About How Many Bags
How Many Bags is a set of calculators for the moment when you are standing in the home center or on the phone with a supplier and need a number. How many bags of concrete, how many yards of mulch, how many tons of gravel, how many deck boards. Every calculator works out the quantity from your measurements and adds the waste that real jobs need. It turns the answer into the units things are actually sold in, with a list you can copy to your phone's notes.
What the calculators do and do not do
They estimate quantities to buy. They do not design anything. A slab's thickness, a footing's size, a deck's joist span and an asphalt driveway's base depth are set by your local code, the product maker and your site's conditions. The calculators use the common figures only as starting points you can change. Where a permit or an engineer is the right call, the page says so.
Where the numbers come from
Bag yields, board sizes, densities and recommended depths are taken from manufacturers' data sheets, government and university publications and industry guides. Each page lists them at the end with the date they were read. The formulas are plain geometry, and automated tests check them against answers worked out separately before any page is published. Where a page gives a cost, it is a range from named documents, with the published figures the range is built from quoted against their document. The calculators take the price from your own store or supplier, so the total matches where you live.
How the pages are made and kept honest
Pages are prepared from those documents in the way the editorial policy describes. Their figures are checked against the sources listed at the end, against the test suite and against competing calculators for gaps. A person will read them over time after they go up, and a page's byline says when that has happened. Nothing here claims to have poured, laid or built the job in the example; the examples are worked arithmetic. The editorial policy page describes the process in full. The contact page is the place to report a figure that looks wrong, which is the most useful thing a visitor can do for the next one.