How accurate does a measurement actually need to be?

Perfectionism about numbers is not the same as good work. A skirting board cut to a tenth of a millimetre against a wall that undulates by six is wasted effort, and a kitchen carcass cut to the nearest five millimetres is a fitting nightmare. Experienced people are not more accurate everywhere — they are accurate in the places that matter and deliberately loose everywhere else.

Step by step

1. Ask what the error is competing against.A gap is visible when it is bigger than the irregularity around it. A 1mm gap in a mitre on a picture frame is glaring; a 1mm gap where a shelf meets a plastered wall disappears entirely because the wall is not flat to 1mm anyway.
2. Separate fit from function.Some tolerances are about appearance and some are about whether the thing works. A door needs a specific clearance all round or it binds or draughts; a shelf just needs to be flat and take the weight. They deserve different amounts of attention.
3. Work to about a millimetre for anything two visible surfaces meet at.Mitres, butt joints, worktop joins, architrave, cabinet doors, tiles. The eye reads a line, and a line that is not parallel to its neighbour is seen instantly even at a fraction of a millimetre of taper.
4. Work to a few millimetres for anything that meets a wall, floor or ceiling.Nothing in a house is straight enough to justify more. Fitted work absorbs the difference with a scribe, a filler strip, caulk or a bead, and the whole point of those details is that they make precision unnecessary.
5. Work to centimetres for setting out and landscaping.Post positions, path widths, bed edges, patio dimensions. What matters there is that lines are parallel and angles are right, not that a dimension is exact. A patio 15mm narrower than planned is invisible; one that is not square is not.
6. Decide where the error is allowed to accumulate.Every job has a place for the leftover: the last board in a run, the corner behind a door, the joint under a bead. Choosing it in advance is what turns an inevitable discrepancy into an invisible one.
7. Cut oversize and fit, wherever fitting is possible.Two millimetres of spare material makes a measurement error recoverable. This is why joiners scribe rather than measure, and why the good habit is measure, cut long, offer up, trim.
8. Match the tool to the tolerance.A caliper for a bearing fit, a knife line for a joint, a pencil line for a stud position, a tape and a wheel for a fence. Using a knife line to mark a post hole is not more accurate, it is slower.

Tips

• If two pieces have to match, make them match each other rather than both matching a number. Clamp them together and cut both at once, or use the first as the template for the second.
• The error you can see is the one that varies. A gap of exactly 3mm along a whole run reads as a deliberate shadow line; a gap that runs from 1mm to 5mm reads as bad work.
• Where something is genuinely critical, say so on the sketch. A note saying 'this dimension is fixed, take up the difference at the other end' saves the argument later.
• Timber moves seasonally by more than most of the tolerances people worry about. Designing for movement matters more than measuring to a tenth.

Common mistakes

• Spending the accuracy budget on the wrong dimension — Time and care are finite. Working to a tenth of a millimetre on a hidden fixing and roughly on a visible joint is exactly the wrong way round, and it is the commonest pattern in amateur work.
• Cutting to the exact measurement with no allowance for fitting — There is no way back from a cut that is short. Every fitted trade cuts long deliberately, and the extra two minutes of trimming is the cheapest insurance available.
• Assuming that because a room measures 3000mm it is 3000mm everywhere — It almost certainly is not, and a piece cut to that single number will only fit where the number was taken.

Questions people ask