Sizing simply means figuring out what size equipment you need to heat and/or cool your home or room.  We calculate how much heating/cooling we need, then we buy equipment that supplies that capacity.  That's true no matter what flavor of heating/cooling equipment you want (central HVAC, mini split, window unit, etc.).

Sizing is important because a unit that's too small won't heat and cool your space well, and a unit that's too big will cost more than necessary (and may have other possible problems).  Most contractors will try to sell you a system that's too big, either because they're trying to make more money, or because they don't know how to properly do the sizing work. (sources 1, 2, 3, 4)

The best way to size your system is to have a "Manual J" calculation done on your space.  Manual J is the gold standard for sizing, taking into account things like how much insulation you have, what kind of windows and what direction they're facing, and everything else.  Many utility companies will do this for free (check with them), and if not, you can hire an energy auditor.  Do not go with an HVAC contractor for the Manual J (who has an obvious conflict of interest in wanting to sell you a bigger system than you need), go with your utility or an energy auditor so you can trust that they did it right.  They'll tell you exactly what size you need, and then you can shop accordingly.

When you buy your equipment, I suggest you match your Manual J to the "Max" figure of the equipment, not the "Rated" figure.  For example, let's say your Manual J says you need 24,000 BTU, and you have these options for equipment:

Shopping Example
Equipment	Rated BTU	Max BTU
Option 1	16,000	25,000
Option 2	24,000	37,000

You'd go with Option 1, matching your Manual J to the Max figure, not the Rated figure for Option 2.  There's a detailed explanation of why, below.

If you have ducts, you should also have your them tested for leaks, because installing properly-sized equipment won't do any good if you're going to gain or lose lots of BTUs through your ductwork.  The national average performance of ducts is only 57%. (source)  If your ducts are a problem, then have them serviced.  Of course, if your ducts are running through conditioned space, then duct losses are not a problem, because you "lose" the air right into the area that you were trying to condition in the first place.

Buying a system larger than what you need is just a waste of money.  That's why I steer you to get a Manual J report so you don't buy equipment that's too big.

Oversized systems might also be less comfortable, but that's becoming less relevant as modern units are moving to inverter technology, which can ramp the output up or down depending on the load.  Without inverter tech, the system might blast you with too-hot air in the winter, and they cause uneven temperatures throughout the house, because it won't run as much as a right-sized sytem, so the air isn't circulated as much.

If you're satisfied with that answer you can skip to the next section.  Otherwise, you can keep reading here about other potential problems with oversizing.

Conventional wisdom is that oversizing also creates other problems besides cost and less comfort, but I consider those to be mostly myths.

1. MYTH: Oversized AC's don't control humidity as well.  The theory here is that oversized systems don't run long enough to dry the air.  This is only sometimes true.  First, modern units with inverters keep running even at low loads.  Second, a university study of homes where grossly oversized units were replaced with right-sized units didn't result in lower humidity.  The researchers theorized that the longer running time of the right-sized units meant that the fans they were drawing in more moisture into the house through the building envelope and the attic. (source PDF)  And Martin Holladay, an icon in the building science field, says, "Oversized air conditioners aren't really as terrible at dehumidification as many energy experts claim." (source)  However, I know of at least one case in which an oversized system reportedly did result in more humidity.
   
   
2. MYTH: Oversized systems wear out faster because of short-cycling.  The theory here is that big systems heat or cool the house so quickly that they shut off after just a few minutes, and the constant on/off cycling is hard on the equipment.  There are a few problems with that:  First, with modern inverter tech, cycling is even less than older systems that are sized perfectly.  Second, it's hard to believe that the extra cycles would really make a big difference in equipment life.  The AC will likely be up for replacement just because it's old before it dies completely.  Third, any stress added by extra cycles should be balanced by the fact that the equipment doesn't run as long.
   
   
3. MYTH: Oversized systems use lots of extra energy.  The theory is that AC's need to run for several minutes to get efficient.  But the study above found that they don't use more energy overall.  However, for mini-split systems, especially heaters, oversized systems can indeed use a lot more energy. (source)

Heat is measured in BTU's (British Thermal Units).  For example, a heater might supply 24,000 BTUs per hour to a house when it's operating.  To size a heating system, we figure how many BTUs of heat the house loses to the outside, and then get a system big enough to replace the BTUs that were lost.

