Slope and Grade Calculator

Two percent — a quarter inch of fall per foot — is the accepted minimum for shedding water off patios, walks, and finished grade, and it is what most municipal details specify. Over a 20 ft run that is 4.8 inches of drop. Pipe can go shallower: about 1% (1/8 inch per foot) is the common minimum for a 4-inch perforated French drain, and the International Plumbing Code permits 1/8 inch per foot on building drains 3 inches and larger. Below roughly 0.5% water stands and silt settles. These are planning figures — your local code and inspector govern the finished work.

The ADA Standards cap a ramp running slope at 1:12 — 8.33%, or 4.76 degrees — with cross slope no steeper than 1:48 (about 2%). A single ramp run may rise no more than 30 inches before a level landing, so a 30-inch rise needs 30 feet of ramp plus landings at least 60 inches long. Those requirements apply to public accommodations and commercial facilities; private homes are not covered by the ADA, although many jurisdictions adopt similar residential rules. Treat this as a planning estimate and confirm slope, landings, handrails, and edge protection with your building official or a licensed design professional before you build.

The International Residential Code (section R401.3) requires the ground to fall at least 6 inches within the first 10 feet from the foundation — a 5% grade. Where property lines, walls, or paving make that impossible, the code allows a shallower grade combined with swales or drains that carry the water away. Impervious surfaces such as patios and walks against the house are generally allowed a 2% minimum instead. Measure from the finished grade at the wall, not from the top of the mulch. Local amendments are common, so confirm the requirement with your building department.

They describe the same slope three ways. Percent grade is rise ÷ run × 100, degrees is the arctangent of rise ÷ run, and ratio is written 1 in N where N = run ÷ rise. So 2% is 1.15 degrees and 1 in 50; 8.33% is 4.76 degrees and 1 in 12; 25% is 14.04 degrees and 1 in 4. A 45-degree slope is 100% grade and 1 in 1 — percent is not capped at 100. Roofers use a fourth form, rise in inches per 12 inches of run, so a 4:12 roof is a 33.33% grade or 18.43 degrees.

Set a level at the high point, stretch a string line dead level to the low point, and measure straight down from the string — that drop is your rise and the string length is your run. A 4-foot level with a 1-inch block under the low end reads about 2%, which is the classic patio and walkway check. For longer runs a rotary laser with a receiver and grade rod, or a builder’s level, beats eyeballing by a wide margin. Always measure the run horizontally: laying the tape on sloping ground inflates the run and flatters the grade.