Calculate slope, gradient, and angle for drainage, landscaping, ramps, and construction. Convert between degrees, percentage, and rise:run ratio.
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Surface water drainage requires a minimum 1:100 (1%) fall. Soakaways and French drains: 1:80 minimum. Sewer connections: 1:40 to 1:80 depending on pipe size. Patio drainage: 1:60 to 1:80. Always check building regulations.
UK Building Regulations Part M: maximum 1:12 for ramps up to 5m. For 5-10m: maximum 1:15. Over 10m: maximum 1:20. Landings required every 5m of ramp run at 1:15 or steeper.
Percentage = (rise / run) × 100. Degrees = arctan(rise / run). Ratio = 1 : (run / rise). A 1:20 slope = 5% = 2.86°. A 1:12 ramp = 8.33% = 4.76°. A 45° slope = 100% = 1:1.
Use a spirit level and a straight edge (1m or 2m). Measure the gap under the level at one end - that's the rise over the length of the level. For a 1m level with a 20mm gap: slope = 20mm/1000mm = 1:50 = 2%.
slope% = (rise / run) × 100
angle = arctan(rise / run)
ratio = 1 : (run / rise)
L = sqrt(run² + rise²)
For drainage connections to public sewers, soakaway design, and any slope stability concerns (retaining walls over 1m, embankments, or sloped building sites), consult a civil engineer or drainage specialist. Building control approval is required for drainage works.
A 1:40 slope means for every 40 units of horizontal distance, the ground drops 1 unit. This equals 2.5% gradient or about 1.43°. It is a common gradient for drainage pipes.
Minimum 1:60 (1.67%) for patios. Recommended 1:50 to 1:80. The fall should direct water away from buildings. A 3m patio needs to drop at least 50mm over its length.
10° = 17.6% = approximately 1:5.7. This is walkable but would feel quite steep. Wheelchair ramps must be much shallower (max 1:12 = 4.76°). A 10° slope is typical for a moderately steep garden.
Degrees = arctan(percentage / 100). For 5%: arctan(0.05) = 2.86°. For 10%: arctan(0.10) = 5.71°. For 100%: arctan(1.0) = 45°. Use a calculator or our tool for instant conversion.