Understanding your result
The headline is the cooling load the equipment, lighting and people add, in BTU per hour, with the equivalent in tons in the sub-line. It is an addition to whatever the room gains through its walls, windows and roof; the central AC size calculator and the mini-split size calculator work out that envelope load, and the two are added for the total. For an interior room with no exterior surfaces, this figure may be the whole load.
The stats split the heat three ways so you can see which source dominates. In a home office it is usually the equipment; in a classroom or meeting room it is the people; in an older shop with fluorescent lighting it can be the lights. The table adds the safety margin as a separate line, the load per square foot, and the heat over 24 hours in BTU and kilowatt-hours, which is the energy the air conditioner will have to remove each day if the load is continuous.
The load-per-square-foot row is the warning sign. An ordinary room gains 20 to 30 BTU/hr per sq ft from everything combined; a room where equipment alone adds 50 or more is a dense load, and the notes say it needs dedicated, year-round cooling. The air changes calculator is the companion for a room that also needs fresh air for the people in it.
How we calculate this
The conversion is the thermal equivalent of a watt: 1 W = 3.412 BTU/hr, because 1 kWh = 3,412 BTU, and the BTU, watts and kW converter applies the same factor to any figure. Electricity used by equipment in a room becomes heat in that room with almost no exceptions; the energy that leaves as light through a window or as sound is negligible, and a computer that “does work” turns all of its input into heat. The per-person figures are the total (sensible plus latent) metabolic heat for a typical adult at each activity level from the ASHRAE Handbook, rounded.
The share-of-nameplate field corrects for the gap between a power supply’s rating and the draw of the device it powers. If you have a measured figure from a plug-in power meter or a panel reading, enter it as the watts and leave the share at 100%.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| BTU/hr per watt | 3.412 | Thermal equivalent of electrical power (1 kWh = 3,412 BTU, EIA) |
| Heat per person | 400 seated, 500 light work, 750 active | ASHRAE Handbook, Fundamentals, metabolic heat gain by activity (total, rounded) |
| Share of nameplate | 100% (enter measured watts), typical 50 to 70% for computers | Measured server and PC loads relative to power supply rating |
| Safety margin | 10% | Common design allowance; 20% for rooms where equipment will be added |
| Dense-load threshold | 50 BTU/hr per sq ft | Judgement: about twice the total load of an ordinary room |
Assumptions last reviewed October 7, 2026.
The calculator treats all equipment heat as sensible heat; it does not split the people’s load into sensible and latent parts, which matters for choosing equipment in humid climates, and it does not model heat from gas appliances, hot water or cooking, which the kitchen allowance in the room-sizing calculators covers in a rough way. The guide on how to size an air conditioner explains where internal gains fit into a full load calculation.
Two worked examples
A home office with two workstations
Equipment nameplate 2,000 W (two PCs, two monitors, a printer and a network switch) at 100% as entered, 300 W of lighting, two people seated, 200 sq ft, 10% margin.
- Equipment: 2,000 W × 3.412 = 6,824 BTU/hr
- Lighting: 300 W × 3.412 = 1,024 BTU/hr
- People: 2 × 400 = 800 BTU/hr
- Subtotal 8,648 BTU/hr; with 10% margin 9,513 BTU/hr (0.79 tons)
- Per square foot: 47.6 BTU/hr per sq ft
This is close to the dense-load threshold, and the note about nameplate watts applies: if the PCs really draw 600 W between them rather than 2,000, the load falls to about 3,400 BTU/hr. Measuring before sizing is worth it here, because the difference is a whole head size on a mini-split.
A server closet
Nameplate 4,000 W of servers and switches, measured to run at 65% of nameplate, 100 W of lighting, one person occasionally, 80 sq ft, 20% margin for growth.
