Fire engineering PD 7974-6BS 7974 v2.0 Reviewed 2026-07-27

ASET and RSET evacuation timeline calculator

A performance-based means of escape design comes down to one inequality: the available safe egress time must exceed the required safe egress time, with a margin. PD 7974-6 equation (1) defines that margin as a plain subtraction:

t_margin = t_ASET − t_RSET

and equation (2) builds RSET from four terms:

t_RSET = Δt_det + Δt_a + (Δt_pre + Δt_trav)

Pre-travel time is normally the dominant term, which is why arguing about walking speeds is usually a waste of effort. Table E.2 gives it as two figures, not one: the time until the first occupants move, and the time until the 99th percentile move. This tool reports both, because a design carrying only the first-occupant figure is not describing the time the building actually takes to empty.

Widths are handled the way Annex G handles them. People keep a boundary layer clear of walls and handrails, so flow is calculated on the effective width: the clear width less the boundary layer from both sides.

Compliant

ASET exceeds RSET with the applied factor

A margin of +425 s at the 99th percentile, and 713 s still inside ASET after a factor of 1.5. Record the tenability criteria that produced ASET alongside this result; the figure is meaningless without them.

RSET: first occupants to 99th percentile

355 – 475 s

ASET

900 s

Margin, ASET − RSET

+425 s

Effective width

750 mm

Working

Step	Value
Detection, Δt(det)ignition to detection	60 s
Alarm, Δt(a)detection to the general alarm	30 s
Pre-travel, first occupantsA, M2 B1 A1–A2PD 7974-6:2019, Table E.2 (Annex E, informative)	1 min (60 s)
Pre-travel, 99th percentiletime until the 99th percentile begin to travelPD 7974-6:2019, Table E.2 (Annex E, informative)	3 min (180 s)
Effective width1,050 mm clear − 2 × 150 mm (stairways, doors, archways)PD 7974-6:2019, Table G.1	750 mm
Walking time45 m ÷ 1.19 m/s (corridor, aisle, ramp or doorway)PD 7974-6:2019, Table G.2	38 s
Flow time at the exit200 ÷ (1.3 persons/s/m × 0.75 m)PD 7974-6:2019, Table G.2	205 s
Travel time, Δt(trav)flow controlled: the exit is the bottleneck	205 s
RSET, first occupants60 + 30 + 60 + 205PD 7974-6:2019, equation (2)	355 s
RSET, 99th percentile60 + 30 + 180 + 205PD 7974-6:2019, equation (2)	475 s
Evacuation time, Δt(pre) + Δt(trav)the last two terms of equation (2), at the 99th percentile	385 s
RSET with safety factor475 × 1.5	713 s
Margin of safetyASET − RSET at the 99th percentile, equation (1)PD 7974-6:2019, equation (1)	+425 s

What this means

The two percentiles are 120 s apart: RSET runs from 355 s for the first occupants to 475 s for the 99th percentile. The design case is the 99th percentile; the first-occupant figure describes when movement starts, not when the building is clear.
The exit is the controlling bottleneck by a wide margin. For comparison, the Approved Document B method, 1.33 persons/s/m applied to the full 1,050 mm clear width, would give 143 s rather than 205 s. PD 7974-6 G.5 sets the two out side by side and the difference is entirely the boundary layer; the effective-width method is the more conservative of the two.
Annex E is informative, and its closing note records a lack of data on evacuation behaviour and warns that the limitations must be borne in mind. These are suggested values for a design behavioural scenario, not measurements of your building.
PD 7974-6 clause 4.7 says a safety factor ought to be considered to allow for uncertainties at each step, and that the acceptable margin of safety should be established during the qualitative design review. Agree the tenability criteria, the design fire and the margin with the approving authority before the analysis; doing it afterwards is the most common reason a fire-engineered design is rejected.

The fire and the warning

Detection time, Δt(det)

s

Ignition to detection. PD 7974-6 puts this outside its own scope; it comes from a detector-response calculation or from fire modelling (PD 7974-1 and PD 7974-2).

Alarm time, Δt(a)

s

Detection to the general alarm in the location being assessed. Effectively zero where an automatic system triggers a general alarm on first detection; several minutes for a staged system or a staff investigation period.

Pre-travel time, Table E.2

Design behavioural scenario

The scenario categories of PD 7974-6 Annex E, set by whether occupants are awake or asleep and whether they know the building.

Fire safety management level

Clause 5.2.4. This is the standard of fire safety management, not the alarm category. Table E.2 prints no M1 row for scenario Ci, so M1 is not offered there.

Building complexity

Clause 5.2.3. Complexity drives the wayfinding time Table E.2 adds on top of the row figure. Only the bands the table gives a figure for are offered.

Override: first occupants

min

Leave at zero to use Table E.2. Enter your own figure, in minutes, where you have observed data or an agreed project value. Both overrides must be set to take effect.

Override: 99th percentile

min

Travel time, Annex G

Controlling route element

Table G.2. Speed and specific flow are a matched pair from the same row, both measured against effective width.

