CAPACITY CALCULATION
SEPTEMBER 2013
ISSUE 04
1.0
1.1
Introduction
The figure used for room capacity is determined by two separate factors:

The maximum number of persons it is designed to hold, this can be
determined by dividing the area of the room (m2) by an occupancy factor
(this varies depending on the use to which the room is to be put and is
summarised in table 1 below).

The number, width and location of the available exits allowing occupants to
escape safely.
1.2
This document summarises relevant guidance from Building Bulletin 100
and DCLG Guidance for Fire Risk Assessment to enable you to calculate
occupant capacity based upon floor space and the capacity of available exit
routes.
2.0
Calculating Exit Capacity
2.1
Two calculations, occupancy factor and available exit capacity should
both be undertaken and the calculation which arrives at the lowest number
will give the maximum permitted capacity.
2.2
Occupancy Factors
Table 1
Room/Area
Occupant capacity based on floor
space factor (m2 /person) 1
Assembly Hall/Dual purpose area
0.45
Dining Room, Restaurant
Sports Hall (not used for assembly or
examinations etc)
Office
0.9
Staff Common Room
1.0
1
5.0
6.0
Figures from Building Bulletin 100
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2.3
Seating and Gangways
As far as practicable, seating should be in regular blocks, having gangways of
uniform width and allowing free and ready access to the exits. see diagram 2





Gangways to be at least 1050mm wide.
In general not more than 14 seats in a row with a gangway each end, Or not
more than 7 in a row with a gangway at one end.
Space between rows (back of one seat to front of seat behind) of at least
305mm.
Where halls are used as examinations room there is generally a
requirement from exam boards for 1.25 metres between chair centres.
All exits to remain unobstructed. Standing and sitting in gangways or in front
of any exit should not be permitted.
Seated persons will take up more space than people standing and thus as an
indication of likely capacity an occupancy factor of 1.0 m2 /person can be used
where persons are seated on chairs in rows.
2.4
Widths of escape routes and exits
Table 2 Number of fire exits needed
Maximum number of persons
Minimum number of escape routes/
exits
60
1
600
2
More than 600
3
Table 3 Exit door widths
The following can be used to determine the general capacities of escape routes in
normal risk premises
Minimum width of doors on escape
Maximum number of persons
route (stated in mm)
750
100
1050
200
Over 1050mm wide then every
additional 75mm increases capacity
by 15 persons (or part thereof)
e.g.
1200 mm
230
1500 mm
290
1800 mm
350
(see p 68 of Fire safety Risk assessment –Educational premises
http://www.communities.gov.uk/publications/fire/firesafetyrisk6)
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The DCLG recommend the minimum width of an escape route in schools should
be 1,050mm (1600mm in dead ends).
Notes:
1.
If only one exit is available from a room then the maximum capacity is 60
persons regardless of the width of the exit (see table 2), 60 people is also
the limit for an exit where the door opens inwards.
2.
When calculating the exit capacity then if a room has two or more exits it
has to be assumed that a fire might prevent the occupants from using one of
these exits.
The remaining useable exit(s) need to be wide enough to allow all
occupants to leave quickly. Therefore when deciding upon the total
available exit capacity the largest exit from the room must be
discounted.
3.
An exit consisting of double doors is regarded as one exit.
4.
If escape route doors are too close to one another and are likely to be
simultaneously disabled by the same fire (e.g. within 45° of each other) then
they should be classed as one exit and it may be necessary to discount
them both from your calculation. ( diagram 1 )
5.
Escape routes that may be classed as ‘true alternatives’ should therefore be
more than 45° apart when viewed from the furthest point in the
room.(diagram 1 )
6.
A number of exits which discharge into a common area cannot be regarded
as alternative to each other.
2.5
You also need to be aware of any specific licensing requirements and
adhere to any capacity limits set by this.
2.6
If having undertaken your capacity calculation you have any queries please
contact the health and safety team healthandsafety@hertfordshire.gov.uk .
2.7
There may be additional fire protection measures in place within the school
which through use of your fire risk assessment and BS 9999 (Code of
Practice for fire safety in the design, management and use of buildings)
allow for some acceptable variations.
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Worked Examples
Example 1
A school hall with a usable area of 210 M2 would have a maximum occupancy of
210 /0.45 = 466 people (from table 1)
Total exit capacity
The hall in question has 2 sets of double doors and 1 single door all at more than
45° apart and with widths as follows.
 1 @ 2.1M
 1 @ 1.8M
 1 @ 0.75M
The widest exit route must now be excluded from all subsequent calculations.
This leaves


1 Door at 1.8M with a capacity of 350 people
1 Door at 0.75M with a capacity of 100 people (from table 3)
Total exit capacity based on exit widths = 450 people
NB Each of the remaining exit routes must be true alternatives at more than 45°
apart. Final exits must be marked as fire exits.
Therefore the maximum number of persons allowed in the hall in this case is
the lowest calculated figure of 450.
Example 2
Should the same hall as in example 1 be set out for a production with chairs in
rows then assuming each person will need approximately 1 M2
Maximum occupancy 210 /1.0 = 210 people
Total exit capacity
350 + 100 = 450 people (as previously calculated)
As a starting point the likely capacity that can be accommodated in the hall in this
case would be 210 as the lowest calculated figure.
However practically the limiting factor is more likely to be ensuring the seating
layout ensures that:




Gangways of 1050mm wide with direct access to exits
No seat is more than 7 seats from a gangway.
Space of 305mm between rows (back of one seat to front of seat behind)
All exits remain unobstructed.
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Example 3
Hall of 165M2
From table 1 the maximum
occupancy = 165 / 0.45 366 persons
Total exit capacity
2 exit routes are required if over 60
people are to occupy the hall, in this
example 3 routes are available.
However exits 2 and 3 are within 45
degrees, see diagram 1, and also
discharge into a common area and
thus must be regarded as one exit.
Should a fire occur as in this example
both exits 2 and 3 would be disabled
and both must be discounted.
Therefore the total capacity is based
solely upon the width of exit 1 which is
1500mm wide and thus from table 3
gives a maximum capacity of 290
persons.
In the event of people being seated
in chairs in rows each person will
need approximately 1 M2 (allowing for
space taken by chair and a 305mm
distance between rows)
Max capacity of 165M2 / 1 = 165
persons
There may be some flexibility in this
figure as detailed in example 2.
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Diagram 1
Doors may be counted as 2 separate exits
Doors must be counted as one exit
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Diagram 2
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