Normally speaking, a tent used for gathering with chairs without any table, each people will take about 1㎡.
Eg: 100 people hold a meeting with 100 chairs will need 100㎡ and 20㎡ lecture desk area, you will need 120㎡ totally, so one set of 8x15m tent is OK.
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A tent used for gathering with chairs and tables, each people will take 1.5㎡.
Eg: 100 people hold a meeting with chair, table, and 30㎡ lecture desk area, you will need 180㎡ totally, so one set of 10x18m tent is OK.
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The above calculation method is just for reference. Different outdoor events usually have different conditions. Pls tell us your requirements, we will provide professional help for you.