A marquee is not a room
Three differences separate it from a function room, and all three cost space. The first is the fall of the roof: the canvas comes down towards the perimeter, so in the last metre there is no height for anybody to stand comfortably or for a waiter to serve from behind.
The second is the poles and legs, which on framed marquees sit at the perimeter but on centre-pole marquees land exactly where you would put the dance floor. And the third is the ground: sloping terrain means decking or levelling, and that changes the access levels.
How much the perimeter costs you
As a working rule, take a metre off the whole perimeter of the marquee before you start placing tables. On a 15 × 30 that is 88 square metres out of 450 — nearly twenty per cent.
It is not wasted space: it is where the service point sits, the perimeter walkway, the side tables and the room for the linings. But it is not covers space, and counting it as such is where marquee capacities that later do not fit come from.
Service access and the technical floor
A marquee is served from outside, and that journey is what shapes the service more than anything. The support kitchen is usually in another marquee or a trailer, so there is an outdoor stretch covered with a tray and sometimes in the rain. It wants to be short, covered and on firm ground.
The technical floor is the other half: sound and lighting cabling, hoses and, if there is a hot buffet, power. All of it should run round the perimeter or under decking, never across the table area.
Evacuation outdoors
An event being in the open air does not remove the evacuation requirement: a marquee closed in with sides is a space with a counted number of exits, and those exits have to be signed, clear and spread out.
What is specific to outdoors is what lies on the other side of the exit. An exit that opens onto a bank, a hedge or an unlit area is not doing its job. The escape route is drawn as far as a safe outdoor space, not as far as the edge of the canvas.
How to draw a marquee on the plan
As a room with two outlines. The outer one is the marquee; inside it you draw the usable perimeter, a metre further in, which is where tables start to fit. Poles go in as obstacles, exactly like the columns of a function room.
With that, the capacity calculation with service gangways works the same as indoors — and, more to the point, you can show it to the client: the conversation about the marquee taking 240 and not 300 is much shorter with the drawing in front of you.