What slope is needed for patio drainage?

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.

How do I measure garden slope?

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.

What is too steep to mow?

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.

Do I need a retaining wall?

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.

What tools do I need to measure a garden slope?

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.

GardenCalcs

Slope Calculator — Garden Gradient & Drainage

Calculate garden slope gradient, angle, and drainage requirements.

Horizontal Distance (run)

Rise (height change)

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

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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.