- Equipment: 4,000 × 65% = 2,600 W × 3.412 = 8,872 BTU/hr
- Lighting: 100 W × 3.412 = 341 BTU/hr
- People: 1 × 400 = 400 BTU/hr
- Subtotal 9,613 BTU/hr; with 20% margin 11,535 BTU/hr (0.96 tons)
- Per square foot: 144 BTU/hr per sq ft; 277,000 BTU (81 kWh) of heat a day
The load is five times that of an ordinary room and runs around the clock, so the note calls for dedicated cooling: in practice a 12k mini-split head on its own condenser, running in cooling mode in January while the house furnace runs. The 81 kWh of heat a day is also the electricity bill the servers generate before the cooling is counted.
Where to find your inputs
Equipment watts. The best source is a plug-in power meter left on each device for a day, or a clamp meter on the circuit. Failing that, the power supply rating on the label, with the share-of-nameplate field set to 50 to 70% for computers and 30 to 50% for networking gear.
Lighting watts. The wattage printed on each bulb or fixture, added up, for the lights that are on when the room is used.
People and activity. Count the people usually present. “Seated” is desk work; “light work” is standing, teaching or cooking; “active” is a home gym.
Floor area. Length × width, for the load-per-square-foot check.
Common mistakes
- Using the power supply rating as the draw. A 750 W supply in a desktop PC typically delivers 150 to 300 W. Measure, or use the share field.
- Forgetting the people. Six people in a meeting room add 2,400 BTU/hr, as much as a 700 W heater.
- Cooling a server closet with the house system. It needs cooling in winter, when the house system is heating; give it its own.
- Counting standby equipment at full power. Printers, chargers and monitors in standby draw a few watts; count what is on.
- Leaving out the margin for growth. Equipment rooms fill up; 20% is cheap insurance.
- Adding this load to a calculator that already includes it. The room-sizing calculators include two or three people and a kitchen; add only the extra equipment and extra people.
Questions people ask
- How many BTU does a computer give off?
- Exactly as many as the watts it draws, multiplied by 3.412. A desktop PC drawing 150 W at the wall adds 512 BTU per hour; a gaming PC at 450 W adds 1,535; a laptop at 40 W adds 136. Nameplate power supply ratings are the maximum the supply can deliver, not the draw; a "750 W" power supply in an ordinary PC typically delivers 150 to 300 W. A plug-in meter gives the real figure.
- How much heat does a person give off?
- About 400 BTU per hour seated and quiet, 500 doing light work standing, and 750 or more exercising, according to ASHRAE's tables of metabolic heat. Roughly half is sensible heat that warms the air and half is latent heat in moisture from breathing and skin, which the air conditioner also has to remove. A room with ten people in it has the heat load of a 1,200 W heater.
- How do I size cooling for a server closet?
- Measure or estimate the real power draw of everything in the closet, treat all of it as heat (3.412 BTU/hr per watt), add lighting and any person who works there, add 10 to 20% for growth, and plan on cooling it 24 hours a day all year, including in winter when the house system is heating. A closet drawing 2,000 W needs about 7,500 BTU/hr of continuous cooling, which is a small dedicated mini-split; the house air conditioner cannot provide it in January.
- Why does the calculator ask for a share of nameplate?
- Because nameplates state the maximum a device can draw, which it rarely does. Servers and workstations typically run at 50 to 70% of their power supply rating, networking gear at 30 to 50%, and appliances cycle on and off. Entering 100% with nameplate figures oversizes the cooling, often by half. If you have measured the load with a power meter or the building's panel, enter the measured watts and 100%.
- Does lighting really matter?
- Less than it did. Ten 60 W incandescent bulbs added 2,047 BTU per hour; the same light from LEDs at 9 W each adds 307. In an office with fluorescent tubes, lighting can still be 1 to 1.5 W per square foot and a tenth of the cooling load. Enter the total watts of the fixtures that are on when the room is in use.
- Do I add this to the room's size from the AC calculator?
- Yes. The central AC and mini-split calculators work out the envelope load from the room's area, climate, sun and insulation and add a standard allowance for two or three people and a kitchen. For a room with unusual equipment, a home office with several computers, a workshop with motors or a home gym, use this calculator for the internal load and add it to the envelope load. For an interior room with no outside walls, this load may be nearly all of it.