Floor Slope Calculator: Rise, Run & Percent Grade

This tool calculates the slope of a floor from the total rise (vertical change) and the run (horizontal length). Enter the rise and run in any consistent unit, and you get back the slope as a percentage, an angle in degrees, and a ratio (1:X) — the three formats most often used in construction and accessibility work.

Formula

The slope is the vertical rise divided by the horizontal run, expressed as a percentage:

Slope (%) = (Rise ÷ Run) × 100

Where:

• Rise – vertical change in height (inches, cm, etc.)
• Run – horizontal distance (inches, ft, m, etc.)

The angle in degrees is: θ = arctan(Rise ÷ Run) (using the same units for both).

The ratio 1:X is: X = Run ÷ Rise (so a 1:20 slope means 1 unit of rise per 20 units of run).

Worked Example

Using the defaults: rise = 1.5 inches, run = 120 feet.

1. Convert run to inches: 120 ft × 12 = 1440 in
2. Slope (%) = (1.5 ÷ 1440) × 100 = 0.1042 × 100 = 0.10%
3. Angle = atan(1.5 ÷ 1440) = atan(0.001042) = 0.06°
4. Ratio = 1440 ÷ 1.5 = 1:960

That matches the calculator output: 0.10%, 0.06°, 1:960.

Why Floor Slope Matters

Floor slope is not just a geometry exercise. It affects drainage, accessibility, and safety. For instance:

• Garage floors are typically sloped 1–2% toward the door for water runoff.
• Shower floors usually need 2% (about ¼ inch per foot) to drain properly.
• Accessibility ramps often require a maximum 1:12 slope (about 8.33%) for wheelchair use.
• Wood or tile floors should be flat within a certain tolerance to avoid tripping or hollow spots.

Knowing the exact slope helps you decide if a floor meets code, if water will pool, or if furniture will wobble.

How to Measure Rise and Run

You can measure the rise and run with simple tools:

• Rise: Use a level and a tape measure. Place the level on the floor, lift one end until it reads level, and measure the gap below the lifted end. That gap is the rise over the length of the level.
• Run: The horizontal distance between the two points where you measured the rise. If you used a 4-foot level, the run is 4 feet (48 inches).

For larger areas like a room, you can measure the height difference between two points along the floor and the distance between them. Always measure over the longest practical distance to average out small bumps.

Common Mistakes and Unit Mix-Ups

The most frequent error is mixing units. If you measure rise in inches and run in feet, you must convert one to match. This calculator expects the run in feet (and converts internally), but if you use a different unit, be consistent.

Another mistake is confusing slope percentage with degrees. A 100% slope equals 45°, not 90°. The percentage is rise/run × 100, while degrees are the arctangent of rise/run. Many people think 2% slope is 2°, but it’s actually about 1.15°.

Also, don’t forget that the run is the horizontal distance, not the distance along the floor surface. If you measure the sloped surface length, that’s the hypotenuse, not the run.

Reading the Results

Here’s what each output means in practice:

• Slope %: The most common way to express floor slope. 1% means a 1-unit rise per 100-unit run. For a 10-foot run, 1% is a 1.2-inch rise.
• Angle: Useful for comparing to tools like a digital protractor. Most floors are under 2°.
• Ratio (1:X): Often used in accessibility guidelines. A 1:20 slope means for every 20 units horizontal, you rise 1 unit. That’s 5%.

If the slope is very small, like 0.1%, it’s essentially flat. If it’s over 5%, it’s a ramp rather than a floor.

Limitations of This Calculation

This calculator assumes a straight, uniform slope between two points. Real floors may have dips, humps, or multiple slopes. For a comprehensive flatness check, you’d need more measurements (e.g., a straightedge and feeler gauge). Also, the formula does not account for deflection under load or material thickness variations. It’s a geometric tool, not a structural analysis.

For drainage, you also need to consider the direction of the slope and whether water has a path to a drain. A 2% slope in the wrong direction can cause pooling.

Frequently Asked Questions

What is a 2% slope in inches per foot?

2% slope means a rise of 2 units per 100 units of run. For 1 foot (12 inches), the rise is 12 × 0.02 = 0.24 inches, or about ¼ inch. So a common rule of thumb is ¼ inch per foot for drainage.

What is the maximum slope for a wheelchair ramp?

In many accessibility guidelines, the maximum recommended slope for a wheelchair ramp is 1:12 (about 8.33%). That means for every 12 inches of horizontal run, you can rise a maximum of 1 inch. This calculator can help you verify that ratio.

How do I check if my floor is level?

Place a 4-foot or 6-foot level on the floor. If the bubble is not centered, lift one end until it is. Measure the gap under the lifted end. Divide the gap by the length of the level (in the same units) and multiply by 100 to get the slope percentage. For example, a ¼-inch gap over a 48-inch level is (0.25/48)×100 = 0.52%.

What does a 1% slope look like?

A 1% slope is subtle. Over 10 feet, it rises 1.2 inches. Over 4 feet, it rises 0.48 inches (about ½ inch). It’s enough to drain water slowly but not noticeable as a ramp.

Can I use this calculator for outdoor slopes?

Yes, the math is the same for any sloped surface—driveways, patios, sidewalks. Just ensure you measure the horizontal run, not the surface length, and use consistent units.

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in rise | Rise (vertical change) | in | 1.5 | 0
in run | Run (horizontal length) | ft | 120 | 0
out slope_pct | Slope | % | (rise / (run * 12)) * 100 | 2
out angle_deg | Angle | deg | atand(rise / (run * 12)) | 2
out ratio | Ratio | 1:X | round(run * 12 / rise, 1) | 1