Grade
Grade is how steep the ground falls, written as a percent: rise ÷ run × 100. One percent is 1 foot of fall over 100 feet of run. On a 4-foot level, 1% shows up as the downhill end sitting about half an inch lower. Codes write grade in percent. Learn it once, in percent.
Percent, ratio, and the level
| Grade | As ratio | Fall in 4 ft | Fall in 100 ft | Where you meet it |
|---|---|---|---|---|
| 0.5% | 200:1 | 0.24 in | 6 in | Bare minimum some codes allow for paved channels |
| 1% | 100:1 | 0.48 in | 12 in | Minimum for pipes and most swales |
| 2% | 50:1 | 0.96 in | 24 in | Comfortable swale grade, lawns drain visibly |
| 4% | 25:1 | 1.92 in | 48 in | Upper end for a plain grass swale |
| 8% | 12.5:1 | 3.84 in | 96 in | Check dam territory or armor |
The trap: a ratio like 50:1 is the same fact as 2%, but codes and inspectors speak percent. Two different-looking numbers, one grade — and the wrong one ends up in your plan.
Why eyeballing fails
Human eyes are bad at shallow slopes; that is the entire reason the level was invented. A “flat” yard that a line level says is 1.2% will carry water fine. A “slightly sloped” yard measuring 0.3% will not. Between 0 and 2%, the eye reports vibes. Only the instrument reports grade.
Which direction counts
Grade is a vector, not a number: it has a direction. The same 2% that drains a swale into the street will, aimed the other way, aim the street into your basement. When this desk says “measure grade,” it means measure the direction of fall along the path water actually takes — toward or away from the building, not the cross-slope of the lawn.
Where to go next
- Measure yard grade with a line level — 30 minutes, tools from a hardware shelf.
- Swale — the grade range a grass channel survives.
- Check dam — what to do when the grade is too steep.
Sources
- Minnesota Stormwater Manual — stormwater.pca.state.mn.us
- ICC International Plumbing Code, drainage slope provisions — iccsafe.org
- USGS Water Science School — usgs.gov