Solve for fall, slope, or run for drain pipes, yards, patios or shower pans - metric or imperial.
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Choose what you want to solve for - fall/rise, slope, or run - based on which two values you already know. Choose an application (Drain/sewer pipe, Yard/landscape, Patio/paving, Shower/wet area, or Custom/general) for a contextual guidance note; this does not change the calculation itself. Enter your slope in whichever format is most convenient (percentage, ratio, mm/m or in/ft) - results always show all four formats together so you can compare against whichever format your plans or code reference use.
Slope is the vertical fall (drop) divided by the horizontal run, commonly expressed as a percentage. The same relationship can be rearranged to solve for any one of the three values if you know the other two.
Run cannot be solved from a slope of zero, since that would require infinite run for any nonzero fall. The result can also be expressed as a ratio (1 : X, where X = Run ÷ Fall), in millimetres of fall per metre of run, or in inches of fall per foot of run.
The underlying math is identical for a drain pipe, a graded yard, a patio, or a shower pan - all are a fall over a run. What differs is the typical reference range for each application, and none of these are universal requirements. Drain pipe commonly references around 1/4 inch per foot (about 2.08%) as a workable starting point, subject to your pipe diameter, material and local plumbing code. Yard and patio grading is highly site- and spec-dependent, following your landscape plan, civil drawing or paving manufacturer's guide. Shower and other wet-area slope is set entirely by your waterproofing system and tile/membrane manufacturer's documentation - always defer to that documentation rather than a general percentage.
Solving for fall. A 10m pipe run at a 2% target slope: fall = 10 × 0.02 = 0.200 m (200 mm), equivalent to a ratio of 1:50, 20 mm/m, or about 0.24 in/ft.
Solving for slope. A 30ft run with 0.75ft (9in) of available fall: slope = (0.75 ÷ 30) × 100 = 2.5%, equivalent to about 0.3 in/ft, or a ratio of roughly 1:40.
Solving for run. A 200mm (0.2m) available fall at a 2% target slope: run = 0.2 ÷ 0.02 = 10 m.
The most common mistake is confusing slope percentage with slope ratio, or inches-per-foot with a percentage, when comparing against a reference - always check which format your reference is using before comparing numbers directly. Another frequent error is assuming more slope is always better: excessive pipe slope can cause liquids to outrun solids and increase clogging risk, while excessive surface slope can create a trip hazard, erosion, or an unusable outdoor space. It's also worth double-checking whether a stated slope applies to the whole run or must be maintained consistently along it, since low or high spots ("bellies") in a pipe run can trap water even if the average slope meets the target.
This calculator performs the run/fall/slope geometry calculation only. It does not check plumbing code compliance, pipe diameter/capacity requirements, structural drainage design, waterproofing system compliance, or site-specific soil and grading conditions. Minimum and maximum slope requirements vary by jurisdiction, application, pipe size and manufacturer - confirm your specific requirement against your local plumbing or building code, the relevant manufacturer's documentation, or a qualified professional.
Divide the vertical fall (drop) over the pipe run by the horizontal run length, then multiply by 100 for a percentage. A slope around 1/4 inch of fall per foot of run (about 2.08%) is a commonly encountered reference point in plumbing guidance, not a universal minimum - your local plumbing code and pipe diameter set the actual requirement. Use the calculator above with Drain/sewer pipe selected.
This is highly site- and spec-dependent, so there isn't one universal number - follow your landscape plan, civil drawing, paving manufacturer's installation guide, or local drainage requirement. Use the calculator above with Yard/landscape or Patio/paving selected for general guidance, and enter your own target slope or fall.
Multiply your run length by your target slope (as a fraction, e.g. 2% = 0.02) to get the total fall. Use the calculator above with "Solve for Fall" selected, enter your run and target slope in whichever format you prefer (%, ratio, mm/m or in/ft), and it calculates the fall automatically.
1/4 inch of fall per foot of run equals a slope of about 2.08% (0.25 ÷ 12 × 100). The calculator above shows your result in percentage, ratio, mm/m and in/ft simultaneously so you can compare against whichever format your plans or code reference use.
Shower and other wet-area slope requirements are set by your specific waterproofing system and tile/membrane manufacturer's documentation, not by a general formula or code checker. This calculator can do the underlying fall/run geometry for you, but always follow your system manufacturer's installation instructions for the actual required slope.
Not necessarily - while more slope moves water faster, excessive pipe slope can cause liquids to outrun solids in a drain line (leading to clogs), and excessive surface slope can create a trip hazard or erosion issue. Follow your local code's or manufacturer's minimum and maximum guidance rather than maximizing slope.
Yes - select "Solve for Run" and enter your available fall and target slope; the calculator works out how much horizontal run that combination allows. A slope of zero cannot be used to solve for run, since it would imply an infinite run for any nonzero fall.
No - it performs the run/fall/slope geometry calculation only. Minimum and maximum slope requirements vary by jurisdiction, pipe diameter, application and waterproofing system - confirm your specific requirement against your local plumbing or building code, or the relevant manufacturer's documentation.