Where our numbers come from

Material weight is quoted per cubic foot in the condition the material is actually in when it reaches you: loose, not compacted, and holding some moisture. That is the honest state for estimating a delivery, because that is what the yard weighs on the truck.

It also means our figures sit slightly above textbook dry densities. Crushed stone and sand are taken at 100 lb per cubic foot, screened topsoil at 76, compost at 45, and wood mulch at 22. Set concrete is taken at 150.

Bag volumes come from the sizes carried by US big-box stores and landscape yards: 2 and 3 cubic foot mulch bags, 0.5 cubic foot (50 lb) stone and sand bags, 0.75 and 1.5 cubic foot soil bags, and 1 and 1.5 cubic foot compost bags.

Concrete bag yields follow the manufacturers' stated figures: 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb.

These do vary between brands and regions. The coverage figure printed on the bag in front of you always beats ours.

Every calculator adds a percentage on top of the exact volume, defaulting to 10 percent. It is covering three separate things: material lost in handling, settlement after watering and compaction, and the fact that ground is never as flat as the measurement assumes.

You can turn it off. We would not, except on a job where running over is worse than running short.

Moisture is the biggest variable. Soaked mulch can weigh half again what dry mulch does, and wet sand is heavier still. Weight figures should be treated as approximate; volume figures are far more reliable.

Compaction is the second. Decomposed granite and crushed stone lose roughly a fifth of their loose volume when properly compacted, which is why those calculators default to a larger allowance.

And nothing here is structural advice. Slab thickness, footing dimensions and reinforcement are governed by local building code, and a calculator cannot know your frost line or your soil.