Short answer: in a temporary structure the practical starting figure for heating is 100 to 150 watts per square metre, and for cooling it is 1 ton, meaning 12,000 BTU or roughly 3.5 kW, for every 9 to 14 square metres. Inside an existing building the maths works differently, because the system is already installed; your question is not what capacity to build but whether the installed capacity can absorb your guest load. Most organisers underestimate how big that load is. According to the occupant heat gain table in Chapter 18 of the ASHRAE Handbook Fundamentals, a seated guest in a theatre layout gives off about 115 watts, a standing or walking guest 160 watts and a guest dancing at moderate pace 265 watts. So the moment dancing starts at a 500 guest gala, someone has effectively switched on a 132 kW heater in the room, except nobody touched a switch. Add stage lighting, LED walls and chafing dishes and the picture gets clearer. Below: the heat load of a crowd, watts per square metre for marquees, how many patio heaters and cooling tons to book, fresh air rates, and the preheat hour that belongs in your build schedule.
How many kW of heating and tons of cooling does an event need?
There are two situations and they do not share a calculation. In a hotel ballroom, a congress centre or an enclosed exhibition hall, heating and cooling live in the building's own system. You are not sizing capacity there, you are testing it. In a marquee, on an open terrace, or in a warehouse type space that has sat unused, there is no system and everything gets built from zero. On the heating side the quick starting figure is 100 to 150 watts per square metre. For a 100 square metre marquee with a floor and an interior lining, that is 10 to 15 kW. If the weather is harsh, the roof is clear or there is wind, the build guides published by the marquee hire trade suggest lifting that figure by 20 to 30 percent, and in real winter hire practice you will see it climb towards 200 watts per square metre. Cooling is measured in tons: 1 ton means 12,000 BTU, roughly 3.5 kW, and the common rule for temporary structures is 1 ton for every 9 to 14 square metres. Both rules give you a starting point, not a finishing one. Three things decide the finishing point: the crowd, the stage load and the sun.
The crowd is a heater: how many watts per guest?
The line item most often left out of a heat calculation is the guest. The occupant heat gain table in the ASHRAE Handbook Fundamentals breaks it down by activity and gives these total heat figures: 115 watts for a person seated in a theatre layout, 130 watts for someone seated doing very light work, 160 watts for a person standing or walking, and 265 watts for moderate dancing. Now map that onto a running order. A 500 delegate conference produces a heat load of roughly 57 kW once the room fills. The same 500 people seated for dinner at a gala push it to 65 kW. When the programme moves to the dance floor, the figure reaches 132 kW. The room's heat load in hour two is more than double hour one. The operational consequence is simple: a team that switches the cooling on after the doors open has already lost. The load peaks at the liveliest point of the programme rather than at doors, and a room takes longer to cool down than it takes to warm up. In winter the same arithmetic works in your favour if you read it correctly. A full room heats itself, and running the heaters flat out only to end up with a sweating room an hour after doors is a common mistake.
How much heat do stage lighting, LED walls and catering add?
One piece of physics does the job: in an enclosed volume, every 1 kW of electricity consumed turns into roughly 1 kW of heat. Lighting, sound and video systems do not destroy that energy, they release it into the room. Which means the total connected load your technical team already worked out for generator sizing is also the input to your heat calculation. A 40 kW lighting and video load on stage is a 40 kW heater running in the room. LED screens are far more efficient than the old halogen rigs, but a large wall still draws real power and radiates all of it forwards, which is why the front rows feel it first. Followspots, moving heads, amplifiers and interpreter booths all join the list. Booths deserve their own note, because two people work inside a sealed box for hours and performance drops fast if the booth has no ventilation of its own. Do not forget catering either: chafing dishes, coffee stations and mobile kitchens generate both sensible heat and moisture. Ask the technical team for the total connected load in kW in writing, then carry that number straight into the heat calculation.
Heating a marquee: watts per square metre, kW per cubic metre
Marquee sizing is done two ways and the two disagree, because they start from different assumptions. The area method says 100 to 150 watts per square metre, so a 9 by 12 metre marquee at 108 square metres lands near 16 kW. The volume method assumes 1 kW heats roughly 10 cubic metres; the same marquee at an average height of 3 metres is 324 cubic metres, which pushes the answer to 32 kW. The gap comes from the temperature lift you are aiming for. If you need to sit 20 degrees above outside air, the power per unit of volume roughly doubles. Hire practice makes the range concrete: trade guides treat a single 25 kW indirect heater as enough for a 9 by 16 metre marquee while recommending two units at 9 by 18 metres, and a 180 square metre clear span tent hosting 120 guests is typically quoted at 35 to 40 kW of forced air heating. The variables driving the decision are easy to check on a site visit. Is there an interior lining, is there a floor, is the roof clear, are the walls closed, which direction does the wind come from. A clear roof radiates heat to the night sky all evening, and so does an unlined tent. Then there is time, which rarely appears on the build schedule: depending on volume, a marquee needs 2 to 3 hours to reach a comfortable temperature. Fire up the heaters half an hour before guests arrive and your first arrivals walk into a cold tent.
