Plumb Level
Press your phone flat against a wall or doorframe and the on-screen plumb bob reads how far off vertical it is.
How to Use This
Carpenters have been hanging weights from strings for about 4,000 years to find true vertical. Tie a brass cone to a length of cord, let it dangle, and gravity does the rest — the string traces a perfect plumb line. This page does the same trick with a string and a tiny brass bob drawn on the screen, except here gravity is your phone's accelerometer telling the bob which way is down.
Stand the phone on its long edge against whatever you're checking — a TV mount, the side of a cabinet, the edge of a picture frame that's already on the wall. The string swings if the surface is leaning. The big number underneath tells you the lean in degrees: 1.4°, 3.7°, whatever it actually is.
If the surface is off-plumb, an arrow lights up next to the number — it points which way to nudge things to bring the string back toward center. Adjust, watch the string swing, repeat until the bob settles inside the green tolerance band at the top. That band is ±0.3° wide, which is tighter than most carpenters work to and plenty for hanging shelves, mounting picture frames, or telling you whether the doorframe was actually set straight.
Freeze locks the current reading so you can lift the phone off the surface and still see what it was — useful when whatever you're measuring is in an awkward corner like behind a couch or above a fridge. Calibrate handles the case where your phone case has a slight back-curve that throws the zero off; press the phone against a doorframe you trust to be plumb, tap Calibrate, and the offset disappears. Reset clears the calibration.
Frequently Asked Questions
What's the difference between level and plumb?
Level is horizontal — parallel to the ground. A countertop should be level. A floor that drains properly is almost level, with a slight intentional tilt. Plumb is vertical — straight up and down, ninety degrees off level. Walls should be plumb. So should the sides of a doorframe and the posts holding up a porch awning. A picture frame needs both: level along the top, plumb along the sides.
How accurate is this?
The accelerometer in a modern phone reads tilt to about 0.1–0.3 degrees, which is the same ballpark as a budget bubble level you'd buy at a hardware store. The tolerance band on this page is ±0.3°. That's tight enough to hang a TV without it visibly leaning, but if you're framing a load-bearing wall, get a proper plumb laser. Most home jobs land in the sweet spot where the phone reading is fine.
Why a plumb bob design instead of just a number?
A bare number reads as abstract. The digital plumb bob gives your eye a literal answer — the brass weight falls toward gravity, you see the lean, you don't have to translate "2.7° right" into a mental picture of which way the wall is tipping. There's a reason carpenters have used a string and a weight since ancient Egypt without much redesign. Numbers tell, the string shows.
My TV is already mounted. How do I check it without taking it down?
Stand the phone on its long edge with the back pressed flat against the side of the TV's bezel. The plumb line on screen runs along the same axis the bezel runs, so the readout is the bezel's tilt. Same trick works on doorframes, cabinet faces, the leg of a wardrobe, or that floating shelf in the kitchen. As long as the phone's back can lie flat against the surface, you can check whether it's plumb without unscrewing anything.
Why is my phone reading off by a degree even on a known-plumb surface?
Two likely culprits. First, your phone case — a lot of cases have a slight curve along the back, which adds a fraction of a degree to every reading. Second, the "known-plumb" surface might not actually be. Builders aim for plumb but settle for close-enough, and a year of seasonal moisture changes can pull a doorframe out by half a degree. Best fix: press the phone against the most trustworthy plumb surface you have (interior doorframes in newer construction tend to be reliable), tap Calibrate, and the offset goes to zero.
Does this work without internet?
Yes, after the first load. The page is a single HTML file plus a sprinkle of JavaScript — once it's open in your browser, the sensor wiring runs entirely on your phone. No tilt data leaves the device. Save the page as a home-screen shortcut on iOS or Android and you can open the level in airplane mode, in a basement, or on a job site without cell signal.