Measure yard grade with a line level

Timebox: 30 minutes. Cost: two stakes, string, a line level, a tape — under twenty dollars at any hardware store. Output: the first of the three numbers, as a percent, measured along the path water actually takes. Not along the fence line. Not across the lawn. Along the path.

Checked 2026-09-10 · procedure re-verified against sources below

Line level, reading by reading. Level the string once; the ground does the rest of the talking. (Desk drawing, not to scale.)

What you need

Two stakes (or a stake and a helper)
Strong string, 50–100 ft
Line level — a bubble vial that hangs on the string
Measuring tape

Steps

Choose the path. The run from where water gathers to where it should leave: gutter corner to street, low spot to swale, back fence to the wet zone. If you don't know the path, this job is how you find out — see grade, direction included.
Set the stakes. One at each end of the run. String between them, snug.
Level the string. Hang the line level at the string's middle, slide one end of the string up or down its stake until the bubble centers. The string is now level.
Measure the drops. At each stake, tape from string to ground. Subtract: that difference is your rise over the run.
Measure the run. Between the stakes, along the ground, in feet.
Compute. grade % = (drop difference ÷ run) × 100. A 6-inch difference over 50 feet: 0.5 ÷ 50 × 100 = 1%.
Write it down with the date and direction. “1.2%, toward the street, 10 Sep 2026” — a number without a direction and a date is half a fact. See how the desk derives grade.

Worked example

Measurement	Value
Drop at high stake	2.5 ft
Drop at low stake	3.1 ft
Difference	0.6 ft = 7.2 in
Run	60 ft
Grade	7.2 ÷ 720 in × 100 = 1%

One percent. Enough for a pipe, comfortable for a swale, invisible to the eye — which is the point of the string.

Where people stop

String sag. A 100-ft string pulled tight still sags a finger's width. Keep runs under 75 ft, or use a third stake mid-run and measure in two legs.
Measuring the wrong axis. Water takes the steepest path down, not the prettiest. Walk the yard during rain once; you will see the path.
Trusting the eyeball. Between 0 and 2% the eye reports opinion. The bubble reports grade. The grade page has the whole argument.

Where to go next

Grade — what the number permits.
Perk test — number two, one day.
Calculate catchment — number three, 30 minutes.

Sources

Minnesota Stormwater Manual — stormwater.pca.state.mn.us
USGS Water Science School, slope and runoff — usgs.gov
US EPA NPDES — epa.gov/npdes