Travel distance to the controlling exit

m

Override the travel speed

m/s

Zero uses the Table G.2 speed for the element above. Override it where the population includes people who move more slowly; Annex G gives separate guidance for occupants with impaired mobility, and averaging them away is not a design.

People using that exit

people

Clear width of the controlling exit

mm

The clear width, not the effective width; the boundary layer is deducted below. G.2 measures clear width wall to wall in a corridor, between the handrails on a stair, and as the actual passage width of a door in its open position.

Boundary layer element

Table G.1. The layer is deducted from both sides, so the effective width is the clear width less twice this figure.

Comparison

ASET (available safe egress time)

s

From the smoke modelling, at the point the tenability criteria are exceeded on the escape route. The figure is meaningless unless the criteria that produced it are recorded with it.

Safety factor on RSET

×

PD 7974-6 prescribes no figure. It says a safety factor 'ought to be considered' to allow for uncertainties at each step, and that the acceptable margin of safety should be established during the qualitative design review. 1.5 to 2.0 is common UK practice, not a published value.

Sources

PD 7974-6:2019, Table E.2 (Annex E, informative)Suggested pre-travel times for the design behavioural scenario categories, in minutes, at the 1st and 99th percentile. Annex E is informative and the table's closing note warns of a lack of data.
PD 7974-6:2019, Table G.1Boundary layer widths. G.2: the effective width of an exit path is its clear width less the sum of the boundary layers.
PD 7974-6:2019, Table G.2Maximum unimpeded travel speeds and maximum specific flow per metre of effective width, for horizontal routes and for four stair geometries.
PD 7974-6:2019, equation (2)RSET = Δt(det) + Δt(a) + (Δt(pre) + Δt(trav)). Equation (1) defines the margin of safety as ASET − RSET, and clause 4.7 requires the acceptable margin to be established at the qualitative design review.
BS 7974:2019Application of fire safety engineering principles to the design of buildings: the framework the ASET/RSET comparison sits within, and where the overall safety margins for the design are assessed.

This is an engineering aid, not engineering advice. The result above is produced from the inputs you gave and the guidance cited. It does not account for everything a competent fire engineer would consider on a real building, and it is not a substitute for reading the current published standard. Check every number before it goes into a design.

Questions people ask

What is ASET and RSET?

ASET is the available safe egress time: how long conditions on the escape route stay tenable. RSET is the required safe egress time: how long it actually takes everyone to get out. PD 7974-6 equation (1) defines the margin of safety as ASET minus RSET, and clause 4.7 says the acceptable margin should be established during the qualitative design review, before the analysis is done rather than after.

What is pre-travel time?

The time between the alarm and people starting to move: recognising the alarm is real, finishing what they were doing, collecting belongings, gathering a group. PD 7974-6 Table E.2 gives it as two figures, the time until the first occupants move (1st percentile) and the time until the 99th percentile move, and in most buildings it is the largest single component of RSET. Note that the older term 'pre-movement time' is not what the 2019 edition uses.

What do M1, M2 and M3 mean in PD 7974-6?

They are fire safety management levels, from clause 5.2.4, not alarm categories and not building complexity. M1 is a well-managed building with trained staff and maintained systems; M2 is ordinary management, where occupants respond but less quickly; M3 applies where there is doubt about an organisation's commitment to fire safety management and staff training. Alarm categories are A1 to A3 (clause 5.2.2) and building complexity is B1 to B3 (clause 5.2.3). All three appear together in the Table E.2 row labels, which is why they are often confused.

Why does my result say 'greater than' instead of a number?

Because Table E.2 says so. Every M3 row, and the 99th percentile for Ci at M3, is printed as an open-ended value such as '>15' or '>40' minutes. An open-ended pre-travel time has no upper bound, so RSET cannot be quantified and no margin against ASET can be demonstrated from the table alone. That is a real finding about poorly managed buildings, not a gap in the tool; the answer is to improve the management level or to justify a figure from project-specific evidence.

What is effective width and why is my exit narrower than I measured?

People keep a boundary layer clear of walls, handrails and obstacles to accommodate lateral body sway. PD 7974-6 G.2 calculates flow on the effective width: the clear width less the sum of the boundary layers, deducted from both sides. Table G.1 gives 150 mm for stairways, doors and archways, 200 mm for corridor and ramp walls, and 460 mm for wide concourses. A 1 m doorway therefore has an effective width of 0.7 m and a maximum flow of 0.91 persons per second, which is the worked example the standard itself gives.

Should the specific flow be 1.3 or 1.33 persons per second per metre?

It depends which width you apply it to, and the two must not be mixed. PD 7974-6 G.5 explains that 1.33 persons/s/m is the value implicit in Approved Document B applied to the clear width, while 1.3 persons/s/m is the more conservative Nelson and Mowrer value applied to the effective width after deducting boundary layers. This calculator uses the second pair. Applying 1.33 to an effective width, or 1.3 to a clear width, is neither method.

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