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How to use a string line level

Updated: October 2026

You have a builders' line, one vial, and two points that ought to match, and the bubble will not sit still. How to use a string line level is a short sequence: anchors, a taut line, one level at the middle, then a mark at each end. The bubble reads the cord where you hung it, not the ground halfway along. Which pack to buy is the [string spirit level comparison](/hub/measuring-and-marking-tools/string-spirit-level). This sequence comes from maker guidance and from amazon.co.uk review faults, set down on 16 October 2026. It is not a lab method. A purchase through our links can earn a commission at no extra cost to you.

You have a builders' line, one vial, and two points that ought to match, and the bubble will not sit still.

Our top picks

Innovess Innovess 8-piece hanging line level

Best overall

Innovess Innovess 8-piece hanging line level
5.8/10about £5.99
View offer
Presch Presch 2-piece line spirit level

Rank 2

Presch Presch 2-piece line spirit level
8.1/10about £11.99
View offer
Bubble Levels Bubble Levels 7-piece hanging line level

Preis-Tipp

Bubble Levels Bubble Levels 7-piece hanging line level
7.3/10about £5.99
View offer
On this page
  1. 1.What the bubble is reading
  2. 2.Two stakes and a tight line
  3. 3.Hang one level in the middle
  4. 4.Mark the ends, then stop
  5. 5.Stop when the next step is not a level line

What the bubble is reading

The vial reads the slope of the line between the two anchors, at the hooks. If the middle has dipped, a level hung there reports that dip, even when the ends are where you wanted them. Johnson Level, a US maker, warns that the slightest sag changes the reading. A bubble that will not centre is often a slack cord. The ground under the middle is a later measurement. Use a tape measure down from the line to a patio board. A centred bubble means the two ends are being treated as level. It is zero fall, not plumb, and not 1:40. When the job becomes a datum or a real fall, move to string spirit level or a laser level.

Two stakes and a tight line

You need the line level, a cord, a tape, and two stakes that will not creep. Secure one end first. The tape does not centre the bubble. Fix the anchors outside the work, and keep the run off a path. Pull until a further tug no longer lifts the middle, and the cord still sits on the hook. A nylon that reviews call too thin, the complaint on the LSGZJZY kit, will not hold that tension. Take the line down when you leave if people are still walking through.
Innovess Innovess 8-piece hanging line level

Testsieger

Innovess Innovess 8-piece hanging line level
5.8/10about £5.99
View offer

Hang one level in the middle

Hook one level at the midpoint so any remaining sag is shared. ShunTool, not a UK publisher, makes that point, and it matches Johnson. DIY Doctor, a UK page, says the level can hang at either end if the line is taut. This page does not follow that. An end hang sits on the anchor, where the cord is higher than a dipped middle, so you can centre a bubble on a line that is still low where you are working. Use one level. A whole pack on the cord adds weight. One Innovess review says hanging all eight will sag the line. Shallow hooks, which Bubble Levels reviewers report, let the vial walk off. Move one stake until the bubble sits between the marks. A creeping line will walk the bubble back out.

Mark the ends, then stop

When the bubble is centred, mark both anchors. Those marks are the level you can transfer. They are not a certificate for the ground between them, and they are not plumb. A post wants a long spirit level. The string only lines up the tops. A knock can split the bubble. That split is not a new height. One UK review of the Hezee pack describes turning the body vertical to join the bubble, which unhooks the level and pings the line. Unhook it on purpose, let the bubble become one, and hang it again at the middle. If two levels disagree on a line you have already tensioned, the pack has failed, and the marks should wait.

Stop when the next step is not a level line

If two vials disagree on that same tight line, do not average them. Stop when the next step is a drainage fall or a wall you are not competent to build. A brick course is a different sequence from two garden stakes.

Common questions

Where on the line should the level hang?
At the middle of the span, on the hooks, with one level. The middle is where a sag shows. Hanging at the end can centre the bubble while the cord is still low where you are working. DIY Doctor allows either end if the line is taut. This page keeps the midpoint, because the middle makes a mistake visible.
How tight is tight enough?
Tight enough that the middle no longer lifts when you pull a little harder, and the cord stays on the hooks. If you can still see a belly under the vial, the bubble is reading that belly. A very thin nylon may never get there. That is a cord problem, not a reading problem.
What if the bubble splits?
A split bubble is usually a knock, not a height. Let it become one bubble again before you trust it. Turning the whole level vertical, which one Hezee reviewer tried, unhooks it and flicks the line. If two levels then sit in two places on the same tight line, stop. Averaging them invents a reading you did not measure.
Can I hang all the levels in the pack?
You can hang two or three once, as a check that they agree, and then take the extras off. Leaving the whole pack on the cord adds weight. One Innovess review says eight at once will sag the line. One level stays for the reading. The extras are spares, not a second opinion you leave hanging.
Can I read a fall for a patio or a drain off this bubble?
You can see that the line is not level. You cannot read 1:40 or 1:80 from where the bubble sits. Those are 25 mm and 12.5 mm per metre in Approved Document H, and they depend on the pipe and the flow. For a short patio, measure a drop with a tape. For a regulated drain, get someone who can set that fall.
Is this the same as plumbing a post?
No. Plumb is vertical. This vial, on a line, is only horizontal. A post can lean and still meet a level string. Check the post with a long spirit level, then use the string for the tops. The hanging vial does not do both.

Related pages

String spirit level buying guide

String spirit level for bricklaying

String spirit level

Best string line level UK: match the listing to the job

String line level vs laser level for a datum or fall

Sources

Public UK sources and the amazon.co.uk listings in the research digest, checked on 16 October 2026. Shelf prices can change.

  1. Approved Document H: drainage and waste disposal (retrieved 16 October 2026)
  2. Consumer Rights Act 2015, section 22 (retrieved 16 October 2026)
  3. Stanley 80 mm line level 0-42-287 (retrieved 16 October 2026)
  4. BS 958:1968, precision spirit levels (retrieved 16 October 2026)
  5. BuildWiz: builders' line (retrieved 16 October 2026)
  6. HSE: slips and trips in construction (retrieved 16 October 2026)
  7. Johnson Level: line levels (retrieved 16 October 2026)
  8. amazon.co.uk: string line level (retrieved 16 October 2026)