Fall / Gradient Calculator
Calculate required fall in millimetres from a 1:X gradient.
Result
- Required fall
- 120.0 mm
- Gradient
- 1:100
FREE FALL & GRADIENT CALCULATOR
Fall / Gradient Calculator
Work out fall over a known run and express the result as a percentage or ratio. This is useful for preliminary checks involving drainage, paving, external surfaces and site levels.
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When this tool is useful
Practical preliminary checks
- Preliminary drainage and stormwater fall checks
- Paving and external surface planning
- Comparing a specified gradient with measured site levels
- Converting between millimetre fall, percentage gradient and ratio
How it works
Gradient percentage is calculated as fall ÷ run × 100, using consistent units. The ratio expresses the same gradient as 1:x. The calculator can also determine the fall required for a nominated gradient over a known run.
Important estimating note
Minimum and maximum falls depend on the specific building element, product, code requirement, drainage design and site conditions. Use the calculator for arithmetic only and confirm the required gradient from the relevant drawings, standards and qualified professionals.
Frequently asked questions
How do I calculate gradient percentage?
Divide the vertical fall by the horizontal run using the same units, then multiply by 100.
What does a 1:100 fall mean?
A 1:100 fall means one unit of vertical fall for every 100 units of horizontal run, such as 10 mm of fall over 1,000 mm.
Can I use this calculator to design drainage?
It can check the arithmetic, but drainage design must follow the project drawings, applicable standards, product requirements and any requirements set by the relevant authority or qualified practitioner.
Related EBC guidance
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