How to Measure Floor Tilt with a Smartphone | What Angle Is 6/1000 and When to Worry
A marble rolls away, a door swings shut on its own, or you just feel slightly off. When you start to wonder whether your floor slopes, the phone in your hand is the obvious first tool. This guide explains what the 3/1000 and 6/1000 thresholds used in Japan mean and what angles they correspond to, how accurate a smartphone level is and where it falls short, how the reversal method cancels the error, other ways to measure such as marbles and a water level, and who to talk to if the tilt turns out to be large.
The benchmarks for floor tilt are 3/1000 and 6/1000
In Japan, floor tilt is usually judged against the “technical standards for reference in housing dispute resolution” issued under the Housing Quality Assurance Act. On a floor with a smooth finish, you measure the height difference between two points at least 3 m apart and sort the tilt into three levels.
| Level | Tilt | Angle | Height difference over 3 m | Likelihood of a structural defect |
|---|---|---|---|---|
| Level 1 | Less than 3/1000 | Less than about 0.17° | Less than 9 mm | Low |
| Level 2 | 3/1000 to less than 6/1000 | About 0.17° to 0.34° | 9 mm to less than 18 mm | Moderate |
| Level 3 | 6/1000 or more | About 0.34° or more | 18 mm or more | High |
These are not lines that prove a defect. They are a guide used in dispute resolution to estimate how likely a structural problem is. A new house can fall into level 2, and a level 3 tilt can turn out to be nothing more than construction tolerance. Still, if the tilt exceeds 6/1000, it is well worth investigating the cause.
How much a smartphone level can tell you
A smartphone's tilt sensor is typically off by about ±0.1–0.5°, varying from phone to phone. 6/1000 is about 0.34° and 3/1000 is about 0.17°, so the error is about the same size as the thresholds themselves. You cannot decide between “level 2” and “level 3” just by laying the phone down and reading the number.
| Tool | Typical accuracy | For judging floor tilt |
|---|---|---|
| Smartphone (uncorrected) | ±0.1–0.5° | Only to see whether there is a large tilt |
| Smartphone (corrected by reversal) | Cancels the sensor's bias and any tilt from the case | A rough idea of the direction and size of the tilt |
| Ordinary spirit level | About 0.5 mm/m (about 0.03°) | A long level is good enough for a rough judgement |
| Laser line level | A few mm per 10 m, depending on the product | Measures the height difference between distant points directly |
On the other hand, for levelling a shelf, a picture frame, a washing machine, a fridge or an aquarium, an error of about 0.5° rarely matters, and a phone is perfectly useful. When measuring floor tilt, correct the error using the steps in the next section and take readings in many places to understand the overall trend.
Steps to measure floor tilt with a smartphone
- 1
Get a long, straight board
A phone only spans about 15 cm, so it is affected by small bumps and joints in the floor. Placing it on a straight piece of timber, an aluminium angle or a leftover floorboard about 1.8 m long averages out the tilt between two distant points.
- 2
Get rid of the camera bump
If the rear camera sticks out, the phone tilts when laid screen-up. Put it in a case that makes the back flat, or stand it on its long edge against the board.
- 3
Correct the error with the reversal method
Put the phone in one spot on the board and note the first reading, then turn only the phone (not the board) 180° horizontally, put it back in the same spot and note the second reading. The true tilt of the surface is (first − second) ÷ 2, and the sensor's bias is (first + second) ÷ 2.
- 4
Measure in several places, lengthwise and crosswise
Measure in several spots, such as the middle of the room, by the window and by the walls, along both the long and short directions of the room. Writing arrows and values on a floor plan shows which way the floor drops.
- 5
Convert to parts per thousand and record it
Take the tangent of the angle and multiply by 1000 to get parts per thousand. 0.25° is about 4.4/1000. The tool shows the “x/1000” value alongside the angle you measured.
Here is a worked example of the reversal method. If the first reading is +0.40° and the second is −0.10°, the sensor's bias is (0.40 + (−0.10)) ÷ 2 = 0.15°, and the true tilt of the surface is (0.40 − (−0.10)) ÷ 2 = 0.25° (about 4.4/1000). Reading the 0.15° bias at face value would have badly misjudged the tilt.
