Lawn Drainage Slope Calculator
Work out the exact grade your yard needs so rainwater runs away from your home instead of pooling on the grass or against the foundation.
Results breakdown
Overview
The Lawn Drainage Slope Calculator tells you whether your yard is graded steeply enough to shed water — and, if you are planning new grading, exactly how much fall you need to build in. Water that sits on a lawn drowns the roots, breeds mosquitoes, and invites fungus; water that runs toward a building instead of away from it is one of the most common causes of damp basements and foundation damage.
It is built for homeowners regrading a yard, landscapers laying out drainage, and anyone installing a patio, retaining wall, or French drain who needs to confirm the ground falls in the right direction at the right rate. Enter a couple of measurements and the tool converts them into slope percent, ratio, angle, and a plain-English verdict against accepted grading standards.
How It Works
- Pick a mode. Choose Check a slope to test ground you have already measured, or Plan a slope to work out the fall you need for a target grade.
- Choose your units — Imperial (feet & inches) or Metric (metres & centimetres).
- Enter the run — the horizontal distance the ground slopes across.
- Enter the drop (Check mode) or a target slope % (Plan mode). Tick the foundation box if the slope sits beside a building.
- Press Calculate. You get the slope, ratio, angle, drop-per-foot, and a verdict. Press Reset to clear everything.
Formula Explanation
Slope is simply how much the ground drops (rise) compared with how far it travels sideways (run), expressed as a percentage:
Slope % = (Drop ÷ Run) × 100(drop and run measured in the same unit)
The calculator also derives:
- Ratio —
Run ÷ Drop, shown as "1 in N" (e.g. a 2% slope is 1 in 50). - Angle —
arctan(Drop ÷ Run), in degrees. - Drop per foot / metre — how much fall you need for every unit of length.
In Plan mode the drop is solved from your target: Drop = Run × (Target % ÷ 100). Because both drop and run share a unit before the ratio is taken, the slope percentage stays valid whether you work in Imperial or Metric.
Practical Benefits
- Protects your foundation — confirms the ground falls at least 6 in over the first 10 ft away from the house (about 5%), the widely used grading target.
- Stops standing water — flags lawns flatter than roughly 1–2%, where puddles and soggy patches form.
- Guides new landscaping — tells you the exact fall to build into a regrade, patio pad, or drainage channel before you move a single wheelbarrow of soil.
- Prevents over-steepening — warns when a slope is too steep to mow safely or likely to erode.
- Saves surveying costs — turns two tape-measure readings into professional-style numbers instantly.
Frequently Asked Questions
A general lawn should fall at least 1–2% so water keeps moving instead of pooling; 2% is a comfortable target. Right next to a building, aim higher — most guidance calls for about 5%, i.e. roughly 6 inches of drop across the first 10 feet from the wall.
Yes. Above roughly 25% (a 4:1 slope), turf gets hard to mow safely and topsoil starts washing away in heavy rain. Steeper areas are usually better handled with terracing, retaining walls, or ground-cover planting instead of grass.
Drive a stake at the top and bottom of the slope, tie a string between them, and level the string with a line level. Measure the string's height above the ground at the low stake — that vertical gap is your drop, and the string length is your run.
Absolutely. The number only helps if the ground falls away from your house and toward a safe outlet such as a swale, drain, or the street. A perfect 2% slope pointed at the foundation still sends water exactly where you don't want it.
Disclaimer: This calculator provides general estimates for planning and educational purposes only and does not constitute engineering, surveying, or construction advice. Slope requirements vary with soil type, rainfall, local building codes, and site conditions. Always confirm grading and drainage designs with a qualified professional and your local authority before carrying out work.
