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What Does 1/8 Inch per Foot Mean?

Understand 1/8 in/ft as a rate of fall, calculate total change over a horizontal run, and distinguish the math from local plumbing requirements.

7 mins read

A slope of 1/8 inch per foot means the pipe changes elevation by one-eighth inch for every foot of horizontal run. Convert the fraction to 0.125, then multiply the horizontal run in feet by 0.125 to find the total fall in inches. Over 32 feet, the total fall is 4 inches: 32 × 0.125 = 4.

Pipe-fall formula

Fall (inches) = horizontal run (feet) × 0.125 in/ft

This notation describes the magnitude of a slope. It does not identify the high or low endpoint or establish the slope required for a particular installation.

How to read 1/8 in/ft

The 1/8 inch is the vertical change, and per foot is the horizontal distance over which that change occurs. The result is a rate, not a total fall.

1 foot of run → 1/8 inch of fall

8 feet of run → 1 inch of fall

16 feet of run → 2 inches of fall

Measure the horizontal run rather than the slightly longer distance along a sloping pipe. Pair the calculation with notes that identify the intended direction of fall and the reference points used on site.

Worked example: 32 feet at 1/8 inch per foot

Horizontal run: 32 feet.
Slope: 1/8 in/ft, or 0.125 in/ft.
Substitution: 32 ft × 0.125 in/ft.
Result: 4 inches of total fall.

The feet cancel, leaving inches. Four inches is one-third of a foot and exactly 101.6 millimeters. This example needs no rounding.

Fall at 1/8 in/ft for common run lengths

Horizontal run	Calculation	Total fall
4 ft	4 × 0.125	0.5 in
8 ft	8 × 0.125	1 in
16 ft	16 × 0.125	2 in
24 ft	24 × 0.125	3 in
32 ft	32 × 0.125	4 in
48 ft	48 × 0.125	6 in

Is 1/8 inch per foot the same as 1%?

Guidance sometimes calls 1/8 in/ft a 1% slope, but the exact mathematical conversion is about 1.0417%. Because one foot is 12 inches, the ratio is 0.125 ÷ 12 = 0.0104167. Multiplying that ratio by 100 gives 1.0417%.

Percent slope = (0.125 in ÷ 12 in) × 100

Percent slope = 1.0417% (approximately)

Metric rate = 10.4167 mm/m (approximately)

Over 32 feet, a true 1% slope falls 3.84 inches, while 1/8 in/ft falls 4 inches. Preserve the original notation from the drawing, specification, or applicable rule rather than replacing it with a rounded equivalent.

Common mistakes

Using 1/8 inch as the total fall. It is the fall for every foot of run; multiply by the full run.
Mixing feet and inches without conversion. Use the rate in inches per foot with a run measured in feet, or first put both sides of the ratio in matching units.
Measuring along the pipe. Slope calculations use horizontal run, not the diagonal pipe length.
Treating 1% as exactly equal to 1/8 in/ft. The values are close but not identical.
Confusing percent with degrees. A 1.0417% slope is about 0.60 degrees, not 1.0417 degrees.
Assuming one rate applies everywhere. Pipe size, system type, jurisdiction, approved plans, and site conditions can change the applicable requirement.

Calculate the fall on iPhone

The free Pipe Slope / Fall tool in Plumbing Calculator & Tools accepts a positive run in feet or meters and a slope in in/ft, percent, or mm/m. For this example, enter 32 feet and 0.125 in/ft. The calculated total fall is 4 inches.

Check the result against the manual formula, then copy or share the valid result as text or save it with an optional note inside a project. Include the source of the specified slope and the high and low reference points with the saved result.

Check the fall over the measured run

Enter the horizontal run and 0.125 in/ft in the free calculator, then verify the output against the job’s applicable requirements.

View Plumbing Calculator on the App Store

What to record

The horizontal run, its unit, and the measurement method.
The original slope notation: 1/8 in/ft or 0.125 in/ft.
The calculated total fall, including any rounding.
The intended high and low endpoints.
The drawing, specification, local rule, or professional instruction used to verify the slope.

Continue with pipe-slope calculations

For a larger in/ft rate, see What Does 1/4 Inch per Foot Mean?. For the general method, see How to Calculate Pipe Fall from Length and Slope.

Sources

NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement — gives the exact relationships of 12 inches to 1 foot, 0.3048 meter to 1 foot, and 2.54 centimeters to 1 inch.
U.S. Geological Survey: To Determine Percent of Slope and Angle of Slope — defines percent slope using elevation change and horizontal distance.
Minnesota Administrative Rules 4714.0813: Pool Deck Drains — provides a jurisdiction-specific example of 1/8 in/ft drain-piping guidance.