Slope Calculator

Calculate slope, gradient, and angle for drainage, landscaping, ramps, and construction. Convert between degrees, percentage, and rise:run ratio.

Garden & Lawn Area Calculator

Calculate plot, lawn, and banked garden areas from dimensions

Enter plan-view dimensions, slope applied

Related Free tools

Free Quote Generator

Turn your measurements into a professional quote

Free Purchase Order Generator

Create supplier order forms from your calculations

Free Invoice Generator

Send professional invoices and get paid faster

Start free trial

Full quoting, takeoff, and job management

Slope calculation tips

Minimum gradients for drainage

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.

Ramp gradients for accessibility

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.

Converting between slope formats

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.

Measuring slope on-site

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%.

Formulas used

Slope percentage

slope% = (rise / run) × 100

Slope in degrees

angle = arctan(rise / run)

Slope ratio

ratio = 1 : (run / rise)

Length on slope

L = sqrt(run² + rise²)

Worked example: 10m drainage pipe at 1:80 gradient

  1. 1Gradient = 1:80 = 1.25%
  2. 2Angle = arctan(1/80) = 0.72°
  3. 3Total drop over 10m: 10 × (1/80) = 0.125m = 125mm
  4. 4Pipe length on slope: sqrt(10² + 0.125²) ≈ 10.001m (negligible difference)
  5. 5At 1:80, a 10m pipe drops 125mm from start to finish

Assumptions & limitations

When to ask a professional

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.

Frequently asked questions

What is a 1 in 40 slope?

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.

What is the minimum slope for a patio?

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.

How steep is a 10 degree slope?

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.

How do I convert slope percentage to degrees?

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.