GardenCalcs.uk
Slope Calculator — Garden Gradient & Drainage
Calculate garden slope gradient, angle, and drainage requirements.
Level (plan) distance across the slope, measured horizontally with a spirit level held flat — not along the sloping ground.
Vertical drop from the high point to the low point over that horizontal distance.
Percentage below which the slope is considered flat (default 1%)
Percentage below which the slope is considered gentle (default 3%)
Percentage above which professional landscaping is recommended (default 15%)
How We Calculate This
Gradient = Rise / Run. The angle in degrees uses arctangent of the gradient. Percentage is gradient x 100.
Slope ratio is expressed as 1:X, where X is the horizontal distance per unit of rise.
Slope length uses Pythagoras: sqrt(horizontal² + rise²). Drainage guidance is based on standard UK landscaping recommendations.
Frequently Asked Questions
A minimum fall of 1:80 (1.25%) is recommended for patios to ensure water drains away from buildings. This equates to roughly 12.5mm per metre.
Use a long spirit level and a tape measure. Place the level horizontally from the top of the slope, then measure the vertical drop at the far end.
Most lawn mowers can handle slopes up to about 15° (roughly 27% gradient). Steeper slopes may need a hover mower or ground cover planting instead.
Slopes over about 30% (17°) in clay or sandy soil typically benefit from a retaining wall. On solid ground, steeper slopes may be manageable with planting.
A long spirit level (or a short one on a straight plank), a tape measure, and string. Or use a smartphone inclinometer app for a quick estimate.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.