Try a Common Scenario
Click to pre-fill the calculator with realistic values.
Work out the required drainage fall — the pipe gradient or slope — for foul and surface water drains. Enter a pipe size and design flow to find the governing gradient, or enter a gradient to check flow capacity. The conversion table below translates common 1 in 40, 1 in 60 and 1 in 80 falls into millimetres per metre, percentages and total fall. The calculator also provides an indicative Building Regulations Part H Table 6 assessment when its foul-drain conditions match.
For educational purposes only. Not a substitute for professional engineering judgment.
Calculator Inputs
Calculation Mode
Choose whether to calculate required gradient or flow capacity
Drainage Type
Table 6 screening applies to foul drains only; surface-water results are hydraulic
Table 6 Nominal Pipe Size
Mapped from supported product diameters; confirm the pairing against product data
Connected WCs
Required because the flatter Table 6 rows are conditional on WC count
Pipe Diameter
Select pipe size in millimetres
Pipe Material & Condition
Select material to determine Manning's roughness coefficient
Manning's n Override (Optional)
Leave blank to use default for selected material
Peak Design Flow Rate
Used for the Manning calculation, capacity check and Table 6 flow band
Drainage Fall Overview
Drainage fall (also called gradient or slope) is the vertical drop of a pipe over its horizontal length — what makes a gravity drain flow. Choose a mode above:
- Calculate Required Gradient — enter a design flow (l/s) to find the gradient needed to convey it, using the Manning equation S = (n Q / (A R2/3))².
- Calculate Flow Capacity — enter a gradient (mm/m or 1:X ratio) to find the full-bore flow the pipe can carry and the corresponding full-bore capacity velocity.
For foul drains, Table 6 is assessed only when the peak-flow, nominal pipe-size and WC conditions match a published row. Surface-water and unmatched cases remain hydraulic results without a Part H compliance claim.
Approved Document H Table 6 — foul drains
| Pipe | Peak flow | Typical minimum gradient | Condition / capacity |
|---|---|---|---|
| 75mm | <1 l/s | 1:40 | 4.1 l/s maximum capacity |
| 100mm | <1 l/s | 1:40 | 9.2 l/s maximum capacity |
| 75mm | >1 l/s | 1:80 | 2.8 l/s maximum capacity |
| 100mm | >1 l/s | 1:80 | Minimum 1 WC; 6.3 l/s at 1:80 |
| 150mm | >1 l/s | 1:150 | Minimum 5 WCs; 15.0 l/s capacity |
Exact rows and conditions from Approved Document H Table 6 for foul drains. The 100mm >1 l/s row is 1:80 (not 1:100). Table 6 does not provide a blanket surface-water minimum. Confirm nominal product size and the current document with building control.
1 in 40, 1 in 60 and 1 in 80 fall in mm per metre
Divide 1000 by the second number in the ratio: a 1 in X fall equals 1000 ÷ X millimetres of drop per metre. These geometric conversions are the same in the UK, Australia, New Zealand, South Africa, Canada and other metric-using regions.
| Gradient | Fall per metre | Percentage | Fall over 5m | Fall over 10m |
|---|---|---|---|---|
| 1 in 40 (1:40) | 25 mm/m | 2.5% | 125 mm | 250 mm |
| 1 in 60 (1:60) | 16.7 mm/m | 1.67% | 83.3 mm | 166.7 mm |
| 1 in 80 (1:80) | 12.5 mm/m | 1.25% | 62.5 mm | 125 mm |
Conversion does not establish compliance. Minimum and maximum gradients depend on pipe diameter, loading, drainage type and the rules adopted where the work is located. The Part H check in this calculator is based on Approved Document H guidance for England; verify the current document and local requirements. Do not substitute it for AS/NZS 3500 or the applicable Canadian, South African or other national, provincial, territorial or local standard.
