# Drainage invert level calculator > Calculate drainage falls, downstream invert levels and chainage schedules, or check the actual gradient between two surveyed invert levels. - Canonical HTML: https://rospower.co.uk/resources/drainage-invert-level-calculator/ - Documentation index: https://rospower.co.uk/llms.txt - Updated: 2026-09-04 > This Markdown version contains the published explanatory content. Use the canonical HTML page for calculators, builders and other interactive controls. ## Page content Apply a drawing-specified gradient to a drainage run, produce a chainage level schedule, or check the actual fall between two surveyed invert levels. **Invert level** is the level at the lowest internal point of a pipe. **Chainage** is distance measured along the run from a stated start point. A gradient written as **1 in 100** means the pipe falls one unit vertically for every 100 units travelled horizontally. **Project datum** is the fixed level reference used across the drawings and survey. A **benchmark** is a stable point with a known level. **Backfall** means the pipe rises in the intended downstream direction. A **traveller** is a fixed-length setting-out aid used to transfer a level. The **pipe crown** is the outside top of the pipe, while **battering** means sloping an excavation side back to reduce collapse risk. ## Level and fall calculator Use levels from the current drainage drawing or survey. The opening figures are an illustrative geometry example, not a recommended drainage gradient. Use metres to the project datum, keeping the same datum throughout. Measure along the straight horizontal run, not the sloping pipe length. Enter the N value from the approved drawing, for example 100 for 1 in 100. Enter the surveyed downstream invert using the same project datum. The final chainage is always included, even when it is not a whole interval. ### Setting-out level schedule Calculated from the start invert and selected gradient. | Chainage | Invert level | Fall from start | | --- | --- | --- | | 0.00 m | 100.000 m | 0 mm | | 5.00 m | 99.950 m | 50 mm | | 10.00 m | 99.900 m | 100 mm | | 15.00 m | 99.850 m | 150 mm | | 20.00 m | 99.800 m | 200 mm | | 25.00 m | 99.750 m | 250 mm | | 30.00 m | 99.700 m | 300 mm | ## What the drainage levels mean Keep the drawing direction, datum and units visible while setting out. Reversing a run or mixing millimetres and metres produces a plausible-looking but wrong answer. ### Invert and cover level The invert is the inside bottom of the pipe. A cover level is normally the finished level at the top of a chamber or surface. They are different references and should not be substituted for one another. ### Chainage Chainage measures horizontal distance from the stated start point. The table gives intermediate calculated levels for a straight, constant-gradient run. ### Gradient A larger denominator is a flatter gradient. For example, 1 in 200 falls half as much per metre as 1 in 100. Use the approved drawing value rather than selecting one from this calculator. ## How the calculation works All calculations use horizontal length and levels in metres. Results are also shown in millimetres per metre and percentage fall. ### Set out a run **Total fall** Horizontal run divided by the gradient denominator **Downstream invert** Start invert minus total fall ### Check surveyed levels **Actual fall** Start invert minus end invert **Actual gradient** Horizontal run divided by the absolute level change ### Worked example A 30m run starting at invert 100.000m and falling at 1 in 100 has a total fall of 0.300m, or 300mm. The calculated end invert is 99.700m. At 5m intervals, each schedule step falls 50mm. These figures demonstrate the geometry only. ## Use the result on site Treat the schedule as a check against controlled project information, not as a replacement for it. Checking more than one run? Use the [manhole schedule checker](https://rospower.co.uk/resources/manhole-schedule-checker/) to review chamber references, downstream connections and levels across a connected schedule. It checks arithmetic and consistency, not drainage design. 1. 1**Confirm direction and datum**Mark the upstream and downstream points. Check that every level uses the same project datum and drawing revision. 2. 2**Check the instrument**Set up the laser or level on firm ground, verify its calibration status and take a benchmark check before measuring the pipe. 3. 3**Measure the pipe invert**Use a suitable staff or traveller at the actual inside bottom of the pipe. Do not confuse the pipe crown, outside diameter or chamber cover with the invert. 4. 4**Record intermediate checks**Check the run at agreed chainages and before covering. Two correct end levels do not prove there is no local high point or backfall between them. ## Limits and safety checks ### Not a drainage design This calculator does not size the pipe, select a minimum gradient, assess capacity, establish cover, design a chamber, or confirm compliance. Use the approved drainage design and project specification. ### A straight-line check The chainage schedule assumes one constant gradient between the entered endpoints. It does not detect deformation, local settlement, incorrect joints or an obstruction within the run. ### Control excavation risk Plan support or battering, safe access, edge protection, spoil position, water control and inspections before anyone works around an excavation. Do not enter an unsupported trench to take a level. ### Locate buried services Plan the work, locate and identify underground services, and use a safe digging method. A setting-out level does not establish that the proposed excavation path is clear. ## Sources and further guidance - [Approved Document H](https://www.gov.uk/government/publications/drainage-and-waste-disposal-approved-document-h): current Building Regulations guidance for foul and surface water drainage in England (HM Government). - [BS EN 1610:2015](https://knowledge.bsigroup.com/products/construction-and-testing-of-drains-and-sewers): construction and testing of drains and sewers (BSI). - [Excavation safety](https://www.hse.gov.uk/construction/safetytopics/excavations.htm): collapse, support, access, services, water, edge protection and inspection guidance (HSE). - [HSG47 avoiding danger from underground services](https://www.hse.gov.uk/pubns/books/hsg47.htm): planning, locating services and safe excavation (HSE). Related resources - [Drainage gradients and falls](https://rospower.co.uk/resources/drainage-gradients/) - [Pipe bedding and trench calculator](https://rospower.co.uk/resources/pipe-bedding-calculator/) - [Percolation and soakaway sizer](https://rospower.co.uk/resources/percolation-soakaway-sizer/) - [Underground services safe digging](https://rospower.co.uk/resources/underground-services-safe-digging/) - [Warehouse and industrial groundworks](https://rospower.co.uk/services/warehouse-groundworks/)