Pipe fall = length × gradient. A 30 m drain at 1 in 80 falls 375 mm, so from an upstream invert of 99.500 the downstream invert is 99.125. In US units, 1/4 in per foot over 100 ft is a 25 in fall.
My Construction Calculator
Pipe Fall Calculator
Inputs
Results update as you type.
Diagram updates as you type.
Results
Check the working before you order.
Pipe fall
375mm
Downstream invert level
99.125 m
Percent
1.25 %
Ratio
1 in 80.0
Angle
0.72 °
mm per metre
12.5 mm/m
Invert at 5.0 m
99.438 m
Invert at 10.0 m
99.375 m
Invert at 15.0 m
99.313 m
Invert at 20.0 m
99.250 m
Invert at 25.0 m
99.188 m
Invert at 30.0 m
99.125 m
Price This optional: add your own prices
Your prices are remembered on this device only.
Send Order WhatsApp, SMS or email to your supplier
Fall = length × gradient = 30.00 m × 0.01250 = 375 mm
Downstream invert = 99.500 m − 375 mm = 99.125 m
Scan to open this calculationThe code links to this calculator with your inputs, so anyone can scan it with a phone camera and see the same result. Save it for a report, a site board or WhatsApp.Test copy: this code opens the live site, myconstructioncalculator.com, so it works on a phone once the site is launched.
Results are estimates for planning and material ordering only. They are not a substitute for structural design or approval by a qualified engineer. Read the full disclaimer.
Scan to open this calculation with the same inputs.
Generated with myconstructioncalculator.com. Estimates for planning and ordering only; verify against drawings and specifications.
Formula
Fall = length × gradientDownstream invert = upstream invert − fallInvert at x = upstream invert − x × gradient
Worked Example
30 m of drain at 1 in 80, upstream invert 99.500 m:
Gradient = 1 ÷ 80 = 0.0125 (12.5 mm/m)
Fall = 30 × 0.0125 = 375 mm
Downstream invert = 99.125 m; every 5 m: 99.438, 99.375, 99.313, 99.250, 99.188
Fall for Common Gradients
Gradient
Fall per 10 m
Gradient (US)
Fall per 10 ft
1 in 40
250 mm
1/4 in per ft
2.5 in
1 in 60
167 mm
1/8 in per ft
1.25 in
1 in 80
125 mm
1/16 in per ft
0.63 in
1 in 100
100 mm
1 in 150
67 mm
Frequently Asked Questions
How do I calculate pipe fall?
Multiply the pipe length by the gradient. At 1 in 80 every metre falls 12.5 mm, so 30 m falls 375 mm.
What is 1/4 inch per foot as a percentage?
About 2.08% (1 in 48). 1/8 in per foot is 1.04% (1 in 96).
What fall does a 100 mm drain need?
Commonly between 1 in 40 and 1 in 80 for house drains (1 in 80 minimum with a WC connected under UK guidance), but follow your local code and drainage design.
Embed This Calculator on Your Website
Free for any website: contractors, suppliers, blogs and schools. Paste this code where you want the calculator to appear. It works in WordPress (Custom HTML block), Wix, Squarespace and plain HTML.
