📐 Drain Slope Calculator

Drop to allow, resulting slope or maximum run: work them out for a waste pipe, a shower, a terrace or a gutter, with the usual slope ranges.

🧮 What do you want to calculate?

Use

Horizontal length

Slope

Picking a use fills in its advised slope. 1% = 1 cm per metre = 10 mm per metre.

📊 Result

Slope expressed as	Value

⚠️ Indicative ranges based on French building standards (DTU) and common practice. For new work or a sewer connection, follow the manufacturers’ instructions, your local regulations and a professional’s advice.

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About the Drain Slope Calculator

A waste pipe without enough fall clogs, a badly set walk-in shower keeps puddles, and a flat terrace sends water against the house. This calculator gives the drop to allow, the resulting slope or the maximum run, and checks that the slope stays within the usual range for the selected use.

How to Use the Calculator

Choose what you need: the drop (length + slope), the slope (drop + length) or the maximum run (available height + slope).
Pick the use: the advised slope is filled in and the usual range is used to check the result.
Enter the slope in the unit you prefer: %, cm/m, mm/m or degrees.
Read the result and every conversion, then copy it.

Formulas and Usual Ranges

Drop (cm): length (m) × slope (cm/m). Example: 4 m at 2% = 8 cm.
Conversions: 1% = 1 cm/m = 10 mm/m; ratio 1:x with x = 100 ÷ slope in %; angle = arctan(slope ÷ 100).
Waste and WC pipes: between 1 and 3 cm/m (French standard NF DTU 60.11); 2 cm/m is a good compromise, often 2 to 3 cm/m for a WC.
Walk-in shower: 1.5 to 2% towards the drain (NF DTU 52.2).
Outdoor tiled terrace: at least 1.5%, 2% in practice, always draining away from the wall (NF DTU 52.1).
Gutter: at least 5 mm per metre towards the downpipe (DTU 40.5).

Tips

Measure the available height between the fixture outlet and the connection point before choosing the route.
Aim for the middle of the range: no installation is perfect, 2 cm/m leaves margin both ways.
Support pipes at regular intervals so they do not sag.
Long runs: if height runs out, a pump may be needed.

Common Mistakes and Limits

Too much fall: water outruns the solids, which stay in the pipe.
Confusing % and degrees: 2% is only 1.15°.
Forgetting bends and fittings: they use height too.
Indicative ranges: the full standards, pipe diameters and local rules take precedence.

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