Guide
Drainage and slope
Concrete is not waterproof in the way a roof is waterproof. Water that sits on a slab finds joints, hairline cracks, and the cold joint at the house. In a freeze-thaw climate that standing water is also a winter problem. Slope is how the surface sends rain somewhere intended.
How much slope
For a home slab, walk, or drive, field talk often lands between about 1/8 inch and 1/4 inch of fall per foot, away from the building. That is a common planning band, not a grading specification. A patio that is “dead flat” can look neat on pour day and still hold a film of water after the first storm.
Steeper is not always better. A drive that dumps water onto a neighbor, into a garage, or across a public walk creates a different complaint. The slope has to meet the door thresholds you already have. If the existing yard already drains toward the foundation, the slab will not fix that by itself.
Where the water goes
Decide the outlet before the forms go in. Options people actually use include a lawn, a swale, a yard drain, or the street if the lot is allowed to shed that way. Downspouts that empty onto a new slab will cut a path in the finish and keep that strip wet.
Control joints are not drains, but they are where water collects if the surface is dished. A slight crown or a consistent plane toward one edge is easier to live with than a slab that looks level and holds puddles in the middle.
What the calculator does not change
The volume tools treat thickness as uniform. A planned slope uses almost the same yardage as a flat slab of the same average thickness. If the low edge is thicker because the subgrade was left flat and the forms step down, enter that thicker average or split the pour. The waste tool will not invent a dish in the middle.
This page is not a drainage design. A lot that already floods, a retaining condition, or a walk that meets the public right-of-way needs the official who owns that rule, not a yardage page.