Furniture dimensional tolerances: what to expect and what to specify

At a glance

Millimetres. Starting points to discuss, not promises. A tighter tolerance needs more machining, checking and rework, which can show in the price and the programme: use it where a fit depends on it, and ask the workshop to price both.

Tolerance, limit and movement are three different things

• Tolerance is the variation either side of a stated size that you agree to accept: a table 745 mm high, plus or minus 3 mm. It belongs to the making, and the workshop controls it.
• A limit is a maximum or a minimum that must not be crossed, such as the 850 mm ceiling on an accessible counter surface. It has one side, and the nominal size has to be set inside it (see below).
• Movement is how the size changes after the piece is made: timber takes up and gives off moisture, an upholstered seat compresses under a sitter, a floor is not level. It belongs to the materials and the room, and no workshop controls it.

Most arguments about tolerances are about the second and third being mistaken for the first. A top that was flat when it left the workshop and bowed by the next winter was not necessarily made badly, and a top that is flat on a bench can still be 4 mm out on a floor that is not.

Name the datum and the method

A tolerance is only usable if two people measure the same thing the same way. Write four things with every number:

• The datum. Heights are taken from finished floor level, at each leg, on a floor that has been checked. Seat heights say loaded or unloaded, and where on the seat (see our seat height, depth and width sheet).
• The method. Squareness is the difference between the two diagonals; flatness is the largest gap under a straightedge of a stated length; a gap is read with a gauge at the corners and the middle. A general phrase such as 'to normal tolerances' measures nothing.
• The conditions. Timber and board are measured at a stated temperature and relative humidity, after conditioning, because they change with both.
• The moment and the person. Say whether it is checked at the workshop before dispatch, on site at handover, or both, and by whom, and record the readings.

A regulation limit is one-sided: keep the whole tolerance band inside it

Approved Document M, Volume 2 (2015 edition incorporating 2020 and 2024 amendments, for use in England), paragraph 4.16(b), asks that part of the working surface of a bar or serving counter is not more than 850 mm above the floor, and paragraph 4.16(c) asks for a clear space of at least 700 mm above the floor beneath a shared refreshment worktop. Each is a one-sided limit: a maximum in the first case and a minimum in the second.

A tolerance written plus or minus around the limit itself puts half the pieces on the wrong side. Instead, set the nominal inside the limit by at least the whole tolerance, and more where the floor varies. With an example tolerance of 5 mm, a nominal of 840 mm gives 835 to 845 mm and stays under 850; a nominal of 850 mm gives 845 to 855 mm and can be 5 mm over. For the clear space, with an example of 10 mm, a nominal of 720 mm gives 710 to 730 mm; a nominal of 700 mm can be 10 mm short. Measure from finished floor level at several points: the limit has to hold at every point along the surface, not on average.

Our bar, counter and stool heights sheet draws this section, and the access consultant sets the figures for a given project.

Wood moves: a tolerance is not a seasonal allowance

The USDA Forest Products Laboratory's Wood Handbook (2021 edition, chapter 4) says wood is anisotropic: its tangential shrinkage is about twice its radial shrinkage, and that difference also distorts pieces. For white oak it gives shrinkage from green to oven-dry of 10.5% tangential and 5.6% radial (Table 4-3). It treats shrinkage as starting at about 30% moisture content and falling in a straight line as the wood dries (Equation 4-9), and gives the moisture content wood reaches in equilibrium with a given temperature and humidity (Table 4-2): at 21 °C, 6.2% at 30% relative humidity, 7.7% at 40%, 9.2% at 50%, 11.0% at 60% and 13.1% at 70%.

Applying the equation to a solid white oak panel 1,000 mm wide at 50% relative humidity gives these estimates. Plainsawn boards use the tangential figure and quartersawn the radial figure, as the Handbook directs.

Our arithmetic from the Handbook's equation and tables. A first estimate for unfinished, unrestrained wood that has reached equilibrium; the Handbook says the shrinkage of an individual piece cannot be predicted accurately and quotes a coefficient of variation of about 15%. The figures are for American white oak, not European oak.

So a tolerance of plus or minus 1 mm on the width of a solid timber top says nothing about it in January and in July. State the moisture content it is made at and the room conditions it is made for, let the fixings allow movement, and where a width has to hold, ask the workshop about a construction that moves less. Tolerances on width and flatness then apply at the stated conditions and not outside them.

