will ask for. Use the AC tonnage calculator to sanity check a quote or a replacement, not to replace the load calculation. Once the system runs, check the charge with the superheat calculator or the subcooling calculator.

Questions

What HVAC techs ask about this

How many tons of AC do I need for 1,500 square feet?
By the ENERGY STAR chart, 1,400 to 1,500 square feet needs 24,000 BTU per hour, which is 2 tons. Very sunny rooms add 10%, extra people and a kitchen add more. It is a starting figure; a Manual J load calculation sets the size you install.
What size AC do I need for 2,000 square feet?
The chart puts 1,500 to 2,000 square feet at 30,000 BTU per hour, or 2.5 tons, before adjustments. Insulation, windows, ducts and climate can move the real load a long way from that, which is why ACCA Manual J exists.
How many BTU are in a ton of air conditioning?
12,000 BTU per hour. ACCA explains the name: melting a ton of ice over 24 hours absorbs heat at roughly that rate, and the industry settled on 12,000.
How many square feet does 1 ton of AC cool?
On the ENERGY STAR chart, 12,000 BTU per hour (1 ton) covers 450 to 550 square feet. Larger spaces get more square feet per ton: the top row, 2,000 to 2,500 square feet at 34,000 BTU per hour, works out to about 700 to 880 square feet per ton.
Is a Manual J load calculation required?
ACCA states that a proper load calculation is required by national building codes and most state and local jurisdictions, and Manual J is the ANSI recognized standard for it. Check what your local code official asks for on a permit.
Can I size a central AC by square footage alone?
Only as a first guess. Square footage cannot see the insulation, the window area and direction, the duct losses or the local design temperatures that a load calculation counts, so two houses of the same size can need different tonnage.

Sources

Where the numbers come from

  1. ENERGY STAR, Room Air Conditioners: sizing chart and adjustments. Area to be cooled against capacity needed, and the shade, sun, people and kitchen adjustments
  2. ACCA, Manual J Residential Load Calculation. The ANSI recognized standard for sizing loads; ACCA notes codes require a proper load calculation
  3. ACCA, A little history behind tons of air conditioner capacity. A ton of AC capacity equals 12,000 BTU per hour

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