The Floor Sag Calculator divides the span between supports by the sag you measured at mid-span to give a ratio such as L/300. It compares that ratio with commonly cited limits, L/360 where tile or a plaster ceiling is involved and L/240 otherwise, and labels the floor Within common limits, Noticeable — assess, or Excessive — engineer. It also shows the sag each limit allows for your span.
- Ratio = span ÷ sag, both in millimetres, shown as L/x; a bigger x means a stiffer floor.
- Tile or plaster ceiling below: compared with L/360. Other finishes: compared with L/240.
- Up to 1.5 times the allowed sag is Noticeable — assess; more than that is Excessive — engineer.
- Measure sag against a string line or laser between supports, not against level.
- Deflection limits are design criteria for new work; an older floor over the limit is a flag, not a verdict.
Floor Sag Calculator
01 /What does the Floor Sag Calculator do?
It puts a number on a sagging floor and compares it with the yardsticks designers commonly use. Floors are usually judged not by how many millimetres they sag but by how much they sag relative to their span, written as a ratio L/x, where L is the span between supports. A floor that sags 10 mm over 3.6 m is at L/360; the same 10 mm over 2.4 m is L/240, a much floppier floor for its length.
You enter three things, and the calculator returns the ratio, a label, and the sag the two common limits would allow on your span. It does not know what your joists are, what they carry or why they sag; for causes and repairs, read sagging floors.
02 /What do you enter?
| Input | Units | Notes |
|---|---|---|
| Span length between supports | Metres | The clear distance between the two supports of the sagging span, such as foundation wall to centre beam |
| Measured sag at mid-span | Millimetres | From the string line or laser method below |
| Finish | Choice | Tile or plaster ceiling below / other finishes |
Converting from imperial: multiply feet by 0.3048 for metres (a 12 ft span is about 3.66 m), and inches by 25.4 for millimetres (half an inch is about 12.7 mm).
Choose tile or plaster ceiling below if the floor carries ceramic or stone tile, or if the ceiling underneath is plaster; both crack at smaller deflections than wood, carpet, vinyl or drywall. Choose other finishes for everything else.
03 /How do you measure sag at mid-span?
Measure sag against a straight line between the supports, not against a level. A whole floor can tilt because one support has settled; that is slope, and the calculator is about bending between supports.
- Find the span
In the basement, find what the joists bear on at each end. Measure between those supports in metres; that is the span.
- String line method
On the floor above (or along the underside of a joist), set two identical blocks, for example 25 mm thick, directly over the supports. Pull a mason's line tight over them. At mid-span, measure from the line down to the floor and subtract the block thickness. The result is the sag.
- Laser method
Set a line or rotary laser to one side. Read the height to the floor at each support and at mid-span. Sag = mid-span reading minus the average of the two support readings (with readings measured down from the laser line).
- Check for humps and local dips
Take a few readings either side of mid-span. The largest reading near the middle is the one to enter; a sharp local dip may be a separate problem such as a cut joist.
- Record it
Mark the points, write down the numbers and date, and repeat later to see whether the sag is changing.
04 /How does the calculator work?
The calculation is deliberately simple so you can check it by hand.
- Ratio. The span is converted to millimetres and divided by the sag in millimetres. The result is shown as L/x. For a 4.0 m span with 14 mm of sag: 4,000 ÷ 14 ≈ 286, so L/286.
- Relevant limit. L/360 if you chose tile or plaster ceiling below; L/240 for other finishes. These are commonly cited serviceability limits, not a code check of your floor.
- Allowed sag. The span divided by the limit: on 4.0 m, L/360 allows about 11.1 mm and L/240 about 16.7 mm. Both are shown for every span.
