Tile grade calculator — one slope, three ways to say it. Free.
Percent grade, inches per 100 feet, feet per 1,000 feet: type any one and the other two update live. Add a run length to turn a total drop into a grade — or a grade into the fall you need over the run. Speak grade with anyone at the table, before a foot of pipe goes in.
Unit math and published rules of thumb here. The full app draws the profile from LiDAR and best-fits the grade line inside your depth limits — no grade math by hand. That’s a flat membership: unlimited fields, no per-acre fees.
A free drainage grade calculator and pipe fall calculator. Convert percent grade to inches per 100 feet and feet per 1,000 feet, work out the fall over a run, and check tile against commonly published minimum grades. Built by Geo-Surface Pro — the flat-fee membership for planning surface drainage, tile and 3D landforming across the US and Canada.
Convert & check in seconds
The same slope, quoted three ways — and a run you can sanity-check
Type a grade in any one unit and the other two follow instantly. Then drop in a run length to see the fall it makes — or how much fall you need to hold a grade.
Convert a grade
Edit any field — the other two update live. All three are the same slope, written differently.
Enter a positive number to convert.
Reading it: 1% grade is a 1-foot drop every 100 feet — that’s 12 inches per 100 ft, or 10 feet per 1,000 ft. Tile and surface drains usually run a small fraction of a percent.
Check a run
Give a run length and a total fall and see the grade — or apply the grade from the converter to see the fall you’ll need.
Grade over the run
Enter a run length and a total fall
Enter a positive run length and a total fall to get the grade.
Minimum grades by pipe size
Commonly published minimum grades for corrugated drainage pipe, based on holding a roughly 0.5 ft/s self-cleaning velocity (ASAE / NRCS-style rules of thumb). Use them as a starting sanity check — local standards, soil, and sediment load can call for more. Verify against the standards you design to before you commit a run.
| Pipe size | Minimum grade | Fall in / 100 ft |
|---|---|---|
| 3" | 0.16% | 1.9 in |
| 4" | 0.11% | 1.3 in |
| 5" | 0.08% | 1.0 in |
| 6" | 0.06% | 0.7 in |
| 8" | 0.04% | 0.5 in |
| 10" | 0.03% | 0.4 in |
| 12" | 0.024% | 0.3 in |
Minimums keep water moving fast enough to carry sediment; flatter is possible but needs care — exactly what best-fit design checks. In Geo-Surface Pro you set your own minimum grade (it defaults to 0.1% for tile, 0.05% for surface) and the best-fit tool holds every run at or above it automatically, inside your depth limits.
How these conversions work
Grade is just rise over run written as a percent. Everything here is exact unit math:
- Grade → inches per 100 ft =
grade% × 12. A 1% grade drops 1 ft (12 in) every 100 ft. - Grade → feet per 1,000 ft =
grade% × 10. A 1% grade drops 10 ft every 1,000 ft. - Run → grade =
(total fall ÷ run length) × 100. A 2.6 ft drop over 1,320 ft is 0.197%. - The reverse of each holds too: divide inches per 100 ft by 12, or feet per 1,000 ft by 10, to get the grade back.
- Values display to a few decimals for readability; the field you’re typing in keeps your exact number.
Frequently asked questions
What is the minimum grade for drain tile?
There’s no single number — it depends on pipe size. Smaller pipe needs more fall to hold a self-cleaning velocity; larger pipe needs less. The minimum grades by pipe size table above lists commonly published ASAE/NRCS-style minimums, from 0.16% for 3" pipe down to 0.024% for 12" pipe. Local standards, soil, and sediment load can call for more — verify against the standards you design to before you commit a run.
What’s the minimum fall for a 4-inch tile?
Commonly published guidance puts a 4" drain tile’s minimum grade at 0.11% — that’s 1.3 inches of fall per 100 feet of run. Flatter is possible but needs care; the minimum-grade table above covers 3" through 12" pipe.
How do I convert percent grade to inches per 100 feet?
Multiply the percent grade by 12: inches per 100 ft = grade% × 12. A 0.2% grade works out to 2.4 inches of fall per 100 feet. To go the other way, divide inches per 100 ft by 12 to get the grade back. Or skip the math — type either number into the converter above and the other two fields update instantly.
Know what this is
Exact math and honest rules of thumb — not a grade design
The conversions here are exact, and the minimum-grade figures are widely published rules of thumb. But this tool doesn’t know your field — how deep you can dig, where the outlet sits, or where water actually pools. Use it to talk grade and sanity-check a run, not to set the grade you’ll build.
Good for
- Converting a grade between percent, inches per 100 ft, and feet per 1,000 ft
- Turning a run length and a total drop into a grade — or back the other way
- Checking a planned run against published minimum grades
- Speaking grade in whatever unit the person across the table uses
Not built to
- Set tile depth or fall over the real field profile
- Account for your outlet, high spots, or how deep you can dig
- Best-fit a grade line inside minimum and maximum depth limits
- Replace a profile design or an RTK survey
Plan and quote from elevation data — survey and grade with RTK when the job goes ahead. Geo-Surface Pro pulls the field profile from LiDAR and best-fits the grade line inside your depth limits, so you don’t do this math by hand. The committed run is then surveyed run-by-run with your machine-control RTK and built however you already work.
This tells you what a grade is. The app decides the grade for you.
No more grade math by hand. Geo-Surface Pro pulls the field profile from LiDAR and best-fits the grade line inside your minimum and maximum depth, holding your minimum grade on every run — on as many fields as you want. One flat membership, unlimited fields and acres, no per-acre fees, ever.
One membership, three jobs, right in the browser: surface drainage · tile design · 3D landforming.
- The grade, best-fit for you. The profile comes from LiDAR; the Auto Best Fit solver fits the grade line inside your min and max depth, holding your minimum grade over the whole run — no by-hand fall math.
- Sized to the grade it lands on. Automatic and stepped pipe sizing follows the fitted profile, with a live profile chart and depth-violation flags to catch a run that won’t grade out.
- When a run won’t grade out, reshape the ground. Pick your outlets, set a max cut, hit Solve — 3D landforming grades the whole field to drain, right in the browser, at one flat price instead of per acre.
$499/yr launch price — moving to $799/yr for new signups on September 1, 2026. Sign up now and your renewal price stays locked at $499/yr for as long as you keep your membership, with every future feature we ship included.
Plan and design from LiDAR. Survey and grade with RTK when the job goes ahead. 30-day money-back, no questions asked.
