guide
How to read a level and trust the answer.
A bubble level is a simple instrument with a few sharp edges. This page covers the four things that decide whether your reading means anything: how a vial is read, how to cancel your device’s bias, where to put the vial, and when to stop trusting a phone. It also indexes the job-by-job guides, from levelling a washing machine to surveying a subfloor before flooring goes down.
Reading a vial.
A spirit vial is a sealed tube of liquid with one air bubble in it. Because the bubble is lighter than the liquid, it climbs to the highest point of the tube. Two lines etched at the centre, called the gate, mark true level. When the bubble sits evenly between them, the surface under the vial is flat along that axis.
The direction matters more than the number when you are shimming something. The bubble moves toward the raised edge. If it drifts right, the right side is high and the left side needs packing. LevelerPro follows the same convention, and the vial turns green once the reading is inside ±0.2°.
Take two readings for anything that matters. Note the result, rotate the device 180° end-for-end in the same spot, and read again. A true surface gives the same answer both ways; a difference between the two readings is your instrument’s error, and the real value sits halfway between them.
Calibrating your device.
Phone accelerometers are mass-produced and factory-trimmed, which means yours reports a small constant offset, often a few tenths of a degree and sometimes more than a full degree. Calibration measures that offset once so every later reading can subtract it.
- Find a reference. Use the flattest surface you have access to and already trust: a machinist’s plate, a saw table, a kitchen counter you have checked with a good spirit level.
- Let it settle. Rest the device flat, screen up, and leave it alone for two or three seconds. Do not hold it.
- Tap Set Zero. The current reading becomes your new zero and is written to local storage on the device.
- Verify end-for-end. Rotate the device 180° in place. A well-calibrated setup reads close to zero in both directions. If it does not, your reference is not flat. Find a better one, or split the difference.
Recalibrate after a big temperature change, after dropping the device, and any time the resting reading looks wrong. Clear Zero restores the raw sensor output.
Choosing a placement.
A single vial only reads the axis it runs along. Placement is how you tell the tool which axis you care about. Put the vial on the edge that lies parallel to the run you are checking.
| You are checking | Turn on | Why |
|---|---|---|
| A shelf, worktop, or picture frame | Top or bottom vial | Reads left-to-right along the span |
| A door jamb, post, or fence rail | Left or right vial | Reads the vertical run for plumb |
| A table, tripod, or appliance | Centre bullseye | Both axes at once, one bubble |
| A drainage fall or ramp | Top vial, mm/m units | Target slope reads directly |
Two vials on opposite edges are a useful cross-check rather than a duplicate: they read the same axis from two positions on the plate, so a rocking or crowned surface shows up as a disagreement between them.
Common jobs.
- Hanging a shelf
- Level the bracket line, not the shelf board, because boards are rarely straight. Mark one end, set the device on the pencil line, and adjust until the top vial centres. Full guide.
- Levelling a washing machine
- Use the bullseye on top of the drum housing and adjust the feet diagonally, front-left against back-right, until the bubble parks in the inner ring. A machine inside ±0.5° walks far less on spin. Full guide.
- Setting a drainage fall
- Switch units to millimetres per metre and aim for the figure in your spec. A fall of 10 mm/m is a 1 % fall. Read along the pipe run, not across it. Conversion table.
- Checking a camera tripod
- Set zero on the camera plate first, then use the bullseye. Panning across a horizon exposes tilt that a single-axis reading hides. Full guide.
Guides for each job.
The sections above cover the instrument. These work through particular jobs end to end, including the tolerances that actually apply and the mistakes that cost the most time.
Appliances
Levelling a washing machine
15 min
Level a washing machine with a phone: which foot to turn, why diagonal rock matters more than tilt, and the tolerance that stops it walking on spin.
Fridges, dishwashers and cookers
20 min
Why a fridge wants a deliberate backward tilt, why a dishwasher wants none, and how to level a freestanding cooker so pans do not slide.
Building & DIY
Hanging a shelf level
30 min
Level the bracket line rather than the board, and use the single-datum trick that keeps a run of shelves straight in a room where nothing is level.
Checking a floor before flooring
45 min
Level and flat are different faults with different fixes. Survey a subfloor with a phone and a straightedge, then read it against flooring tolerances.
Outdoors & travel
Levelling a camera tripod
10 min
Level the tripod base, not the camera, and learn why a panorama needs it. Covers ball heads, levelling bases, bowl mounts and the pan test.
Levelling a caravan or RV
15 min
Level side-to-side with ramps before you unhitch, then front-to-back on the jockey wheel. Includes the tolerance an absorption fridge really needs.
When to reach for glass.
A calibrated phone is a good ±0.2° instrument. That covers most carpentry, appliance setting, and hanging work. It does not cover machine setup, optical benches, or anything specified in arc-minutes. A 0.02° machinist’s level exists because those jobs need it.
A phone also cannot span a distance. A 1.2 m spirit level averages a whole run in one reading, while your device only sees the few centimetres under it. On a wavy surface, lay the device on a straightedge and level the straightedge.