The local village events team are putting on a barn dance in a barn on a usually fully working farm. Somebody knowing that I’m vaguley involved in health and safety (asbestos training) has roped me and without thinking I’ve agreed to help with risk assessments. Most of this is not a problem.  I am now however panicking. I have no experience of fire risk assessments especially around a public event. Is there any idiot proof HSE guidance out there? Is there a numbers per square meter ratio or is that a bit of a myth Any help would be massively appreciated.  I realise I may be out of my depth. It is possible though that previous risk assessment have been carried out.  I will find out more in due course.  I just would to arm myself with a bit more information before I join there next meeting

Frankly I would be very cautious about taking on this FRA unless I was competent and 100% sure I knew what I was doing (No disrespect Sibob) 

Look at the risk:

Members of the public (maybe children too) at an event where alcohol is being served, located in an agricultural building with perhaps few if any of the pieces of fire safety infrastructure that one would to require to afford legal compliance and more importantly, to keep people safe - this FRA is not what I call a walk in the park 

In addition to the link Kate has provided, HM Govt also supply ‘Fire Safety Guides’ for different types of premises, including ‘Places of Assembly’, where there are two guides, one each for small and large premises (Large - > 60 persons)

https://www.gov.uk/government/publications/fire-safety-risk-assessment-small-and-medium-places-of-assembly

https://www.gov.uk/government/publications/fire-safety-risk-assessment-large-places-of-assembly

This might be a good place to start

As you said, working out the maximum occupancy by use of a floor space factor and providing suitable exit width in the right location are indeed a critical part of the FRA.  But there are lots more.

Emergency lighting inside and outside (assuming its a night time gig)

The condition of electrics inside

Heating facilities (gas or electrical)

Signage to escape routes

A method of raising the alarm 

Ensuring final exit doors open easily and perhaps in the direction of escape

Final exit door furniture 

Staffing and training/instruction

Emergency plan, including one of less abled persons 

Fire fighting equipment 

This list is the bare minimum I have compiled on the train going home! There’s lots more

Consider insurance companies and their requirements (the building’s, the organisers’ and yours) and any requirements from the local authority if there’s a temp event notice licence in place. If not, does there need to be one? I might be imagining a completely different scenario that there will be in practice so forgive me if that is true. Its the fact that this event is being held in a farm building that makes me think that completing a FRA will not be a 5 minute job, so please take care