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Yard Drainage Fixes

Yard Slope Calculator

Most slope calculators just convert numbers. This one tells you what the number means for drainage — whether your fall is too flat to drain, enough to move water, or steep enough for grading away from the house.

1.3%

grade

0.15"

per foot

1:80

ratio

Enough to move water across the yard

Around 1 to 2% is the working minimum — water will flow toward a lower point, and it clears the bar for a drain or swale. Out in the yard this is fine; right next to the house you want more.

How to measure: drive a stake at each end, tie a string at ground level on the high stake, level it with a line level, and measure from the string down to the ground at the low stake. That drop and the distance are your two inputs. More on slope and drainage →

What this doesn't tell you

This converts and interprets a measurement — it can't confirm your string was truly level, whether the ground has a dip between the stakes, or that the outlet is where you think. For anything you're installing, measure along the whole run, not just end to end, and confirm the discharge point is genuinely lower.

How to measure your slope

You need two numbers: the drop (fall) and the distance (run). The reliable way to get them costs about ten minutes and a few basic tools:

  1. Drive a stake at the high end of the run and another at the low end.
  2. Tie a string to the high stake, right at ground level.
  3. Run it to the low stake, pull it taut, and slide it until a line level on the string reads level.
  4. Measure from the string straight down to the ground at the low stake — that's your drop, in inches.
  5. Measure the distance between the stakes — that's your run, in feet.

Enter those two numbers above and the calculator gives you the slope as a percentage, in inches per foot, and as a ratio — plus what that fall actually means for drainage.

How the calculator works

Slope is simply fall divided by run, in the same units. Expressed as a percentage, slope % = (drop ÷ run) × 100. One percent works out to about an eighth of an inch of drop per foot, or 1¼ inches over 10 feet. The "how much drop do I need" mode runs the same math in reverse: give it a distance and a target grade, and it returns the fall you need.

What the numbers mean for drainage

The interpretation is the part that matters, and it's where generic slope calculators stop short. Here's the short version of what each range tells you:

  • Under 1% — too flat to drain reliably. Water sits, and a French drain or swale won't work.
  • 1% to 2% — enough to move water across the yard, and the minimum for a drain or swale.
  • 2% to 5% — a comfortable drainage fall; water won't linger.
  • About 5% — 6 inches over 10 feet, the target right next to the house.
  • Steeper than that — sheds water fast, but watch for erosion on open ground.

The full reasoning behind these numbers is in how much slope you need for drainage.

Frequently asked questions

How much slope does a yard need to drain?
About 1% out in the yard — roughly an eighth of an inch of fall per foot, or 1¼ inches per 10 feet — is the practical minimum to keep water moving. Right next to the house you want more: about 6 inches of fall over the first 10 feet, a 5% grade, to push water decisively away from the foundation.
What is the minimum slope for a French drain?
About 1% — an eighth of an inch of fall per foot. Below that, water sits in the pipe instead of flowing out, and a pipe holding standing water silts up and stops working. The 'how much drop do I need' mode here tells you the fall required over your drain's length.
How do I measure my yard's slope?
Drive a stake at each end of the run, tie a string at ground level on the high stake, level it with a line level, and measure from the string down to the ground at the low stake. That drop and the distance between the stakes are the two numbers you enter — the calculator does the rest.
What does 1% slope mean?
It means the ground drops 1 unit for every 100 it runs — which works out to about an eighth of an inch per foot, or 1¼ inches over 10 feet. You'll often hear the rule of thumb 'an inch per 10 feet,' but strictly that's 0.8%, just under 1%. For the yard the difference is negligible; near the house, use the exact 6 inches per 10 feet.
Is my yard too flat to drain?
Under about 1% slope, water won't reliably move and a gravity drain won't work. But most 'flat' yards have more fall than they look — measure over a longer run toward a far corner before concluding it's flat. If it genuinely is, the answer shifts to storage or regrading rather than a drain to daylight.
How much drop do I need over a given distance?
Switch to the 'how much drop do I need' mode, enter the distance and a target slope, and it gives the required fall in inches. For example, reaching 1% over 30 feet needs about 3.6 inches of drop from the high end to the outlet.

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