How do I make and use a water level?

Before lasers, this is how a level was carried round a building, and it is still the best answer when the two points cannot see each other. It works round corners, through doorways, up a garden and behind obstructions, it costs a few pounds, and unlike every other levelling tool it has no accuracy specification because it is not an instrument — it is physics.

What you'll need

• Clear plastic tubing, 8 to 12mm bore, longer than the distance
• Water, ideally with a drop of food colouring
• Two clothes pegs or clips to hold the ends (optional)
• A helper, unless you clip one end (optional)
• Pencil

Step by step

1. Get tubing of the right bore.8 to 12mm internal diameter. Too narrow and surface tension holds the water up unevenly; too wide and it is heavy and slow to settle. Clear, not opaque, and long enough for the job plus a couple of metres.
2. Fill it slowly from a tap.Run water in from one end with the other end low, letting it fill without splashing. Filling fast traps air. A drop of food colouring makes the meniscus far easier to read, especially in poor light.
3. Chase every bubble out.Hold the tube up in loops and work each bubble to an end by squeezing and shaking. A single trapped bubble breaks the continuous water column and puts the two ends out by an unpredictable amount. This is the one thing that makes a water level wrong.
4. Mark your datum at the first point.Hold or clip one end at the height you want to transfer — a marked line on a wall, the top of a post, a finished floor level — and let the water settle so its surface sits exactly on that mark. Clip it there.
5. Take the other end to the second point.Route the tube however you like: along the floor, round a corner, through a doorway, up a garden, behind a chimney breast. None of it matters. Let it settle for a few seconds and mark where the water surface sits.
6. Read the surface, not the tube.Read at the bottom of the meniscus, consistently, at both ends. Get your eye level with the surface — looking down at it from above shifts the apparent reading by several millimetres.
7. Check both ends are open.A finger over an end, a kink in the tube or the tube pressed against the ground seals it and stops the two surfaces equalising. If a reading looks wrong, this is the second thing to check after bubbles.
8. Prove it by going back.Take the free end back to the datum and confirm the water still sits on the original mark. It takes ten seconds and it catches a lost bubble or a leak, which is the only failure mode worth worrying about.

Tips

• Working alone: clip or tape the datum end at the right height with a clothes peg and carry the other end yourself. There is no requirement for anyone to hold it, only that both ends stay open.
• Very cold water in a warm room, or a tube lying half in the sun, produces a small genuine error because water expands with temperature. Over a domestic run it is negligible; over a long garden run in full sun it is worth avoiding.
• This is the classic way to level a run of fence posts, a raised bed, decking bearers or a patio datum across a garden where a laser is unusable in daylight.
• Leave the tube full between uses if you can, with the ends taped up. Refilling and de-bubbling is the slow part of the job.
• For garden work, a small bottle taped to each end with the tube going into it gives you a bigger, calmer surface to read.

Common mistakes

• Leaving an air bubble in the tube — The column is no longer continuous, so the two surfaces no longer find the same height. It is the only way a water level gives a genuinely wrong answer, and it is easy to miss in a long tube.
• Capping one end to carry it — A closed end cannot equalise. The reading is then whatever the trapped air pressure happens to be, and it looks perfectly plausible.
• Reading from above rather than at eye level — Parallax on a curved meniscus is worth several millimetres, and it will be a different error at each end because you stand differently.
• Assuming the tube route matters — It does not, and people waste a lot of effort keeping the tube straight or off the floor. Only the two open ends matter.

Questions people ask