How do I mark out repeated spacings without the error building up?

Set out six shelf pins at 200mm centres by measuring 200 from each previous mark and the last one will be visibly out. Nothing went wrong at any individual step; the errors simply had nowhere to go but forward. Every measurement carries a fraction of a millimetre of pencil width, tape stretch and eye position, and measuring from a mark that is already wrong inherits that error and adds a new one on top. The drawing shows the same run marked both ways and where the drift ends up.

What you'll need

• Tape measure
• Sharp pencil or a marking knife
• A straight batten for a story rod (optional)
• Dividers (optional)

Step by step

1. Pick one end and call it the datum.One physical point, marked clearly, that every other measurement refers back to. On a wall it is usually the corner you want the work to look right against; on a board it is one end. Everything downstream is a distance from that point.
2. Work out the running dimensions before you pick up the pencil.For six positions at 200mm centres, write down 200, 400, 600, 800, 1000, 1200. Adding up in advance, sitting down, is where the arithmetic belongs — not on your knees with a tape in one hand.
3. Hook the tape once and leave it there.Hook or clamp it at the datum and mark every position along the blade without lifting it. The tape is now a ruler rather than a repeated measurement, and no error can be passed from one mark to the next.
4. Mark with a V, not a stroke.A pencil line is a millimetre wide and you cannot later tell which edge you meant. The point of a V is unambiguous, and for anything that matters, follow it with a knife line.
5. Absorb the leftover deliberately.A run rarely divides exactly. Decide in advance where the odd bit goes — usually split equally at the two ends, or hidden at the least visible end — rather than discovering it at the last position.
6. Use a story rod for anything you will repeat.Mark the positions once onto a straight batten, then use the batten to transfer them to every piece. Every copy is now identical to the rod rather than to a fresh measurement, which is exactly how staircases, window frames and kitchen runs are set out.
7. Use dividers for equal spaces rather than a tape.Set a pair of dividers to the approximate spacing and step them along. If the last step misses the end, adjust by a fraction of the miss divided by the number of steps and go again. Three passes usually lands it, and there is no arithmetic at all.
8. Check the last position before you commit.Measure from the far end back to the last mark and confirm it matches what you expect. This one check catches the whole class of error in five seconds.

Tips

• This is why tapes have numbers all the way along and not just at the end. Read the number at each position rather than resetting the tool.
• For stud or joist centres, mark the centre and also mark which side of the line the timber goes, with a small cross in the waste. A run of studs set alternately either side of the line is a classic and expensive mistake.
• If two people are marking out the same job, use the same tape and the same datum. Two datums on one job is the same problem in a different form.
• The dividers method is the fastest way to space anything evenly — dovetails, balusters, screw holes, fence pales — and it never needs a calculator.

Common mistakes

• Moving the tape along and re-hooking it at each mark — Every re-hook introduces a fresh error and inherits all the previous ones. It is the mechanism behind almost every run that is fine at one end and wrong at the other.
• Measuring from the edge of the pencil line rather than a defined point — A line has two edges and a width. Over ten repetitions, consistently reading the far edge shifts the run by a full centimetre.
• Leaving the remainder to fall wherever it falls — The odd space always ends up at the most visible end, because that is where you finished. Decide where it goes at the start.

Questions people ask