A plumber’s level, usually a short torpedo-style tool, has two or three glass vials filled with liquid and an air bubble. You read it by checking where the bubble sits relative to the lines marked on each vial. When the bubble is perfectly centered between the two lines, the surface is either level (horizontal) or plumb (vertical), depending on which vial you’re looking at. The third vial, found on most plumbing levels, measures pitch, which is the slight slope needed for drain pipes to flow properly.
The Three Vials and What They Tell You
Most plumber’s levels have three vials, each oriented differently. The horizontal vial sits flat along the length of the tool and tells you whether a surface is level. When the bubble rests exactly between the center lines, the surface has zero slope. The vertical vial is mounted at 90 degrees and tells you whether a pipe or surface is plumb, meaning perfectly straight up and down. This is the vial you watch when installing vertical drain stacks or supply lines.
The third vial is what makes a plumber’s level different from a standard carpenter’s level. This is the pitch vial, set at a 45-degree angle, and it measures the slight downhill slope that drain pipes need to carry wastewater by gravity. On many pitch vials, the markings are calibrated so each increment represents 1/8 inch of drop per foot of pipe. When the bubble touches the zero line, the surface is level. As you tilt the pipe, the bubble moves toward the outer marks. If the bubble reaches the fourth line from center, for example, that indicates a slope of about 1/2 inch per foot.
Reading the Bubble Correctly
The key to reading any vial is your eye position. Look at the vial straight on, not from an angle, since even a slight shift in your viewing angle can make the bubble appear off-center when it isn’t. The bubble should float freely and settle quickly. If it’s sluggish or sticks, the vial fluid may have degraded and the level needs replacing.
For a level reading, center the bubble precisely between the two parallel lines etched on the vial. The bubble doesn’t need to fill the entire space between the lines. It just needs to be evenly positioned so the gaps on both sides of the bubble are equal. If the bubble drifts toward one end, that end of the pipe or surface is higher. Lower the high end or raise the low end until the bubble centers.
For a plumb reading, rotate the level vertically and hold it against the pipe or surface. The same rule applies: the bubble centered between the lines means the surface is perfectly vertical.
Setting the Right Drain Slope
Drain pipes can’t be installed perfectly level because water won’t flow without gravity. Plumbing codes require specific slopes depending on pipe diameter. Pipes sized 2 inches or smaller need a minimum slope of 1/4 inch per foot. Pipes between 3 and 6 inches need at least 1/8 inch per foot. Pipes 8 inches and larger require a minimum of 1/16 inch per foot.
To set this slope with your level, place the tool on top of the pipe and tilt the pipe until the bubble in the pitch vial reaches the correct mark. If your pitch vial is calibrated in 1/8-inch increments, you’d move the bubble to the second mark for a 1/4-inch-per-foot slope (common for smaller residential drains) or to the first mark for 1/8 inch per foot (standard for 3- and 4-inch pipes). Some levels have the measurements printed directly on the vial housing, which makes this easier.
If your level only has a standard horizontal vial with no pitch markings, you can create the slope manually. For a 1/4-inch-per-foot drop, place a 1/4-inch shim under the low end of the level for every foot of level length. A 9-inch torpedo level would need roughly a 3/16-inch shim. When the bubble reads level with the shim in place, the pipe is at the correct slope.
V-Grooves and Magnets
Plumber’s torpedo levels have a V-shaped groove running along the bottom edge. This groove lets the level sit securely on round pipes without rolling off. Place the groove directly on top of the pipe so it cradles the curve, and the vials will give you an accurate reading of the pipe’s orientation.
Many plumbing levels also have built-in magnets along the base. These grip onto steel and iron pipes, letting you work hands-free. You can stick the level to a vertical cast-iron drain stack, step back, and read the plumb vial without holding the tool in place. The magnets won’t help on PVC or copper, so on those materials you’ll need to hold the level steady yourself or use the V-groove to keep it seated.
How to Check if Your Level Is Accurate
A level that reads incorrectly is worse than no level at all, especially when you’re setting drain slopes where even a small error causes standing water or slow drainage. You can test accuracy in under a minute.
For the horizontal vial, place the level on a flat surface (it doesn’t need to be perfectly level). Note exactly where the bubble sits. Then rotate the level 180 degrees, like spinning a helicopter blade, so the left end is now where the right end was. Place it back in the same spot. If the bubble is in the exact same position relative to the lines, the vial is accurate. If the bubble shifts, the level is off and should be replaced.
For the vertical vial, hold the level against a wall and mark the wall along the top edge and at both ends. Flip the level end-over-end so the top becomes the bottom, then align it with your marks. Again, the bubble should read identically. Any difference means the plumb vial is unreliable. Inexpensive torpedo levels are not worth recalibrating. Replace them and retest the new one before trusting it on a job.