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How to use a post level

Updated: 22 October 2026

The post is in the hole, the concrete is still soft, and the post moves as soon as you let go to look at a vial. You strap it on, you read plumb on two faces, and you keep both hands for the brace. This is how to use a post level once it is already in your hand. It is the procedure, not a ranking of listings. If you are still deciding whether a long straight level would do this job, that split is [post level vs spirit level](/hub/measuring-and-marking-tools/post-spirit-level/sub/post-spirit-level-vs-spirit-level). Prices and review counts stay on the [post spirit level comparison](/hub/measuring-and-marking-tools/post-spirit-level). Peter F. used maker how-to notes and stored amazon.co.uk review themes on 22 October 2026, not a workshop test. A qualifying purchase can earn a commission at no extra cost to you.

The post is in the hole, the concrete is still soft, and the post moves as soon as you let go to look at a vial. You strap it on, you read plumb on two faces, and you keep both hands for the brace.

Our top picks

LSGZJZY LSGZJZY triple bubble

Best overall

LSGZJZY LSGZJZY triple bubble
5.6/10about £5.99
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TIMCO TIMCO 125 mm post level

Alternative

TIMCO TIMCO 125 mm post level
5.4/10about £7.52
View offer
Bubble Levels Bubble Levels 2-pack

Rank 3

Bubble Levels Bubble Levels 2-pack
8.2/10about £9.99
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On this page
  1. 1.Fit the level before you read it
  2. 2.Read plumb on two faces
  3. 3.Know a bad vial
  4. 4.When the bubble is not the problem

Fit the level before you read it

Seat the corner of the tool on the corner of the post so there is no gap. A gap moves the vials off the faces you think you are reading, and a centred bubble then describes the air, not the timber. Johnson Level’s post-level note says to pull the rubber band tight. Swanson’s version is the same split: magnets for steel, the elastic for wood, vinyl, or composite. In the stored UK reviews of a folding two-pack, the magnet is the part that fails first. Reviewers describe magnets that slide on a steel post or a Durapost unless the elastic is on, and they also describe a strap that arrived short or missing. If the band is slack, the body creeps while you look away. Pull the band until the tool does not drop when you take your hand off. Then try the magnet only as a helper. If it slides, ignore it and trust the band. On timber, concrete, copper, or a thick coating there is nothing honest for a magnet to hold. You can leave the level strapped on while you nudge the post. Take it off if the band is so tight that it bows a thin post, and refit it one notch looser before you trust the bubble.

Read plumb on two faces

Johnson’s reading is the one to keep. When the horizontal vials are centred, the post is plumb. When the vertical vial is centred, the member you have put the tool on is level, which is a rail or a shelf, not the usual fence post. You need both horizontal vials, because they look at adjacent faces. One centred vial and one ignored vial is how a post ends up leaning towards the neighbour while it looks straight from the pavement. Stand so your eye is level with the vial. Johnson’s straight-level note flags parallax: if you look down into the tube, the bubble appears to sit in a different place from the one it occupies. The same habit applies on a post. If the bubble is off the marks, move the post, then look again. Brace the post while the concrete is soft, and look once more before you walk away, because a brace that slips will walk the bubble off centre. The third vial is for later, when a rail has to be level between two posts that are already plumb. Rotating the tool to chase a third axis is optional. It does not replace the two plumb vials.
LSGZJZY LSGZJZY triple bubble

Testsieger

LSGZJZY LSGZJZY triple bubble
5.6/10about £5.99
View offer

Know a bad vial

A bubble that is larger than the marks cannot be centred, because it no longer fits the gap the marks describe. One stored review of the two-pack describes exactly that. Another describes vials that had been glued in and then fell out. Those are reasons to stop using that vial, not to average it with a guess. An under-filled tube is the same fault in a milder form. If the bubble is the wrong size, or the vial is loose in the frame, the reading is not a reading. Johnson’s reverse check is written for a straight spirit level: mark the position, turn the level end for end, and the bubble should agree. The surface does not have to be level for that check to mean something. On a post level you can sometimes turn the tool onto the opposite corner and see whether the plumb vials still agree. If they contradict each other on a post you have not moved, the tool is the problem. Do not turn one reviewer’s bad vial into a failure rate for every post level. Do stop when the vial in your hand is empty, oversized, or loose.

