Updated
March 27, 2026
Reviewed against the current calculator logic, structured content, and internal linking used on BuildCostLab.
Drainage Estimating
Work out how much ducting to buy before you order stock lengths
A good buying number for length-based materials depends on the run, the stock size, and how much cutting waste the job will create.
Methodology
Planning-first estimate
Use this guide to build a rough material estimate, then confirm it against product coverage data, site conditions, and supplier pack sizes before you order.
Trust
How we publish
See the calculator methodology and editorial policy for the standards behind these pages.
Use the calculator first
The quickest path is to start with Ducting Calculator, then use this guide to sense-check the result and decide what to buy next.
Why this page exists
Best for trim, drainage, roofline, pipework, and edging products where the real order is based on whole stock lengths. This one is tuned for ducting jobs.
Core assumption
Linear calculators assume materials are bought in stock lengths and the job can be reduced to a total run with a reasonable cut allowance.
Common mistake
Common misses include forgetting joints, corners, mitres, end conditions, and the waste created when standard stock lengths do not divide neatly into the run.
Next step links
Open the full Drainage Estimating cluster or go straight to the Ducting Calculator.