Theatre seating rows, risers and sightlines

The datum: what the sightline is drawn to, and from where#

Every sightline drawing needs a focal point, and in a theatre it is the bottom edge of the projected image, not the centre of the screen. That is the part of the picture the head in front covers first, and it is where subtitles and the bottom third of every composition sit. Drawing to the centre of the screen produces a riser roughly half the height it should be, and nothing reveals the error until the first film.

The two heights the calculation uses are both measured from the finished floor of the row in question, which means the front row is measured from the room floor and the back row from the top of its riser. Eye height is taken as 46 in (117 cm) for an adult in a theatre chair with a 17 to 19 in (43 to 48 cm) seat height, which is the middle of the range given in seat height, depth and angle. Reclining lowers the eye by 4 to 8 in (10 to 20 cm) and moves it 6 to 10 in (15 to 25 cm) backwards, which helps the sightline and hurts the viewing distance. Design to the upright position, because that is the worse case for seeing over a head. The wider set of seated and standing eye figures is in eye level and sightlines.

Distances are horizontal, from the screen surface to the eye. If those numbers are not yet fixed, settle them first using screen size and viewing distance, because the riser depends on them and not the other way round.

The sightline calculation, worked in full#

Call the height of the image base above the front row floor hF, the front row eye height E1, the sightline clearance C, and the two viewing distances D1 and D2. The sightline from the back row eye to the image base must pass at least C above the front row eye, which gives

E2 = hF + (E1 + C - hF) x D2 / D1

and the riser is simply E2 - E1.

C is the theatre designer's C-value. Sixty millimetres (2.4 in) is the usual minimum where seats are staggered laterally, so the view runs between two heads. Ninety millimetres (3.5 in) is comfortable. One hundred and twenty to 150 mm (4.7 to 5.9 in) is what a clear view straight over an unstaggered head needs, and in a domestic room with three seats directly behind three seats, that is the honest figure.

Work a real case. A 120 in screen with its base 32 in (81 cm) above the floor, a front row at the THX distance of 161 in (409 cm), a 66 in (168 cm) row pitch so that D2 is 227 in (577 cm), an eye at 46 in (117 cm) and a staggered C of 2.5 in:

E2 = 32 + (46 + 2.5 - 32) x 227 / 161 = 32 + 16.5 x 1.410 = 55.3 in

The riser is 55.3 minus 46, which is 9.3 in (236 mm). Change nothing but the C-value, taking it to 4.7 in for unstaggered seats, and E2 becomes 58.4 in and the riser 12.4 in (315 mm). Three inches of clearance over one head has cost 3 in of platform.

Riser height as it actually varies#

Front row at 161 in, eye at 46 in, C at 2.5 in. Everything else moves.

Two patterns are worth reading off it. Row pitch has a modest effect: 24 in (61 cm) of extra pitch buys 3.6 in (91 mm) of riser at the worst screen height and almost nothing at the best. Image base height has an enormous effect: every inch the screen goes up removes roughly half an inch of riser, and the last row of the table is barely a riser at all.

Row pitch is set by the seat, not by the room#

The chain is short. A power recliner 40 in (102 cm) deep closed extends to about 70 in (178 cm) open. The seat behind it must begin beyond that, and a passing gap of at least 12 in (305 mm) has to remain, which is the minimum clear passage between rows that the IBC requires in assembly seating. A home theatre in a dwelling falls under the IRC rather than the IBC, so this is a borrowed figure rather than a rule that applies, but it is the right floor to hold. Add the 70 in of open seat to a nominal back-of-seat allowance and the pitch lands at 66 in. The recliner footprints behind these numbers, including the wall clearance a non-hugging frame needs, are in recliner dimensions and the clearance behind.

Note what the table implies about room length. Choosing power recliners over fixed cinema seats adds 18 to 24 in (46 to 61 cm) per row, so a two-row theatre in recliners needs 2 ft more length than the same room in cinema seats, before anyone has sat down.

Riser depth, and the step nobody counts#

The riser has to hold a seat that opens to 70 in (178 cm) and leave 4 to 6 in (10 to 15 cm) behind the back legs, so the platform runs 72 to 84 in (183 to 213 cm) deep. Build it shallower and the seats get pushed forward, which changes the viewing distance the whole layout was set from.

Height is where the trouble starts. The IRC caps a stair riser at 7.75 in (196 mm), which means a 9 to 12 in (229 to 305 mm) platform is not a single legal step. The fix is two risers of 4.5 to 6 in (114 to 152 mm) with a tread of at least 10 in (254 mm) between them, which pushes the front face of the riser forward by that tread and has to be drawn before the pitch is finalised. The same rise, run and headroom logic that governs stair dimensions applies to a platform in a dark room, with more force rather than less.

Aisles, widths and how many seats fit across#

Home theatre recliners are 28 to 32 in (71 to 81 cm) wide overall, against 20 to 24 in (51 to 61 cm) arm to arm for a fixed cinema seat. Three recliners at 32 in occupy 96 in (244 cm); four occupy 128 in (325 cm).

A three-wide row with 30 in aisles either side needs 156 in, which is 13 ft (3.96 m) of room width. Push to four seats and it becomes 15 ft 8 in (4.78 m). The aisle figures come from the same body of work as ordinary walkway and circulation widths, and if the room has to accommodate a wheelchair user the clear space and turning dimensions are set out in wheelchair turning and manoeuvring space.

Keep the back row 36 in (91 cm) off the rear wall for a second reason: heads pressed into a wall boundary sit in the pressure maximum of every length mode, which makes the bass in the worst seat both loudest and least accurate. The positions the loudspeakers then take around that seating block are covered in speaker placement for 5.1 and Atmos, and the whole set of figures belongs to media room and home theatre dimensions.

Before anything is ordered, put it on the floor. Full size tape lines, as described in taping out a layout, will show a 66 in pitch and a 72 in riser in a way no plan does, and a stack of books at the riser height under the back row will tell you in five minutes whether the sightline works.