Pipe Gradient and Fall Calculator: How to Work Out 1:40, 1:80 and Invert Levels

Why gradient matters

A gravity drain only works if it falls steadily in the direction of flow. Too shallow and solids settle and the pipe blocks. Too steep and water can run away from the solids it should carry. Getting the fall right is a daily check for drainage and groundworks teams, and an error is expensive once the pipe is bedded and backfilled.

KaelSite’s free Pipe Gradient and Fall Calculator converts between gradient ratios, total fall, fall per metre and invert levels, so you can check a design or a survey in seconds.

Key terms

• Invert level: the level of the lowest inside surface of the pipe, usually shown on drawings in metres above datum (for example 52.450 m AOD).
• Fall: the vertical drop along the pipe.
• Gradient: the fall expressed as a ratio, percentage or fall per metre.
• 1:N: for every N units of length, the pipe falls 1 unit.

The formulas

Use the same units for length and fall.

• Total fall = pipe length ÷ N
• Ending invert = starting invert − total fall (for a gravity run)
• Fall per metre = total fall ÷ length, or 1,000 mm ÷ N
• Gradient from invert levels: N = length ÷ (start invert − end invert)
• Percentage gradient = 100 ÷ N

Some common conversions:

Worked examples

1. Design from a gradient
A 25 m run at 1:40 with a starting invert of 52.450 m:
Total fall = 25 ÷ 40 = 0.625 m (625 mm)
Ending invert = 52.450 − 0.625 = 51.825 m
Fall per metre is 25 mm.

2. Gradient from two invert levels
Starting invert 52.450 m, ending invert 52.050 m, pipe length 32 m:
Total fall = 52.450 − 52.050 = 0.400 m
N = 32 ÷ 0.400 = 80, so the gradient is 1:80 (12.5 mm per metre).

3. Setting a laser from fall per metre
You want 20 mm per metre over 18 m, starting at 48.300 m:
Total fall = 20 × 18 = 360 mm
N = 1,000 ÷ 20 = 50, so 1:50
Ending invert = 48.300 − 0.360 = 47.940 m

4. Spotting an adverse gradient
Start invert 50.100 m and end invert 50.250 m. The pipe rises by 150 mm along the direction of flow. This is an adverse gradient. A gravity foul or surface water drain must fall in the direction of flow unless it is a pumped rising main, so check the survey data or the design.

Metres or millimetres?

Invert levels on UK drainage drawings are normally in metres to three decimal places. Falls over short runs are easier to discuss in millimetres. Mixing them is a classic error: 625 mm entered as 625 m gives a nonsense result. The calculator accepts either unit and shows total fall in both.

Choosing a gradient

The required gradient depends on the pipe diameter, the type of drain (foul or surface water), the flow and the design standard. Steeper falls drain more aggressively, and shallower falls need larger pipes or enough flow to keep solids moving. In England, Approved Document H of the Building Regulations sets out minimum gradients for drains serving buildings, and BS EN 752 covers drain and sewer systems outside buildings. Always follow the drainage design and check the current edition of the documents that apply to your job.

Checking falls on site

• Check the starting and ending invert levels against the drawing before laying.
• Use a laser or string line set to the calculated fall per metre.
• Check the level at each chamber and at intermediate points, not just at the ends.
• Record as-built invert levels so the finished drain can be compared with the design.

Common mistakes

• Wrong direction. Subtracting the fall from the wrong end gives a rising drain.
• Mixing units. Metres and millimetres in the same calculation is the biggest source of error.
• Confusing 1:40 with 1:4. One extra digit changes the answer completely.
• Checking only the ends. A dip in the middle can hold water even when the end levels are correct.
• Ignoring pipe length along the slope. For normal drainage gradients the difference is negligible, but use the true run length from the drawing.

Frequently asked questions

How do I calculate pipe fall from a gradient?
Divide the pipe length by N. A 40 m run at 1:40 falls 1 m. The ending invert is the starting invert minus that fall.

What does a 1:40 or 1:80 gradient mean?
The pipe falls 1 unit for every 40 or 80 units of length. That is about 25 mm per metre for 1:40 and 12.5 mm per metre for 1:80.

How do I find the gradient from two invert levels?
Subtract the ending invert from the starting invert to get the fall, then divide the pipe length by that fall.

What is fall per metre?
It is the fall divided by the length, a handy figure for setting lasers and string lines.

What if the ending invert is higher than the start?
It is an adverse gradient for a gravity drain. Check the survey or design, unless the pipe is a pumped main.

Related calculators

Setting falls is one part of a drainage job. These tools help with the rest:

• Excavation and Spoil Removal Calculator: plan the trench dig and the spoil to remove once you know your depths.
• Aggregate and MOT Type 1 Calculator: work out the bedding and sub-base material.
• Concrete Volume Calculator: calculate concrete for pipe surrounds and chamber bases.
• Construction Cost and Labour Calculator: estimate the labour hours and cost of the drainage works.

See all of them in our complete guide to free construction calculators.

Keep your drainage records in one place

Falls, levels, photos and test results all need recording as the work goes in. KaelSite helps UK site teams manage inspections, safety and reporting from one mobile-first app. Explore KaelSite features.

References

• GOV.UK, Approved Document H: Drainage and waste disposal: https://www.gov.uk/government/publications/drainage-and-waste-disposal-approved-document-h
• BS EN 752, Drain and sewer systems outside buildings (BSI): https://www.bsigroup.com
• CIRIA, guidance on drainage and sustainable drainage: https://www.ciria.org