How the Drainage Gradient Calculator Works

Enter your pipe length and either gradient you plan to lay or fall you have available. The calculator converts between formats and tells you whether result meets the Approved Document H minimum for your pipe size and use.

The core formula is simple: fall = length ÷ gradient ratio. A 10 metre run at 1:40 needs 10,000 ÷ 40 = 250mm of fall, while same run at 1:80 needs 125mm. Working backwards is just as useful on site, divide your available fall by run length to find the gradient you can actually achieve.

The Gradients Part H Expects

Approved Document H sets minimum graFdients by pipe size and flow and these are figures Building Control checks against:

• 100/110mm foul drain, peak flow under 1 litre per second: 1:40 minimum, which is 25mm per metre
• 100/110mm foul drain serving at least one WC with flow over 1 l/s: 1:80 minimum, 12.5mm per metre
• 150mm foul drain serving at least five WCs: 1:150 minimum, 6.7mm per metre
• 100mm surface water drain: 1:100 minimum
• 150mm surface water drain: 1:150 minimum

The logic behind numbers is self cleansing velocity, foul water needs to move at roughly 0.7 metres per second so solids stay suspended. Trade practice adds a rule of thumb worth knowing, sometimes called Maguire's rule, divide pipe diameter by 2.5 for a comfortable working gradient, which gives 1:40 for a 100mm pipe. The regulations minimum will pass inspection, steeper figure gives margin for real world flows.

Too Steep Is Also a Problem

The counterintuitive rule of foul drainage, steeper is not automatically better. Beyond roughly 1:20 the water outruns solids it carries, stranding them in pipe where they build into blockages. Foul runs are best kept in the 1:40 to 1:110 window.

Where a site slopes so much that pipe cannot stay in that range, answer is not a steeper pipe but a backdrop manhole or tumbling bay, which drops the level vertically in a controlled step and lets each pipe section run at a proper gradient. Surface water is more forgiving since it carries no solids, though very high velocities above 3 m/s risk scouring the pipe.

The Upsizing Trick When Fall Is Tight

The most common drainage problem on site is not enough fall to reach connection point and there is a legitimate design escape. A 100mm foul pipe needs 1:80, but a 150mm pipe only needs 1:150, so stepping up pipe size nearly halves fall required. Over a 20 metre run that is difference between 250mm and 134mm of fall, which can be difference between a gravity drain and an expensive pumped station. The larger pipe costs more per metre but shallower excavation usually wins overall. Note 150mm gradient assumes adequate flow, so check design with Building Control before committing.

Setting Out the Fall on Site

Levels are set from invert, inside bottom of pipe. The professional method is a rotating laser and receiver on a staff, working along trench and adjusting bed until each point reads designed drop. For a quick check between joints there is an old site trick, for 1:80 over a 6 metre pipe you need 75mm of fall, so tape a 75mm block to one end of a spirit level and when bubble centres with the block in place pipe sits at gradient.

Remember that all below ground drainage work needs Building Regulations approval, laid on proper granular bedding with inspection access at changes of direction, so involve Building Control before digger arrives rather than after.