Seat slope and back rake: how many degrees

At a glance

Degrees. Seat slope is from the horizontal, with the seat rising to the front; back rake is from the vertical, with the back leaning away from the sitter; seat to back is the angle between the two surfaces. Basis: 'teaching guide' is the University of Melbourne design guide; HSE is the Health and Safety Executive; 'to confirm' means conventional practice for our partner upholsterers to confirm; 'not stated' means the source gives no figure. Design guidance, not clinical advice.

Three angles, three datums

A seat has two surfaces that matter, the seat and the back, and there are several ways to quote their angles. Published guidance mixes them without saying so, which is how one source can give 5 degrees and another 100 degrees for the same chair. Choose one convention, draw it, and write it into the specification.

The four are tied together: c = 90 + b - a and d = 90 + b. A seat that slopes back 10 degrees with a back raked 20 degrees gives 100 degrees between the two surfaces and a back 110 degrees from horizontal. This sheet quotes slope from the horizontal (a) and rake from the vertical (b), because those two can be set with a level and read straight off a section drawing; the angle between the surfaces (c) is given for information.

Formal versus relaxed: what each degree costs

The two ends are easy to describe. A formal seat is shallow and close to level, with an upright back: the sitter is brought forward and square to the table, and can rise easily. A relaxed seat slopes back and leans back: the sitter settles in and stays longer, and finds it harder to get out. The figures below are those one published design guide gives for each end.

The practical trade is three things at once: how the seat feels over a long stay, how much floor area it takes, and how easily people stand up. Moving towards relaxed gains the first and loses the other two. Where a room needs both, such as a banquette that serves a quick lunch and a long dinner, the usual answer is a position between the two with a modest slope and a few degrees of rake, tested on a full-size mock-up with the real foam and cover. The upholstery side of the same choice is on the formal versus relaxed seating sheet.

Degrees to millimetres

Angles are hard to check on a mock-up; millimetres are not. The height difference between the rear and front of a seat is its depth times the tangent of the slope, and the lean at the top of a back is its height times the tangent of the rake. For other sizes use that formula.

One check against a published figure: the same design guide limits a dining seat's front-to-rear slope to about 25 mm, which on a seat 400 to 420 mm deep is about 3.5 degrees, inside its 2 to 5 degree range for dining chairs.

Five checks before you fix an angle

• Measure the loaded seat: foam and feather compress, so the angle as sat on can differ from the unloaded drawing. Fix angles on a mock-up built with the production foam and cover.
• Check the cover against the slope: a US Department of Defense ergonomics fact sheet for office chairs (2002) notes that a slippery seat covering lets the sitter slide away from the backrest. Test hospitality covers such as leather and velvet on the mock-up.
• Match depth to rake: the Health and Safety Executive's guidance says a seat should not be so deep that shorter people cannot use the backrest. A deep seat with an upright back leaves them perched on the front edge. The depths are on the seat height, depth and width sheet.
• Check the table edge: the lean of the back (see the table above) moves the sitter's torso away from the table. On a banquette the back is fixed, so decide the rake with the table in place.
• Check access: Approved Document M gives no seat slope or back angle for dining or lounge seating. Where a project needs accessible seating, take the figures from BS 8300-2 and the project's access consultant.

Seat slope [3] degrees back from horizontal, tolerance [+/- 1] degree, measured along the seat centreline on the finished, loaded seat. Back rake [5] degrees from vertical, tolerance [+/- 2] degrees, measured on the finished front face of the back with the seat loaded. Seat-to-back angle [92] degrees, for information. Angles drawn on the shop drawing section and confirmed on an approved mock-up.

Seat slope [10] degrees back from horizontal, tolerance [+/- 2] degrees, measured along the seat centreline on the finished, loaded seat. Back rake [25] degrees from vertical, tolerance [+/- 3] degrees. Seat-to-back angle [105] degrees, for information. Angles drawn on the shop drawing section and confirmed on an approved mock-up built with the production foam and cover.

All back angles are measured from the vertical and all seat angles from the horizontal. Angles quoted between the seat and the back, or from the horizontal at the back, must be marked as such on the drawing.

Standards and sources

• University of Melbourne, Ergonomics and product interactions: teaching guide, secondary source.
• Health and Safety Executive, Seating at work (HSG57), 3rd edition 1997, reprinted 2011: Figure 1(a), paras 20, 23 and 24.
• US Department of Defense Ergonomics Working Group, Fact Sheet 15 (March 2002): office chairs.
• Approved Document M, volume 2 (2015 edition with 2020 and 2024 amendments): gives no seat angles.
• BS EN 16139:2025 and BS 8300-2:2018: named, not read.

Questions we are asked

What is the difference between seat slope and back rake?

Seat slope is how far the seat tilts back from the horizontal, with the front higher than the rear. Back rake is how far the back leans away from the vertical. They are measured from different directions, so always say which you mean.

How many degrees should a dining chair back lean?

One published design guide gives no more than about 5 degrees from vertical for a dining chair, with the seat sloping back 2 to 5 degrees. A relaxed lounge chair is much further back. Check the figure against the partner upholsterer's frame.

What angle should a relaxed lounge seat be?

The same guide puts relaxed seating at 8 to 15 degrees of seat slope and gives 95 to 120 degrees between seat and back for an easy chair, without stating where that angle is measured. Draw it and check it on a mock-up.

What does 105 degrees mean on a seat drawing?

Usually the angle between the seat surface and the back, so a seat sloping back 10 degrees with a back raked 25 degrees. It can also mean the back measured from the horizontal, which would be only a 15 degree rake. The drawing has to say which.

Why does a back that leans further sit the diner away from the table?

Because the top of the back moves backwards as it leans: a 400 mm back leaning 25 degrees moves its top about 187 mm away from the table, against 35 mm at 5 degrees.