Approved Document H Table 6 gradients for foul drains
Table 6 does not give one minimum for every drain. Its flatter rows depend on peak flow, nominal pipe size and connected WCs; its published maximum capacity is shown alongside each row.
| Pipe | Peak flow | Minimum gradient | Condition | Maximum capacity |
|---|---|---|---|---|
| 75mm | <1 l/s | 1:40 | — | 4.1 l/s |
| 100mm | <1 l/s | 1:40 | — | 9.2 l/s |
| 75mm | >1 l/s | 1:80 | — | 2.8 l/s |
| 100mm | >1 l/s | 1:80 | Minimum 1 WC | 6.3 l/s |
| 150mm | >1 l/s | 1:150 | Minimum 5 WCs | 15.0 l/s |
Source: Approved Document H (England), Table 6. The document labels the peak-flow bands <1 and >1 l/s, so this calculator does not infer a Table 6 minimum at exactly 1 l/s. Table 6 covers foul drains and does not establish a blanket surface-water value. Confirm the current document and how a product's outside diameter maps to nominal size.
Foul vs surface water drainage
Foul drainage
Carries waste water and solids from WCs, sinks and appliances. Table 6 ties its recommended minimum gradients to peak flow, nominal pipe size and, for the flatter 100mm and 150mm rows, connected WC count.
Surface water drainage
Carries rainwater run-off from roofs and paving. Table 6 does not provide its minimum gradient. Size and grade it for the applicable rainfall, hydraulic capacity and local requirements, and keep it separate from the foul system.
Worked example
A 10 m run at 1:80, after that gradient has been selected under the applicable design conditions:
- Total fall = length ÷ ratio = 10 ÷ 80 = 0.125 m = 125 mm
- Fall per metre = 1000 ÷ 80 = 12.5 mm/m
- Percentage grade = 100 ÷ 80 = 1.25%
- If the start invert is 30.000 m, the end invert is 30.000 − 0.125 = 29.875 m
What is drainage fall?
Drainage fall (also called gradient or slope) is the vertical drop of a drainage pipe over its horizontal length. It is what makes a gravity drain work. It is expressed as:
- Ratio (1:X): e.g. 1:80 means 1 unit of drop for every 80 units of length
- Fall per metre (mm/m): e.g. 12.5 mm/m
- Percentage (%): e.g. 1.25%
The table above converts the common ratios. The calculator uses the Manning equation to relate gradient, pipe size and flow, then applies a Part H Table 6 row only when its foul drain, peak-flow, nominal-size and WC conditions are satisfied.
Using this outside the UK
The maths behind drainage fall is universal: a gradient of 1:80 means one unit of drop for every 80 units of pipe length everywhere, so the ratio and fall-per-metre conversions in the reference table are valid regardless of where you are working.
What changes between countries is the minimum gradient a code allows for a given pipe size and use. The compliance table follows Approved Document H guidance for England, but rules in other markets set their own comparable minimum grades — for example AS/NZS 3500 in Australia and New Zealand, and the International Plumbing Code (IPC) in much of the United States. Use the reference conversions for geometry, but design against the standard that applies in your jurisdiction rather than assuming the Part H assessment carries over.
Frequently asked questions
What is the minimum fall for drainage?
There is no single Part H minimum for every drain. Approved Document H Table 6 gives 1:40 for 75mm and 100mm foul drains below 1 l/s peak flow. Above 1 l/s it gives 1:80 for 75mm, 1:80 for 100mm only with at least one WC, and 1:150 for 150mm only with at least five WCs. Table 6 does not set a blanket surface-water minimum. Confirm the current guidance and project requirements.
What happens if a drain is laid too steep?
Counter-intuitively, a foul drain that is too steep can be as problematic as one that is too flat. If the water runs away faster than the solids it is carrying, the solids can be left stranded in the pipe and cause blockages. This is why foul runs are usually kept within a sensible range (roughly 1:40 to 1:110) rather than simply made as steep as possible.
What fall does a 110mm drain need?
