Measure the floor
Measure how much a floor is out of level and see what it means. Enter the drop and the span, and the tool gives you the slope four ways and reads it against the tolerances — and the distress signs that tell original construction from active settlement.
What it does
This tool interprets a measurement. It does not diagnose structural safety or replace an inspection.
Why it matters
Almost every home has some slope in its floors. What separates a harmless original-construction slope from active settlement isn't the reading alone — it's whether the slope comes with distress(cracked drywall, sticking doors, gaps at the trim) and whether it's changing over time. This tool gives you the measurement in comparable units, then reads it against both the tolerance bands and the distress signs you observe.
It never says your foundation is safe, failing, or settling. Those are decisions for an in-person review and sometimes an engineer. What it does is turn a fuzzy "this floor feels off" into a number you can track and take to a professional.
How to measure it
The 4-foot level method is easiest: set a level on the floor, lift the low end until the bubble centers, and measure the gap you had to shim — that gap is the drop over 4 feet. The drop-over-span method works for longer runs: measure the height difference between two points and the distance between them. Either way, use the longest clear stretch of floor and measure it the same way each time so you can compare later.
Reading the result
| Slope | Typical read |
|---|---|
| Under 1/4" per 10 ft | Normal — usually original construction |
| 1/4"–1/2" per 10 ft | Mild — monitor; watch for distress |
| 1/2"–1" per 10 ft | Meaningful — worth an evaluation |
| Over 1" per 10 ft | Significant — get it evaluated |
These are guidelines, not limits — a small slope with distress can matter more than a larger one without it, which is why the calculator weighs both.
At your measuring run
The bands above are stated per 10 feet, but nobody measures a 10-foot run by accident. Here is the same thing as an actual drop across the runs people really measure — find your span, and see how many inches of drop it takes to leave normal.
| Span you measure | Mild from | Meaningful from | Significant from |
|---|---|---|---|
| 4 ft | 0.1" of drop | 0.2" of drop | 0.4" of drop |
| 10 ft | 0.25" of drop | 0.5" of drop | 1" of drop |
| 15 ft | 0.38" of drop | 0.75" of drop | 1.5" of drop |
| 20 ft | 0.5" of drop | 1" of drop | 2" of drop |
| 30 ft | 0.75" of drop | 1.5" of drop | 3" of drop |
Derived from the same thresholds the calculator classifies against. A floor that lands in a band and has no cracks, sticking doors, or separations is usually original construction; the same slope alongside those signs is the pairing worth an evaluation.
Measure the height difference between two points and the distance between them — the longest clear run of floor gives the most reliable reading.
Slope is one clue, not a diagnosis. It reads more reliably alongside the distress signs and a look at the whole home.
FAQ
Two easy ways. Set a 4-foot level on the floor, shim the low end until the bubble centers, and measure the gap you shimmed — that gap is the drop over 4 feet. Or measure the height difference between two points and the distance between them. Use the longest clear run of floor for the most reliable reading.
As a rough guide: under about 1/4 inch per 10 feet is normal, 1/4 to 1/2 inch is mild, 1/2 to 1 inch is meaningful, and over 1 inch per 10 feet is significant. The NAHB notes a floor can slope up to about 1/2 inch over 20 feet without being a defect. Context matters as much as the number.
No. Many homes have some slope from the way they were originally built, and a slope with no cracks, sticking doors, or separations is often just that. A slope that comes with those distress signs, or that grows when you re-measure, is the kind more likely to reflect active movement.
It's the drop divided by the distance, written as '1 in N.' Around 1 in 300 (about 0.8 inch over 20 feet) is a commonly cited threshold before structural concern in a wood-framed home, and cosmetic damage like cracked drywall can start near 1 in 500. The calculator shows this ratio for your measurement.
Re-measure the same run over time and watch for distress. Original slope tends to be stable and undamaged; active settlement grows between readings or shows up alongside new cracks, sticking doors, and gaps at trim. One measurement can't tell the difference — a second one months later can.
A 4-foot or 6-foot level, a laser level, or a water level for long spans. A rolling marble only hints that a floor isn't level — it doesn't measure how much. Whatever you use, measure the same run the same way each time so readings are comparable.
Reach out for a significant slope, a slope that's growing, or any slope paired with cracks, sticking doors, or separations. A foundation contractor can explain options; a structural engineer is useful for an independent opinion when repairs are expensive or recommendations conflict. This tool organizes a measurement — it does not diagnose your foundation.
Related
What causes floors to slope and when it matters.
OpenThe other symptoms that travel with movement.
OpenHow settlement is corrected if it's active.
OpenTrack a crack's width alongside the floor slope.
OpenThe same idea applied to a wall that is leaning in.
Open