About Grading & Drainage Slope Calculator (IRC R401.3)
The grading slope calculator checks a yard grade against the foundation drainage rule of the 2021 International Residential Code. R401.3 requires lots to be graded so surface water drains away from foundation walls, with the grade falling not fewer than 6 inches within the first 10 feet — a 5 percent equivalent slope. Where lot lines, walls, slopes, or other physical barriers prohibit 6 inches of fall within 10 feet, the R401.3 Exception directs drains or swales to be constructed to ensure drainage away from the structure, and the same Exception requires impervious surfaces within 10 feet of the foundation — patios, driveways, sidewalks — to slope not less than 2 percent away from the building.
Enter the fall you measured in inches and the run it was measured over in feet, and toggle whether the surface is impervious. The calculator reports the slope as a percentage, the fall the governing rule demands for your run, and a pass/fail verdict with the margin in inches and the section reference. Runs longer than 10 feet get a reminder that the code measures the drop within the first 10 feet, and runs shorter than 10 feet get the swale/drain note, because a lot line or wall that cuts the run short is exactly the case the Exception covers. A separate converter card translates any grade between percent, inches per foot, and 1:N ratio.
How It Works
- Measure the fall: drive a stake at the foundation and another at the end of the run (10 ft out is the code basis), stretch a string line between them with a line level, and measure the height difference from the string to grade at each stake — the difference in inches is the fall.
- Enter the fall in inches and the run in feet. The calculator computes the slope as fall ÷ (run × 12) × 100 and compares the fall against the governing requirement: 6 inches for ground within the first 10 feet (R401.3), or 2 percent of the run for impervious surfaces (R401.3 Exception).
- Read the verdict: PASS means the fall meets the rule exactly or better — a grade exactly at the minimum passes. FAIL shows how many inches short you are, which is the amount of soil to add at the foundation (never regrade by digging at the footing).
- If your run is longer than 10 feet, the badge keeps checking your entered fall against 6 inches but reminds you the code measures within the FIRST 10 feet — re-measure the drop at the 10-ft point if you recorded it farther out.
- If your run is shorter than 10 feet — a lot line, retaining wall, or neighboring slope cuts it off — the calculator checks the 5 percent equivalent fall (0.6 in/ft) and flags that the R401.3 Exception applies: build a drain or swale at the low point to carry water away from the structure.
- Use the converter card for any other slope you meet in the field: enter a value as percent, inches per foot, or the N in a 1:N ratio, and the other two forms are computed instantly — 5% = 0.6 in/ft = 1:20, 2% = 0.24 in/ft = 1:50.
Worked Example
A new lawn grade next to a foundation drops 6 inches over the first 10 feet. Slope = 6 ÷ (10 × 12) × 100 = 5.0%, which meets the R401.3 requirement of 6 inches within the first 10 feet exactly — PASS with zero margin. Had the drop measured 5.9 inches, the check would fail 0.1 inches short, calling for about an inch of soil feathered in at the foundation. The same grade in other forms via the converter: 5% = 0.6 inches per foot = a 1:20 ratio. For the concrete patio on the other side of the house, the impervious rule of the R401.3 Exception governs instead: over its 10-ft width it needs only 2% — a 2.4-inch fall — away from the building.
Formulas
- Slope from fall and run
S [%] = fall [in] / (run [ft] × 12) × 100- Required fall (ground, R401.3)
fall_req = 6 in within the first 10 ft (≡ 5%); for runs < 10 ft, 0.6 in/ft × run with drains or swales per the Exception- Required fall (impervious, R401.3 Exception)
fall_req [in] = 2/100 × run [ft] × 12 = 0.24 × run- Slope form conversions
in/ft = % × 12/100 · ratio 1:N with N = 100/% · % = 100/N
Standards & References
- IRC 2021 R401.3 — lots graded to drain surface water away from foundation walls; the grade shall fall not fewer than 6 in (152 mm) within the first 10 ft (3048 mm)
- IRC 2021 R401.3 Exception — where lot lines, walls, slopes or other physical barriers prohibit 6 in of fall within 10 ft, drains or swales shall be constructed to ensure drainage away from the structure
- IRC 2021 R401.3 Exception — impervious surfaces within 10 ft (3048 mm) of the building foundation shall be sloped not less than 2 percent away from the building
- IRC 2021 R401.3 — surface drainage shall be diverted to a storm sewer conveyance or other approved point of collection that does not create a hazard
Frequently Asked Questions
Why does the code require 6 inches of fall in the first 10 feet?
Water standing against a foundation is the root cause of wet basements, hydrostatic pressure on walls, and frost heave. R401.3 requires the grade to fall not fewer than 6 inches within the first 10 feet — a 5 percent slope — because that is steep enough to shed roof runoff and rain away from the backfill zone before it can soak down along the wall. The backfill next to a new foundation also settles for years, so builders typically start above 5 percent to stay compliant after settlement.
What if my lot line or a wall makes 6 inches of fall impossible?
That is exactly what the R401.3 Exception covers: where lot lines, walls, slopes, or other physical barriers prohibit 6 inches of fall within 10 feet, drains or swales shall be constructed to ensure drainage away from the structure. In practice that means grading a shallow swale parallel to the wall that intercepts water and carries it around the house, or installing a surface drain at the low point piped to daylight or a storm connection. The calculator flags this case automatically whenever your run is shorter than 10 feet.
Do patios and driveways need the full 5 percent slope?
No. The R401.3 Exception treats impervious surfaces within 10 feet of the building foundation separately: they must be sloped not less than 2 percent away from the building — 0.24 inches per foot, or about 2.4 inches across a 10-foot patio. Concrete flatwork at 5 percent would be noticeably tilted, so the code accepts the shallower 2 percent because impervious surfaces shed nearly all of the water that lands on them. Toggle the impervious switch in the calculator to check against the 2 percent rule.
How do I measure the fall of my existing grade?
Drive a stake at the foundation and a second stake 10 feet out. Tie a string tightly between them and hang a line level at mid-span, adjusting the string until it reads level. Measure from the string down to grade at each stake: the difference between the two measurements is your fall in inches. A 4-foot level on a straight board with a tape measure under the low end works for shorter checks, and a laser level speeds up whole-yard surveys. Measure after rain if you can — puddles reveal the low spots a string line can miss.
Is 5 percent the same rule as the 6-inch fall, and which should I use?
The code states the fall — 6 inches within the first 10 feet — and 5 percent is just the arithmetic equivalent over that basis. The distinction matters when your run is not exactly 10 feet: on a run longer than 10 feet, the requirement stays 6 inches within the FIRST 10 feet (not 5 percent of the whole run), and on a shorter run the code cannot be prorated — the Exception with its drains or swales applies instead. The calculator handles both cases and notes which basis it used.