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Slope and Grade Calculator

Convert between percent grade, slope ratio (H:V), degrees, pipe fall (inches per foot), and rise over run. Enter any one and see all the others instantly. Includes OSHA trench cut slopes, IPC pipe minimums, ADA grades, and common site drainage references. Built for excavation contractors grading sites and laying pipe, not civil engineering students.

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The math, shown transparently

No black box. Here is exactly what the calculator computes on every input change. All conversions are standard trigonometry.

// Percent grade to slope ratio (H:V)

ratio_H = 100 / percent

ratio_V = 1

// Percent grade to degrees

degrees = atan(percent / 100) × (180 / π)

// Degrees to percent grade

percent = tan(degrees × π / 180) × 100

// Percent to pipe fall (inches per foot)

in_per_ft = percent × 12 / 100

// Ratio to percent

percent = (V / H) × 100

// Rise from horizontal distance

rise = distance × percent / 100

Worked example

A 1/4 in/ft pipe slope over a 200 ft run:

in_per_ft = 0.25

percent = (0.25 / 12) × 100 = 2.083%

ratio = 100 / 2.083 = 48:1

degrees = atan(0.02083) × 57.296 = 1.19°

rise = 200 × 2.083 / 100 = 4.17 ft over 200 ft

The pipe drops 4.17 feet over a 200-foot horizontal run. Set a string line at those elevations before the pour.

OSHA cut slope requirements

OSHA 29 CFR 1926 Subpart P requires trenches 5 feet or deeper to be sloped, shored, or shielded. The maximum allowable slope depends on soil classification, determined by a competent person on site. These are maximums, not recommendations. Soil that has been previously disturbed, is subject to vibration, or has water seepage may require flatter slopes.

Soil typeRatio (H:V)DegreesPercent
Stable rockVertical90°-
Type A3/4 : 153°133.3%
Type B1 : 145°100%
Type C1.5 : 134°66.7%

Source: 29 CFR 1926 Subpart P, Appendix B. Type A includes cemented soils and stiff clays. Type B includes angular gravel, silt, and previously disturbed Type A. Type C includes granular soils (sand, gravel), submerged soil, and soil from which water is freely seeping.

Minimum pipe slopes

Gravity-fed pipe needs minimum slope to maintain flow velocity and prevent solids from settling. The International Plumbing Code (IPC) sets minimums by pipe size. Smaller pipes need more slope because less water is available to carry solids.

Pipe typein/ftPercentSource
Sanitary, 3" and smaller1/42.08%IPC Table 3005.1
Sanitary, 4"-6"1/81.04%IPC Table 3005.1
Sanitary, 8" and larger1/160.52%IPC Table 3005.1
Storm drain1/81.04%IPC / local codes
French drain1/42.08%Industry standard

Frequently asked questions

How do I convert percent grade to a slope ratio?

Divide 100 by the percent. 8.33% = 100 / 8.33 = 12:1 (12 horizontal for every 1 vertical). 50% = 100 / 50 = 2:1. 100% = 1:1, which is a 45-degree angle.

What slope does OSHA require for trench cuts?

Depends on soil type. Type A soil: 3/4:1 (53 degrees). Type B soil: 1:1 (45 degrees). Type C soil: 1.5:1 (34 degrees). Stable rock can be cut vertical. These are maximum allowable slopes per 29 CFR 1926 Appendix B. Soil must be classified by a competent person before cutting.

What is pipe fall in inches per foot?

Pipe fall is the vertical drop per horizontal foot of pipe run. 1/4 in/ft means the pipe drops 1/4 inch for every foot of horizontal run. This is the standard minimum slope for small sanitary drains (3 inches and smaller) per the International Plumbing Code. Larger pipes can use less slope because the greater cross-section maintains flow velocity.

What is the minimum slope for a parking lot?

1 to 2 percent for ADA accessible parking areas and routes. 2 to 4 percent for standard parking lots. The slope must be steep enough for stormwater drainage but not so steep that shopping carts roll or accessibility is compromised. Most jurisdictions follow ADA Standards and ASCE drainage guidelines.

What is the difference between H:V and V:H ratio?

This calculator uses H:V (horizontal to vertical), the standard convention in excavation and earthwork. 3:1 means 3 feet horizontal for every 1 foot of vertical rise. Some engineering texts and structural specs use V:H (vertical to horizontal), where 1:3 means the same thing. Always check which convention your project spec uses before cutting or grading.

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