Patio heaters outdoors: how many units, how many cylinders?
For arrival areas, terraces, smoking areas and outdoor bars the maths is rougher but easier. A standard mushroom style gas heater is rated at 13 kW and works over a radius of 3 to 4 metres, which gives a usable warm zone of roughly 28 to 50 square metres. The practical rule the hire trade uses is to divide the floor area by 35, round up, then add 1 or 2 units for the spots where people cluster. On an 80 square metre terrace that comes to 3 heaters. Cylinder life is the second half of the plan: a unit running at full output empties a bottle in about 6 to 8 hours, so budget 1 spare cylinder per heater for a 4 hour event and 2 spares for anything past 6 hours. Two mistakes repeat on site. The first is heating empty floor. What needs warming is people, not square metres, so put the units at the queue, the welcome desk, the bar and the smoking area. The second is ignoring wind, which cuts the effective radius sharply; in a crosswind a 13 kW unit will not cover even half its paper area. Arrange heaters as a screen around the crowd rather than facing into the wind. Never bring a gas patio heater inside a closed marquee, because the products of combustion stay in the room with your guests. The right answer for enclosed volumes is a ducted indirect heater, where the burner sits outside and only warm air is blown in.
Cooling: how many tons, and what does the Istanbul data say?
Cooling is counted in tons and 1 ton is 12,000 BTU, roughly 3.5 kW. For marquees and temporary structures the common rule is 1 ton for every 9 to 14 square metres, dropping to as tight as 7 square metres per ton when it is very hot and the crowd is dense. On a 400 square metre marquee that works out at 29 to 43 tons, so roughly 100 to 150 kW of cooling. If the number looks inflated, go back to the per person figure: 400 guests dancing in that same marquee already produce 106 kW of heat. The rule is not exaggerating, it is counting the crowd. Choosing your design point from a monthly average is where people go wrong. Turkish State Meteorological Service normals for 1991-2020 put Istanbul's average August maximum at 29.7 degrees and its August mean at 24.7 degrees, with an average January maximum of 9.6 degrees and 11.8 degrees in December. Yet across the 1950-2025 measurement period the highest recorded temperature is 40.6 degrees in July and the lowest is minus 9 degrees in February. You are designing for a bad day, not an average one. Add solar gain on top: under midday sun, the inside of a clear roof marquee sits well above outside air temperature. For a summer event, choosing a white or opaque roof is a cheaper decision than the extra tons of cooling you will otherwise book late.
No cooling calculation survives without ventilation
A stuffy room is not always a hot room. Humidity and carbon dioxide build-up make air feel heavy even while the thermometer reads 23 degrees. The international reference for fresh air is ASHRAE 62.1, and its figures for public assembly spaces are specific: auditorium seating areas and lobbies call for 2.5 litres per second per person of outdoor air, plus 0.3 litres per second for every square metre of floor. For 500 guests in a 600 square metre room that comes to 1,430 litres per second, roughly 5,150 cubic metres of fresh air per hour. Most portable cooling units cannot deliver that on their own; a recirculating machine chills air without renewing it. Ask about the venue's ventilation capacity during the site visit and insist on an answer in cubic metres. On the Turkish side the duty is written down as well. The Regulation on Health and Safety Measures in Workplace Buildings and Extensions, published in the Official Gazette of 17 July 2013 numbered 28710, requires employers to provide adequate lighting, ventilation and thermal comfort conditions, states that the temperature of the working environment must suit the type of work and the energy employees expend, and requires heating and cooling equipment to be positioned and maintained so that it neither disturbs employees nor creates a risk of accident. That clause protects staff rather than guests, but on site both breathe the same air.
Electrical load, safety and the hour that goes in the build schedule
Cooling changes your generator calculation, heating usually does not. The electrical rating on an air conditioning unit and its cooling capacity are not the same number, so put the nameplate kW on the generator list rather than the tonnage. Gas heaters add no electrical load but they do add fuel logistics: cylinder count, spares, a storage position and the distance from that position to the decor. Indirect heaters need diesel consumption and an external placement worked out in advance. Safety comes down to three lines. Combustion products must not stay inside an enclosed volume, which is why marquees get indirect systems. The manufacturer's clearance distance between heaters and fabric, drapes, backdrops and decor must be respected and drawn onto the floor plan, not left to the crew on the night. Cable and hose runs stay off pedestrian routes. Two entries belong in the build schedule as well: start heating a marquee at least 2 to 3 hours before doors, and start cooling earlier still, because a warmed mass takes longer to bring down. One last practical warning: measure the temperature at guest height, not at the ceiling. A system reading 26 degrees at roof level can easily leave 30 degrees around a seated guest's head. Tales Event is based in Istanbul and delivers dealer meetings, product launches, openings, gala nights and graduation ceremonies across Turkey, running stage, sound, lighting and LED screen production with a single team. Send us your venue, guest numbers, date and running order, and we will work out the heat load alongside your stage load and put heating, cooling and generator planning onto the floor plan.
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