How marbles, a water level and a laser compare
- Marbles: on a very smooth floor a marble rolls with the slightest tilt, while on a floor with joints or a rough surface it may stop even when the tilt is large. It tells you which way the floor drops, but not by how much.
- Water level (clear hose): a traditional method that uses the fact that the water surfaces at both ends of a clear hose filled with water settle at the same height, to measure the height difference between two distant points. A hose from a DIY store measures to within a few millimetres from one end of the room to the other. The trick is to leave no air bubbles inside.
- Laser line level: project a horizontal laser line onto the walls and measure the height from the floor to the line in several places. Inexpensive models exist, and they show the height differences across a whole room at once.
- Professional inspection: a home inspection measures the tilt of floors and columns with a laser level and also checks for things like cracks in the foundation.
For columns and walls, 3/1000 and 6/1000 are also the benchmarks. A plumb bob (a weight on a string) shows how far they lean from vertical. At a height of 2 m, 6/1000 is a 12 mm lean.
What to do if the tilt is large
- Look for other signs: check whether doors or windows open and close on their own or stick, whether gaps around doors differ top and bottom, and whether there are cracks in the foundation or exterior walls.
- Keep records: notes of the date, place, direction and value of each measurement, plus photos of symptoms, help when you consult an expert and when you check later whether the tilt is getting worse.
- Within 10 years of a new build or purchase, contact the seller or builder: in Japan, the Housing Quality Assurance Act makes the seller of a new house liable for defects in the main structural parts for 10 years from handover.
- Talk to a third party: in Japan, the Center for Housing Renovation and Dispute Settlement Support runs a phone consultation service (Sumai-ru Dial) where you can ask architects and other experts about problems with a house.
- If you rent, contact the management company or landlord: share the values you measured and ask for advice before doing any investigation or work yourself.
It is also important to check that the floor is level before placing heavy furniture or an aquarium. The relationship between aquarium weight and floor capacity is explained in aquarium weight and floor load capacity.
Measure floor tilt on the spot with your phone
The slope and angle meter (smartphone level) works in the browser with no app to install: put your phone on the floor or a board and read the tilt. Laid flat screen-up, it shows the tilt in two directions with a round bubble; with the long edge against a surface, it shows a tube-style bubble. When the tilt is small it also shows the “x/1000” value and how it relates to the 3/1000 and 6/1000 benchmarks. It can correct the error with the reversal method, hold a value, and set the current surface as the 0° reference.
For converting parts per thousand like 6/1000 into degrees and percent, see converting slope between percent and degrees. If you want to measure a roof pitch, also see Japanese roof pitch (sun) in degrees with a rafter-length table.
FAQ
- What angle is a 6/1000 floor tilt?
- About 0.34°. That is a height difference of 6 mm over 1 m, or 18 mm over 3 m. 3/1000 is about 0.17° (9 mm over 3 m).
- How much floor tilt is acceptable?
- Under Japan's technical standards for housing disputes, measured between two points at least 3 m apart, a tilt of less than 3/1000 means a structural defect is unlikely, and 6/1000 or more means it is likely. Even new houses can have a tilt of around 3/1000.
- Can a smartphone level measure floor tilt accurately?
- A smartphone sensor is off by about ±0.1–0.5°, which is not enough to judge the 6/1000 (about 0.34°) threshold precisely. If you place it on a long board, correct the error with the reversal method and measure in many places, it gives a useful idea of the direction and rough size of the tilt.
- Does a marble rolling mean the floor is tilted?
- It is a clue to the direction of the tilt, but on a smooth floor a marble rolls with even a tiny tilt, so it tells you nothing about the size. To get a number, measure with a spirit level, a water level or a laser.
- What is the reversal method?
- It is a way to cancel the error by measuring twice in the same spot with the phone or level turned 180° the second time. The tilt of the surface is (first − second) ÷ 2, and the sensor's bias is (first + second) ÷ 2.
Tools in this article
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