Checking level tells you whether a surface is horizontal; checking plumb tells you whether it is vertical. With a spirit level and a few simple checks, you can spot a tilt before fitting shelves, hanging a door, setting a post, or starting another project.
Level and plumb are related but not interchangeable: a shelf can be level while its supports are not plumb, and a wall can be plumb even if a nearby floor is not level. Start by choosing the right reference surface and a level long enough to span the area you need to assess. If you are checking a large structure or a task with specific building requirements, follow the project instructions and applicable local requirements.
What You’ll Need
- A spirit level, long enough to bridge the surface where practical. A shorter level can check a small area, but it may not reveal a gradual slope across a longer span.
- A straightedge, if the surface is too wide or uneven for the level to sit steadily. It should be rigid and straight.
- A pencil or removable marker for noting high spots, low spots, and readings.
- A stable surface or support for the workpiece, if you are checking a loose item.
- For a tall wall or post, a plumb line or a level with a vertical vial can help check plumb.
Use a level that is clean and undamaged. If you are unsure whether it reads accurately, test it before relying on it, as described below. Keep fingers clear of pinch points when positioning a level against heavy or moving materials.
Step-by-Step
- Decide what you are checking. Identify the exact surface and the result you need. A countertop, shelf, or floor is usually checked for level. A wall, post, door jamb, or cabinet side is usually checked for plumb. A stair rail or drain may need a deliberate slope, so do not assume that every surface should be level. Check the plans or project instructions when a slope or tolerance is specified.
- Prepare a stable contact area. Brush away dust, grit, dried adhesive, and loose debris from both the surface and the level’s edges. A small particle under one end can make the tool rock and give a misleading result. If the surface is rough, place a straightedge across it and set the level on the straightedge. Confirm that the straightedge itself is not bowed or twisted.
- Check the level before use. Put the level on a reasonably flat surface and note where the bubble sits between its marks. Turn the level end for end in the same position. If the reading changes noticeably, the level may be inaccurate, the contact surface may be uneven, or the tool may not have been placed the same way. Repeat the comparison carefully. If it still disagrees, use a verified tool rather than making adjustments from a doubtful reading.
- Check for level. Place the level flat on the surface, with its horizontal vial facing upward. Keep it straight and centered over the area you want to check. Wait for the bubble to settle. The surface is level in that direction when the bubble sits between the vial’s marked lines. If it moves toward one mark, the surface is tilted toward that end. Do not lift or twist the level while reading it.
- Check the other direction. A surface can be level from front to back but slope from side to side. Move the level to another position across the surface, or rotate it ninety degrees and check again where there is enough room. For a long worktop or shelf, take readings at more than one location. This helps reveal a twist, dip, or rise that a single reading would miss.
- Check plumb on a vertical face. Set the level’s edge against the wall, post, jamb, or cabinet side. Use the vertical vial and hold the tool upright without leaning it forward or backward. The face is plumb in that plane when the bubble is centered between the marks. Check near both sides of a broad surface if you need to find whether it is twisted as well as tilted.
- Use a plumb line when the level cannot reach. Hang a plumb line from a stable point near the top of the surface. Let the weight settle and stop swinging before judging the result. The line gives a vertical reference. Compare the surface with the line at the top and bottom, without letting the line touch or catch on the surface. A visible gap that changes from top to bottom indicates that the surface is not parallel to the vertical reference.
- Mark the reading and decide what needs changing. Note which end or side is high, and where the bubble sits. For a loose shelf or frame, adjust its position and repeat the checks. For a fixed structure, identify how it is supported before changing anything; do not force, cut, or shim a load-bearing component without understanding the design. When a project specifies an allowable deviation, compare your reading with that requirement rather than relying on a general rule.
- Recheck after adjustment. Make changes in small, controlled increments, then check level and plumb again. Check more than one direction and confirm that correcting one reading has not made another worse. Once the item is fixed in place, take a final reading. Fasteners and tightening can shift a piece slightly, so a check made only before fastening may not represent its finished position.
Common Mistakes
Confusing level with plumb: Level is horizontal; plumb is vertical. Use the horizontal vial for a horizontal surface and the vertical vial for a vertical face. Do not judge a wall by comparing it to a floor that may itself be sloped.
Relying on one reading: One position checks only the area and direction covered by the tool. A long surface may have a dip in the middle or a twist across its width. Move the level and check the other direction before concluding that the whole surface is true.