When the bubble is not the problem

A fencer should take over when the problem is no longer the bubble: a cable on the plans, a hole near a neighbour’s foundation, a height that needs permission, or a post that will carry a gate on an exposed site. Digging, fence height, and buried services belong on the fence post level page. The straight level comes back into your hands when you need to see a rail or a panel, not when the post vial has already failed. If the tool is not in your hand yet, the post spirit level buying guide ranks the strap and the vials.

Common questions

Which vials mean the post is plumb?
The horizontal vials. Johnson Level’s post-level guide treats a centred horizontal vial as plumb and a centred vertical vial as level. On a corner tool you read both horizontal vials, one for each adjacent face. A single centred vial is only half the post. If your tool does not mark which vial is which, watch which bubble moves when you tilt one face and leave the other still.
How tight should the strap be?
Tight enough that the body does not slide when you take your hand away, and seated so the corner of the tool touches the corner of the post. Johnson says to pull the band tight. A slack band lets the tool creep, and the next glance is a different place on the post. If the band is short, frayed, or missing, do not invent a fixing with a weak magnet and call the post plumb.
Do I read one face or two?
Two adjacent faces, not one face twice and not two opposite faces. Opposite faces can both look plausible while the post leans on the axis you never checked. The two horizontal vials are there so you see both axes before the concrete stiffens. If you only own a straight level, you still check two adjacent faces, one after the other, and you need a hand free to hold it.
What if the magnet slides down the post?
Stop trusting the magnet and use the strap. Stored reviews describe magnets that would not hold on steel or on a Durapost unless the elastic was fitted, and other cases where they held on railing posts. A slide means the bubble is travelling with the tool. Refit the band, seat the corner, and read again. Timber and concrete were never a magnet job.
What if the bubble is larger than the marks?
Retire that vial. A bubble bigger than the marks cannot sit between them, so a nearly centred look is not a plumb post. One stored review describes this fault, and another describes vials that fell out after being glued in. That is a fault in the tool you are holding. It is not a rate you can apply to every listing, and it is not something you correct by squinting.
How do I check the tool rather than the post?
Use a reverse check where the body allows it. On a straight level, Johnson’s method is to mark the spot, turn the level end for end, and see whether the bubble agrees. The surface need not be level. On a post level, turn the tool to the opposite corner without moving the post. If the vials disagree with the first reading, the post did not change and the tool did.
When do I pick up a straight level instead?
When the length matters: a rail between posts, or a panel where a 125 mm body cannot show a bow. The post level stays on the corner of the post. It does not span 1.8 m. If the vial itself is bad, a straight level is also the backup for that one post, held on two adjacent faces, until you replace the corner tool.

Related pages

Best post spirit level

Post spirit level

Post spirit level buying guide: strap, vials, fold

Fence post level: plumb, height and buried services

Post level vs spirit level when you already own one

Sources

Public UK sources from the research digest, then the amazon.co.uk listings compared on this page. Shelf prices in this four are the stored listing figures. Draper and Faithfull prices outside the set were captured on 21 October 2026. This is not a laboratory test.

  1. England, GPDO 2015, Class A (gates, fences, walls) (retrieved 21 October 2026)
  2. Consumer Rights Act 2015, section 9 (retrieved 21 October 2026)
  3. BS 958:1968, spirit levels for use in precision engineering (retrieved 21 October 2026)
  4. HSE HSG47, avoiding danger from underground services (retrieved 21 October 2026)
  5. LSBUD, line search before you dig (retrieved 21 October 2026)
  6. TIMCO post level 125 mm, model 468165, trade page (retrieved 21 October 2026)
  7. Draper 75042 on amazon.co.uk, outside this set (retrieved 21 October 2026)
  8. Faithfull FAIPOST on amazon.co.uk, outside this set (retrieved 21 October 2026)