How do I measure something longer than my tape measure?

Sooner or later a dimension exceeds the tape, and the instinctive response — measure five metres, put a finger on the wall, shuffle along and measure again — is where a surprising number of ordering mistakes come from. There are four better answers depending on what you are measuring and how accurate it has to be.

What you'll need

• Tape measure
• Pencil or masking tape to mark the join
• Laser distance measure (optional)
• A length of non-stretch line (optional)

Step by step

1. Decide how accurate this actually has to be.A fence run to order panels for needs to be right to a few centimetres. A worktop needs to be right to a millimetre. That decision picks the method, and it is worth making consciously rather than defaulting to the fussiest one.
2. For a long distance, use a laser.A laser distance measure takes the whole span in one reading with no join at all, which removes the entire class of error. Fold out the rear foot into the corner so it measures from the pivot rather than the case.
3. If you must join two tape readings, mark the join physically.Pull the tape to a round figure — five metres exactly, not 4,873 — and put a pencil mark or a piece of masking tape at that point. Then measure from the mark to the end. You are adding two numbers you can both see, not one number and a fingertip.
4. Get a second person to hold the hook.Over anything more than a few metres a tape sags, and a sagging tape reads long. Two people pulling it taut and flat against the surface is the single biggest accuracy improvement available, and it is free.
5. For setting out, use a line rather than a tape.A non-stretch builder's line pulled between two pegs, with the required points marked on the line itself, transfers a long dimension with no joins. Ordinary string stretches enough to matter, so use braided line and pull it to the same tension each time.
6. For repeated long spans, make a rod.A straight batten cut to the exact length you need, or marked with the positions you need, is faster and more reliable than any tape. It carries the dimension physically, so it cannot be misread or mis-added.
7. Record it on a sketch, with units, immediately.Draw the shape roughly, write each number on the line it belongs to, and note what it was measured to — face of wall, centre of post, outside of frame. A column of numbers on a phone note is unusable a week later and is a common cause of ordering the wrong thing.
8. Sanity-check the total.Pace it out, or compare it against something whose size you know. A number that is a metre out from your rough expectation is almost always a joining or transcription error, and catching it takes ten seconds.

Tips

• Two tapes hooked together is not a method. The hooks slip, and neither hook is designed to be pulled against another hook.
• A 10m or 30m open-reel tape costs very little and removes the problem entirely for garden and boundary work. It is the right tool for fences, patios and plots.
• Measuring on your own: hook the tape over a screw driven into a scrap block, or use the nail-grab slot in the hook over a nail head. That frees both hands and removes the sag.
• Photograph the sketch as soon as you have made it. The paper is the thing that gets lost, and the numbers are the expensive part.

Common mistakes

• Holding a finger on the wall at the end of the tape and shuffling along — The finger moves, and you cannot see by how much. It is the least accurate method available and it is the one almost everyone uses first.
• Adding an awkward number to another awkward number in your head — The join point should always be a round figure so the addition is trivial. Most joining errors are arithmetic, not measurement.
• Letting the tape sag over a long span — A curved tape is longer than the straight line it is measuring. Over ten metres a lazy sag can easily add a centimetre or two, always in the same direction.

Questions people ask