- Label. If the ratio is equal to or better than the relevant limit, the result is Within common limits. If the sag is more than the limit allows but no more than 1.5 times that amount, it is Noticeable — assess. Beyond 1.5 times, it is Excessive — engineer.
| Finish | Within common limits | Noticeable — assess | Excessive — engineer |
|---|---|---|---|
| Tile or plaster ceiling below (L/360) | L/360 or better | Between L/360 and L/240 | Worse than L/240 |
| Other finishes (L/240) | L/240 or better | Between L/240 and L/160 | Worse than L/160 |
| Span | Sag at L/360 | Sag at L/240 |
|---|---|---|
| 2.4 m (7 ft 10 in) | 6.7 mm | 10.0 mm |
| 3.0 m (9 ft 10 in) | 8.3 mm | 12.5 mm |
| 3.6 m (11 ft 10 in) | 10.0 mm | 15.0 mm |
| 4.2 m (13 ft 9 in) | 11.7 mm | 17.5 mm |
| 4.8 m (15 ft 9 in) | 13.3 mm | 20.0 mm |
| 6.0 m (19 ft 8 in) | 16.7 mm | 25.0 mm |
05 /Worked examples
| Span | Sag | Finish | Ratio | Result |
|---|---|---|---|---|
| 4.0 m | 8 mm | Tile or plaster | L/500 | Within common limits |
| 4.0 m | 14 mm | Tile or plaster | L/286 | Noticeable — assess (allowed 11.1 mm; 1.5 times is 16.7 mm) |
| 4.0 m | 14 mm | Other finishes | L/286 | Within common limits (allowed 16.7 mm) |
| 4.0 m | 22 mm | Other finishes | L/182 | Noticeable — assess (1.5 times is 25.0 mm) |
| 4.0 m | 30 mm | Either | L/133 | Excessive — engineer |
| 3.6 m | 12 mm | Tile or plaster | L/300 | Noticeable — assess (allowed 10.0 mm; 1.5 times is 15.0 mm) |
The second and third rows show why the finish choice matters: the same 14 mm dip that is comfortably acceptable under carpet is the kind that cracks tile and grout.
06 /What are the limits of the result?
- Design limits and measured sag are different things. Designers usually check deflection under a calculated live load for a new floor. What you measure in an existing house is total accumulated sag: original tolerances, decades of creep, shrinkage and any settlement of supports together.
- It says nothing about strength. A floor can be within limits and still have a cracked joist or rot at a bearing, or be over the limit and structurally sound. Look underneath.
- Change matters more than size. A stable L/250 in a 90-year-old house is less concerning than an L/400 floor that was L/600 last year.
- Supports must be real supports. If a centre post has sunk, the "span" you measured is really two spans with a dropped middle; the fix is at the post, not the joists.
If the result is Noticeable or Excessive, check the framing for the causes in sagging floors, compare repair approaches in sistering floor joists and beams, posts and footings, and see planning costs in the structural repair cost guide. To combine sag with other signs, run the Structural Warning Sign Triage, or send a request with your numbers and photos.
FAQQuestions people ask
How much floor sag is acceptable?
Commonly cited limits are L/360 under brittle finishes such as tile or plaster and L/240 otherwise, which on a 4 m span is about 11 mm and 17 mm. They are design criteria for new work; an older floor beyond them deserves a closer look, not panic.
What does L/360 mean?
It means the sag at mid-span is no more than the span divided by 360. On a 3.6 m span that is 10 mm; on a 4.8 m span, about 13.3 mm.
Why does tile use a stricter limit?
Tile, grout and plaster are brittle and crack at smaller movements than wood, carpet or drywall, so floors under them are usually designed to deflect less.
Should I measure sag with a level?
Use a level to see the shape, but measure sag from a straight line between the supports, such as a tight string over equal blocks or a laser. A level reading mixes sag with any overall tilt.
My floor is Excessive. Is it unsafe?
Not necessarily. The label says the floor is well beyond common stiffness limits. Safety depends on the framing's condition and whether it is moving. Check for split members, crushed bearings, rot and change, and get an engineer's opinion.