How to write a tolerance

• General note plus critical dimensions. A general tolerance note on the drawing covers everything, naming a class from a standard where one exists, for example for welded metalwork (BS EN ISO 13920). Individual tolerances are then written only where a fit or a limit depends on them. Where the two disagree, the individual tolerance governs.
• Tolerances add up. Three parts each within 1 mm can stack to 3 mm in the worst case. Statistical stacking, the square root of the sum of the squares, gives about 1.7 mm for the same parts, but it is a probability argument for independent variations and is not a safe basis for a flush joint. Decide which dimension carries the tolerance and let the others absorb it.
• Colour and grain have no number. Approve a sample, say in what light it is viewed, and agree whether a limit sample sets the range. Our sample hierarchy and approval sheet sets out the order.
• Say what happens outside tolerance. Rework, replace, or accept with a recorded concession, and who decides. See snagging acceptance criteria.
• Remember the building. Allowances between a piece and the wall, floor or ceiling are a different question: see joinery tolerances and scribing and the site survey sheet.

Unless a dimension carries its own tolerance on the approved shop drawing, finished dimensions are to be within [± 2 mm up to 1,000 mm and ± 3 mm from 1,000 to 3,000 mm]. Squareness: difference between diagonals not more than [ ] mm per metre. Flatness of tops and panels: not more than [ ] mm under a [1,000] mm straightedge. Gaps between doors and drawer fronts [ ] mm, even to [± ] mm. Welded metalwork to BS EN ISO 13920 class [ ] [edition to be confirmed]. Measured at [20] °C and [ ] % relative humidity, on the finished piece before dispatch, by [party], and the readings recorded.

Finished table height [745] mm ± [3] mm, measured from finished floor level at each leg. Finished seat height [450] mm ± [10] mm as sat on, measured at [ ]. These are critical dimensions: checked before dispatch and again at handover, and recorded.

Accessible section of the working surface: nominal height [840] mm above finished floor level, tolerance ± [5] mm, so that the surface is never more than 850 mm above the floor at any point along its length. Knee space: nominal clear height [ ] mm, tolerance + [10] mm / − [0] mm, so that it is never less than [the figure taken from BS 8300-2:2018 or the access consultant's drawing]. Measured from finished floor level at [ ] points and recorded before handover.

Solid timber tops and panels are to be made at [ ] % moisture content for a room conditioned at [ ] to [ ] % relative humidity at [ ] °C. Tolerances on width and flatness apply when the piece is measured at its delivery conditions and do not cover seasonal movement outside that range, which the fixing details allow for as shown on the shop drawing.

Standards and sources

• USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021), chapter 4: Tables 4-2, 4-3 and Equation 4-9, read.
• Approved Document M, Volume 2 (2015 with 2020 and 2024 amendments, England): paragraph 4.16(b), (c) read.
• Named, not read: BS EN ISO 13920:2023 (welded constructions); BS EN 1529:2021 and BS EN 1530 (door leaves); BS EN 1334 (beds); BS 5606:2022 (accuracy and tolerance); BS 8000-5:1990 (joinery workmanship). Check current editions.
• Make Bespoke Studio journal: How to specify bespoke joinery

Questions we are asked

What tolerance is normal for furniture?

It depends on the material and the process, and no UK furniture standard we could read sets figures. As a typical starting point to discuss with the workshop: about plus or minus 2 to 3 mm on overall sizes of timber and board pieces, 1 to 2 mm per metre on squareness and flatness, and gaps of 2 to 3 mm even to about 1 mm. Upholstered pieces are softer: about plus or minus 10 to 20 mm overall.

Why is plus or minus 1 mm not realistic on a solid timber table?

Because timber changes size with humidity. A 1,000 mm wide plainsawn white oak panel moves by an estimated 11 mm narrower at 30% relative humidity and 7 mm wider at 60%, relative to 50%. A tolerance only means something at stated conditions, so write the moisture content at manufacture and the room humidity.

How should I specify the height of an accessible counter?

As a nominal below the limit, with a tolerance that keeps the whole band under it. Approved Document M, Volume 2, paragraph 4.16(b), asks for part of the working surface at not more than 850 mm above the floor, so a nominal of 840 mm with a tolerance of plus or minus 5 mm stays under it. Measure from finished floor level at several points.

What is the difference between flatness and squareness?

Flatness is how much a surface departs from a plane, read as the largest gap under a straightedge. Squareness is how close a frame or top is to a true rectangle, read as the difference between its two diagonals. A top can be flat and out of square, or square and bowed.

Who should set the tolerances, the designer or the workshop?

The designer sets the ones that matter to the design, such as a flush face or a regulatory limit, and the workshop says what it can hold and at what cost. Agree both on the shop drawing, with the datum and the way of measuring each one.

What happens if a piece is outside tolerance?

Agree it in advance: rework, replace, or accept with a recorded concession, and who decides. Without that, the only measure is argument at handover.