Using a short tool on a long span: A short level may show that a small patch is level while missing a gradual change over the full length. Use a longer level where it fits, or a straightedge that can bridge the span. Avoid joining loose objects end to end as a substitute; their combined edges may not provide a reliable reference.
Reading while the tool is moving: A bubble needs time to settle. Vibration, a rocking level, or pressure from your hand can shift the reading. Set the tool down gently, release unnecessary pressure, and wait before deciding.
Assuming the bubble proves the tool is accurate: A bubble centered in a faulty level can still give a false result. The end-for-end comparison helps reveal disagreement. Also inspect the tool for damage and make sure it rests evenly on the surface.
Forcing a result where slope is intentional: Some surfaces are designed to drain or guide water. A sloped surface is not automatically a mistake. Confirm the intended direction and requirement before adjusting it.
Tips
- Keep the level’s measuring edges clean and do not drop it. Damage can affect how it sits or how its vial reads.
- Use the longest practical level for the span, but make sure it is supported evenly and does not sag.
- When checking a wall, press the level gently against it at two separated locations. If the face is textured, choose stable contact points rather than balancing the tool on raised bumps.
- For a broad panel or cabinet, check both vertical directions and compare the corners. This can help distinguish a simple lean from a twist.
- Mark high and low areas before adjusting. Clear marks make it easier to see whether a change improved the intended reading.
- Recheck after moving, fastening, or loading the item. A shelf, for example, may shift as its fasteners are tightened or as weight is placed on it.
- If the job depends on precise alignment, follow its specified tolerance and use a suitable measuring method. A basic spirit level may not be sufficient for every installation.
Troubleshooting
The bubble will not settle: Check whether the surface or level is vibrating, rocking, or being pressed unevenly. Move away from nearby movement, clean the contact edges, and place the tool down gently. If the vial is damaged or the bubble appears stuck, do not rely on that level.
The reading changes when you flip the level: Make sure you turned it end for end without moving to a different part of the surface. Repeat the test on a stable, flat area. A persistent change suggests that the tool or test surface may be unreliable; compare with another checked level before adjusting the work.
The level says the surface is true, but it looks crooked: The reference may be misleading, or the problem may be a visual alignment issue rather than level or plumb. Check another direction and location, then compare with a reliable nearby reference. A wall, floor, or adjoining structure is not automatically straight or level.
One part reads level and another does not: The surface may be bowed, twisted, uneven, or supported unevenly. Record readings at several positions and use a straightedge to locate changes. Do not try to correct only one point if the underlying support is moving or damaged.
You cannot fit the level against the surface: Use a rigid straightedge as a bridge, provided it can sit steadily. For a tall or obstructed vertical surface, a plumb line can provide a reference. If the shape, height, or access makes the check unsafe or unreliable, use an appropriate measuring tool or ask a qualified tradesperson to assess it.
An adjustment fixes one reading but worsens another: Stop and check whether the object is twisted or whether the supports are uneven. Return to the intended reference, adjust the support arrangement methodically, and repeat checks in both directions. For fixed structural work, do not keep adding force or improvised packing; establish what the design allows first.
Frequently Asked Questions
What is the difference between level and plumb?
Level means horizontal, like a correctly aligned shelf. Plumb means vertical, like a properly upright post. A surface can meet one condition without meeting the other.
How can I tell which way a surface slopes?
Place the level on it and observe which direction the bubble moves. Read the vial markings and note the end or side that is high. Confirm the direction by turning the level or checking another position; do not infer the slope from the bubble alone without knowing the vial’s markings.
Can I use a phone as a level?
A phone with a level or inclinometer function may help with a rough check, but its result depends on the device, app, calibration, and how the phone contacts the surface. It is not a substitute for a suitable, checked level when accurate alignment matters.
How do I check whether my spirit level is accurate?
Set it on a stable surface and note the bubble position. Turn it end for end in the same place and compare. A substantial change means the tool or its contact with the surface needs investigation. Repeat the test or compare it with another reliable level before using it for important adjustments.
Does a centered bubble mean a wall is straight?
No. A centered vertical bubble indicates plumb only where the level is placed and in that plane. A wall may still bow, have a hollow, or twist elsewhere. Check multiple positions and use a straightedge if you need to assess flatness.
Should every surface be perfectly level?
No. Some installations require a deliberate slope, and a finished surface may be judged against a stated tolerance rather than exact level. Check the project instructions before adjusting anything, especially where drainage, structural support, or adjoining components are involved.