We use BTUs for cooling, too, to measure how much heat an air conditioner removes from a house or a room.  For example, a small window-unit AC will remove about 5000 BTU per hour from a room.  When we size for cooling, we'll be figuring out how many BTUs of heat enter the house, and then get a system that can remove that much heat.

AC equipment is also measured in tons.  This has nothing to do with the weight of the equipment, it's just a measurement of heat.  One ton is 12,000 BTU.  So a 2.5-ton system would be 2.5 tons x 12,000 BTU/ton = 30,000 BTU.

Easy mistakes to make when sizing gas heaters

First off, think twice before buying a gas heater, or replacing an old one.  Modern heat pumps almost always save energy versus gas, even in cold climates. (source)  However, if you're dead-set on going with gas, then keep reading.

Many homeowners who buy gas-fired equipment are bitten by a couple of big misunderstandings.  The first is that they don't know that gas furnaces are rated by their input, not their output.  So a "60,000 BTU" gas furnace that's 80% efficient will supply only 48,000 BTU to your home.  If you've figured that you need a 60k BTU of heating and you buy a 60k BTU gas furnace, you're going to wind up with only 48k BTU of heat which is not enough.  So you need to calculate the output when you buy a gas furnace.  (Oil furnaces, on the other hand, are rated by their output, so a 60,000 BTU oil furnace will supply 60,000 BTUs.)

The other mistake is to ignore the efficiency when replacing old equipment.  Let's say you have a 70k BTU gas furnace at the end of its useful life.  Your contractor points out that your old unit was only 80% efficient, but he can sell you a new 70k unit that's 95% efficient and it will cost only $350 more, promising that you'll make the difference back quickly with the energy savings.

But regardless of the energy savings, the unit is bigger than what you need, so if you buy it you'll be paying more than you need to.  Your old unit put out 70k BTU x 80% = 56k BTU.  The new one puts out 70k BTU x 95% = 66.5k BTU.  So the new one is effectively 10.5k BTU larger than what you need—and you'll be paying for it.  Here again, you must shop by output, not input.  If you need only 56k BTU of heating and you go with a 95% efficient unit, then you should buy one in the neighborhood of (56k BTU ÷ 95% = ) 59k BTU.

A "Manual J" calculation is the gold standard for sizing HVAC equipment.  This is something you hire an energy auditor, local utility, or engineer to do.  (Again, don't get an HVAC company to do it.  Contractors frequently churn out bad Manual J's by using the wrong inputs—either because they're trying to upsell bigger equipment, or because they sincerely don't know how to use the software properly.)  You can't really do a Manual J yourself.  Even if you bought the software, without training on how to use it, the chance of making a mistake is high, and as they say, garbage in = garbage out.  (If your input data is bad, then the result is worthless.)

There are various calculators on the net that try to help you size your system.  They are not as accurate as a good Manual J, and they also require that you enter all the data perfectly.  If you make a mistake, garbage in = garbage out.  I tried several but the interfaces and/or accuracy were so bad I wouldn't recommend any of them.

The worst way to size a system is by square feet, though many contractors do just that.  The problem is that two homes that are the same size can have drastically different needs, depending on climate, insulation, shading, foundation, etc.  An engineering company which does Manual Js pulled the data for the last 40 reports they did for buildings in hot climates, and for the buildings that were <3000 sf, the results ranged from 624 to 1900 square feet per ton of cooling—quite a range. (source) 

But for what it's worth, which is not much, the formula is 1 ton (12,000 BTU) of cooling for 1200 square feet in hot climates (based on my eyeballing the median from the engineer's data mentioned above).  Contractors typically oversize by recommending something like 1 ton of cooling for only 500 sq. ft.

We're throwing lots of numbers around so let's put them in a list:

• 1900 sf/ton:  Best example from a sample of ≤3000 sf buildings in hot climates as calculated by an engineer
• 1200 sf/ton:  Median for above engineer's data
• 624 sf/ton:  The worst result from the engineer's sample
• 500 sf/ton:  What you're likely to hear suggested by a contractor

Note that you could invert the figures if that's more comfortable:  Another way of saying 1 ton for every 1200 s.f., is 10 BTU per square foot.

If it's bad to estimate the cooling load by square feet (and it is), then estimating heating by square feet is even worse.  As you can see from the table, the range between efficient and inefficient homes is huge.  This underscores how useless it is to estimate heating loads by house size alone.

You might be tempted to just pick the number in the middle for your house.  Don't.  Get the Manual J.