Calories to BTU for Occupant Heat Gain in Cooling Loads

Turning a Metabolic Rate into the Cooling Load a Room Sees

Every person in a conditioned space is a small, permanently running heater. Comfort research measures that output metabolically — in met units, in watts per square metre of skin, in kilocalories per hour — while the cooling load sheet on an HVAC designer's screen wants BTU per hour per occupant so it can be added to the glazing, the lighting and the plug load. Getting between those two vocabularies is the first arithmetic step in almost every occupancy-driven load calculation, from a twelve-seat meeting room to a lecture theatre.

Conversion factor: 1 cal = 0.003 965 651 24 BTU. A seated adult dissipating about 100 W releases 360 kJ each hour, which is 86 040 cal/h; entering that gives 341.2 BTU/h. That single figure is why load software treats an occupant as roughly a third of a thousand BTU per hour before the activity level is even considered.

What Sits Behind the Per-Person Number

One Met Is One Seated Adult

The met is defined as 58.2 W per square metre of body surface. Applied to the Dubois surface area of an average adult, about 1.8 m², one met lands close to 105 W — the origin of the familiar “a person is a 100-watt bulb” shorthand.

Two Halves With Different Destinations

Part of the output warms the air by convection and radiation; the rest leaves as water vapour from the lungs and skin. The coil has to deal with both, but only one of them shows up on a thermometer, and their ratio moves with activity.

Diversity Keeps the Peak Honest

Design occupancy and simultaneous occupancy are rarely the same. Open-plan floors are commonly loaded at 60–80 % of nameplate headcount, while a conference room or auditorium is sized for a genuinely full house.

Published Values Assume 75 °F

The tabulated gains in the ASHRAE Handbook — Fundamentals are stated at a 75 °F room dry-bulb. Raise the room to 80 °F and the total is unchanged, but roughly a fifth shifts out of the sensible column and into the latent one.

Sizing Occupant Gain for a Room Schedule

The workflow below assumes you already have a metabolic figure in calories or kilocalories per hour — from a comfort standard, an ergonomics report or a physiological study — and need it expressed the way a load form expects it.

Type the metabolic figure as printed

Comfort literature is largely European, so a value may arrive as 100,9 rather than 100.9 and may carry spaces between thousands. Either decimal mark is read correctly and spacing is ignored, so nothing has to be reformatted before it goes in.

Multiply by the occupancy you are designing for

Take the BTU/h result for one person and apply the head count, then the diversity factor for the room type. A boardroom takes its full seat count; a floorplate of desks usually does not.

Reverse it to audit someone else's number

When a load report already states BTU/h per person and you want to know what activity level it implies, the swap arrows turn the pair around. Typing into either box drives the other, so the check runs in whichever direction the document is written.

Move the value into the load sheet

The copy control above a field hands over the digits alone, with no unit and no separators, so a room-by-room schedule receives a value it can sum rather than a label it has to strip.

Total heat is not the number you size a coil on: the sensible portion drives supply air temperature and airflow, the latent portion drives the dehumidification duty, and an equipment schedule wants them separately. Convert the total, then split it with the ratio published for that activity — never assume the split you used for a quiet office still applies in a gym.

Heat Gain per Person by Activity Level

Total and sensible gains below are the adjusted per-occupant values published in the ASHRAE Handbook — Fundamentals for a 75 °F room, with a normal mix of adults and children. The kcal/h column is the same total expressed metabolically, and the met figure is the approximate activity level from the thermal-comfort literature.

Activity	Typical space	Rate (met)	Total (kcal/h)	Total (BTU/h)	Sensible (BTU/h)
Seated at rest	Theatre, evening	≈1.0	88.3	350	245
Seated, very light work	Office, hotel, apartment	≈1.2	100.9	400	245
Moderately active office work	Office, meeting room	≈1.4	113.5	450	250
Standing, walking slowly	Bank, pharmacy, shop floor	≈1.7	126.1	500	250
Moderate dancing	Dance hall, function room	≈2.4	214.3	850	305
Walking 3 mph, light machine work	Factory floor	≈2.6	252.2	1,000	375
Heavy machine work, lifting	Factory floor	≈4.0	403.5	1,600	635
Athletics	Gymnasium, sports hall	≈5.0	453.9	1,800	710

Read down the last two columns and the design consequence is obvious: total output rises more than fivefold from a seated audience to a sports hall, but the sensible part only about triples. Everything else went into moisture. A gymnasium coil is therefore doing far more dehumidification per degree of temperature drop than an office coil, and a plant sized on totals alone will be badly matched to the latent duty in exactly the spaces where people are working hardest.

