The room is the equipment plus a person, and the person is the bigger number
Add up what goes in a typical well house: a 24-inch tank, a 30-inch control and pump assembly, a 24-inch filter. That is 78 inches of equipment, six and a half feet. Put a foot of clearance between and around each and you are at 126 inches — ten and a half feet — before anyone stands up.
The depth is worse, because it is not additive. The deepest item might be 24 inches, but you need a foot behind it to reach a fitting and two and a half feet in front of it to stand, so a 2-foot object produces a room five and a half feet deep. That ratio is why pump houses built to the equipment footprint are unusable and pump houses built with an aisle are fine.
The door is a one-way test
A pressure tank goes in on the day the building is framed, when there is no wall on one side. It comes out eleven years later through a door. If the door clear width is less than the tank diameter, the answer on that day is a reciprocating saw.
The tilt row on this page is the part people miss. A 60-inch tank cannot go through a 78-inch door standing up if the ceiling is 72 inches, so it tilts — and tilting it needs the diagonal, √(24² + 60²) = 64.6 inches, swinging clear inside the room. A room sized exactly to the upright height has nowhere to swing.
Where the heat loss number comes from, and what it is not
Conduction through each surface is area ÷ R, summed. Air leakage is 0.018 × volume × ACH, where 0.018 is the heat capacity of a cubic foot of air per degree F — 0.075 pounds per cubic foot times 0.24 BTU per pound per degree. Add them and you have UA, in BTU per hour per degree. Multiply by the temperature difference for the design-day loss; multiply by 24 × heating degree days for the season.
Notice how large the leakage term gets in a small building. A 10 × 6 × 7 room is 420 cubic feet, so at one air change per hour the infiltration UA is 7.6, which at the default R-values on this page is more than double what the whole roof loses. Small buildings are nearly all surface and nearly all leak, which is why they lose heat out of proportion to their size.
What this does not give you is a freeze-protection answer. Whether water in a pipe freezes depends on where the pipe is in the room, whether it is moving, what it is wrapped in and how long the cold lasts — the pipe freeze risk calculator works that side, and the pipe insulation heat loss calculator covers the wrap. This page states no freeze-protection figure at all.
What belongs elsewhere
The well and its pump are a separate subject: yield and drawdown on the well yield and drawdown calculator, and the tank sizing itself on the well pressure tank calculator, which is where the cut-in and cut-out pressures decide how much of that tank is actually water. If the building also holds bulk storage, the cistern sizing calculator is the volume question. Degree days to a base other than 65 come from the heating degree day calculator.
The building itself — base, framing, roof — is not sized here either. For the base, the shed foundation comparison calculator is the nearest thing.
Questions people ask
What clearance should I leave around the equipment?
This page will not tell you, and you should be suspicious of anything that does from a distance. There are at least three separate clearances in play: what the manufacturer requires for each piece of equipment, what the electrical code your jurisdiction has adopted requires as working space in front of anything energised, and what you personally need to swing a wrench. The first two are stated in documents specific to your equipment and your jurisdiction. Take the largest of them, put it in the field, and read the room size off.
Can a well house be smaller if I never plan to work in it?
Everyone plans not to work in it. Then a pressure switch fails at two in the morning in January, or the tank waterlogs, or a fitting weeps and the floor is wet. The realistic question is not whether you will work in it but whether the work will happen kneeling in a doorway or standing inside. The clearance table on this page prices that difference in square feet, and it is usually a smaller number than people expect — often two or three feet of extra length.
How do I get degree days for a room held at 40 degrees?
Not by using the published 65-degree figure, which will overstate the heat by a large multiple. Degree days are the sum over the season of the shortfall below your base temperature, so a base of 40 counts far fewer degree days than a base of 65 in the same winter — frequently a quarter as many or less. The heating degree day calculator on this site rebases them. If you only have the 65-degree number, understand that using it here produces an answer several times too big.
Does this size the heater?
No. It gives a heat loss in BTU per hour for the geometry and R-values you entered, at the temperature difference you entered, which is one input to a heater decision rather than the decision. It does not account for thermal mass, for warm water arriving from the well, for solar gain, for the fact that a small building swings quickly, or for what happens when the power that runs the heater is the thing that failed. Freeze protection design is a job for someone who can see the building.
Should the pump house be insulated at all, or just heated?
That is a trade this page can inform but not settle. Insulation lowers UA, which lowers both the design-day load and the season energy, and in a building this small it is cheap because the surfaces are small. Heat with no insulation works, and it works until the power fails, at which point insulation is the only thing buying you hours. Run the numbers at a couple of R-values and look at what happens to the total UA line; in a leaky building you will find the air-leakage term dominates and sealing pays better than more insulation.