BCBuildCalc
BuildCalc Project Guide

Drainage Fall & Gradient Guide

Understand fall, gradient and level difference calculations for drainage and site setting-out

In this guide
  1. Fall and run
  2. Gradient forms
  3. Level difference
  4. Worked example
  5. Quantity vs design
  6. Regional requirements

Fall and run

Fall is the vertical change over a horizontal run. Keep both measurements in compatible units before calculating gradient.

Gradient forms

A gradient may be expressed as 1:n, a percentage or an angle. These forms are related but should not be mixed without conversion.

percentage fall = vertical fall ÷ horizontal run × 100

Level difference

If the required gradient and run are known, calculate the target vertical difference before setting levels.

Worked example

A 100 mm fall over 5,000 mm gives 100 ÷ 5,000 × 100 = 2%, equivalent to 1:50.

Quantity vs design

A mathematically correct fall does not prove that a pipe size, drainage layout or discharge arrangement is compliant or adequate.

Regional requirements

BuildCalc supports the United Kingdom, United States, Canada, Australia, New Zealand, Ireland and South Africa. The geometry is universal, but terminology, units, pack sizes and common product dimensions differ. Use the dimensions printed on the product you intend to buy rather than assuming a regional default is exact.

Check applicable drainage standards, manufacturer instructions and authority requirements for regulated work.

Use the related calculators

Use the calculator for your measurements, then return to this guide to check assumptions, allowances and ordering considerations.

Drainage fallDrainage gradient
Important: BuildCalc provides indicative quantity estimates, not structural, regulatory or professional design advice. Check manufacturer data, site conditions and applicable local requirements before ordering or construction.