What the Converter Does for a Load Calculation

Occupancy Counts Tried One After Another

Neither field is a locked output. Convert one person, overwrite with the room total, then try a lower diversity assumption — each pass is a keystroke rather than a fresh calculation.

Clipboard Values the Load Form Accepts

Copying by button or with Ctrl+C inside a field yields the bare digits, which a room schedule or equipment selection form can take without any cleaning up.

Watt-Based Comfort Standards in the Same List

Both dropdowns are searchable and carry all 23 energy units, so a figure quoted in watt-hours by a comfort standard or in kilojoules by a physiology paper reaches BTU without a second tool.

Full-Auditorium Totals Stay Legible

A thousand-seat hall runs into the hundreds of thousands of BTU per hour and millions of calories. Thousands are spaced instead of crowded, and very large results switch to exponent form rather than filling the box.

Occupant-Load Questions from the Cooling Load Sheet

Why is a person modelled as a 100-watt heat source?

Because that is very close to what a resting adult actually dissipates. Basal metabolism for an average adult runs somewhere near 1,700–1,900 kcal a day, and spread over 24 hours that averages about 85 W; sitting quietly and awake pushes it slightly above 100 W. The met unit formalises it: 58.2 W/m² across roughly 1.8 m² of skin is about 105 W. The body cannot store that energy, so essentially all of it crosses into the room. One hundred watts is a coincidence worth keeping, because it makes headcount-to-load estimates something you can do mentally.

Why does the latent half change which coil I select?

Because moisture removal only happens where the coil surface sits below the dew point of the air passing over it. A high latent fraction forces a lower apparatus dew point, more rows or a slower face velocity, and it lowers the sensible heat ratio the equipment must be selected at. Feed a selection program a single total figure carrying an office-like ratio and it will offer a coil that hits the temperature target and leaves a dance floor or a full lecture theatre uncomfortably humid — the room reaches setpoint and still feels wrong.

How does the met unit line up with kilocalories per hour?

One met is 58.2 W/m², which happens to be almost exactly 50 kcal per hour per square metre — the older physiological definition that the watt figure replaced. For the standard 1.8 m² adult that is about 90 kcal/h at one met, and every kcal/h carries 3.97 BTU/h with it. So a 1.4-met office worker sits near 126 kcal/h of whole-body output. Tabulated design gains run a little below the raw metabolic arithmetic because they are averaged over men, women and children rather than taken from an adult male alone.

A full meeting room or the lighting — which loads the system harder?

The people, and it is not close in a modern building. Twelve occupants at moderately active office work add 5,400 BTU/h, close to half a ton of refrigeration. The same 400 ft² room lit to a current LED allowance of about 0.8 W/ft² draws 320 W, or roughly 1,090 BTU/h. Lighting power densities have fallen by two-thirds in twenty years while people have stayed exactly as warm, so occupancy has quietly become the dominant internal gain in conference rooms, classrooms and training suites.

Are the Calories someone eats the same number as the heat they give off?

Over a day, and for a person whose weight is steady, they are close — almost all food energy ends up as heat, because the mechanical work a body does on its surroundings is a small fraction of intake. A 2,000 Calorie day is about 7,931 BTU, which averaged over 24 hours is roughly 97 W. Two cautions for load work. The dietary Calorie is a kilocalorie, a thousand of the calories this converter takes on the left, so the decimal point has to be right. And a daily average is useless for peak sizing: the room has to cope with the activity level happening at 3 p.m., not with a 24-hour mean.

Calories to BTU