Drain size

1. Type your measurements. Feet-inch (12' 6 1/2"), decimal feet and metric all work; a bare number means feet or metres.
2. Need is the exact amount. Buy rounds up to whole bags, sheets or pieces, with the waste % included.
3. "How we got it" shows the formula, so you can check every number.
4. A "Check this" note means a value looks unusual. Have a look before you buy.

Worked example — change any value to try your own job.