It depends on peak flow, connected appliances and the pipe’s nominal size. For the 100mm nominal foul-drain row in Approved Document H Table 6, a peak flow below 1 l/s uses 1:40. Above 1 l/s, 1:80 is listed only where at least one WC is connected. A 110mm outside-diameter product is not itself a separate Table 6 row, so confirm its nominal size and application with the manufacturer and building control.
Is drainage fall the same as gradient and slope?
Yes — fall, gradient and slope describe the vertical drop over horizontal length in different forms. A 1:80 gradient, a 1.25% slope and 12.5mm of fall per metre are equivalent. The reference table on this page compares the common ratios; the calculator accepts mm/m or 1:X when checking flow capacity.
What is the difference between foul and surface water drainage falls?
Foul drainage carries solids, so connected appliances and self-cleansing behavior matter as well as capacity. Approved Document H Table 6 provides conditional foul-drain rows; it does not provide a blanket surface-water minimum. Surface-water drainage must be designed to the applicable rainfall, capacity and local requirements. Keep the systems separate and do not carry a foul-drain Table 6 value across automatically.
How do I calculate fall per metre?
For a 1:X gradient, fall per metre = 1000 ÷ X in millimetres. Total fall = run length in metres × fall per metre. For example, 1 in 80 is 1000 ÷ 80 = 12.5mm per metre, so a 10m run drops 125mm. The ratio conversion is universal, but the minimum permitted gradient depends on the drainage system and local rules.
What is a 1 in 40 fall in mm per metre?
A 1 in 40 fall is 25mm per metre, which is a 2.5% gradient. The total drop is 125mm over 5m or 250mm over 10m. This is a metric conversion, not a universal design recommendation: confirm that 1:40 is appropriate for the pipe, use and code that apply to your project.
What is a 1 in 60 fall per metre?
A 1 in 60 fall is 16.7mm per metre (16.67mm/m before rounding), or a 1.67% gradient. That gives about 83.3mm of total fall over 5m and 166.7mm over 10m. Required drainage gradients vary by pipe type, diameter and jurisdiction.
What is a 1 in 80 fall per metre?
A 1 in 80 fall is exactly 12.5mm per metre, or a 1.25% gradient. The total fall is 62.5mm over 5m and 125mm over 10m. It is commonly associated with 110mm foul drainage in Part H guidance when the stated conditions are met, but it should not be carried across to another pipe or jurisdiction without checking the applicable rules.
What fall do I need for a 110mm soil pipe?
Check the product’s nominal size, peak flow and connected appliances. Table 6 has a 100mm nominal row: below 1 l/s it lists 1:40; above 1 l/s it lists 1:80 only with at least one WC. It does not separately list 110mm outside diameter, so verify how the selected product is classified before using that row.
How do I measure drainage fall on site?
Set the invert level (the inside bottom of the pipe) at the start and end of the run, then check that the measured drop matches the approved design — for example with a laser level, a dumpy level and staff, or boning rods and a sight rail.
What fall do I need for a waste pipe?
Internal waste pipework is outside the foul-drain Table 6 screening performed here. Its permitted gradients depend on pipe size, branch arrangement, appliances and the rules for above-ground sanitary pipework. Use the applicable plumbing guidance rather than applying a buried-drain value from this calculator.
What fall do I need for a soil pipe?
For a below-ground foul drain, use the Table 6 row only when all its conditions match. The 100mm nominal 1:80 row requires peak flow above 1 l/s and at least one WC; the 150mm 1:150 row requires peak flow above 1 l/s and at least five WCs. Above-ground soil pipework follows different guidance.
Standards & related tools
Approved Document H (gov.uk)
Official Building Regulations guidance for drainage and waste disposal.
Manning's n Reference
Roughness coefficients for common pipe materials.
Manning's Pipe Calculator
Full-bore pipe flow capacity when the gradient is known.
Building Regulations Part H Guide
How Approved Document H addresses foul drainage and its conditional Table 6 gradients.
BS EN 752 Drainage Guide
The European standard for drain and sewer systems outside buildings.
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Last verified: February 2026