Published August 17, 2026 · Updated August 23, 2026 · By Tupep
First, should this even be a slab?
A 4×8 patio is 32 square feet, roughly the footprint of a grill and two chairs. At 4 inches thick that is about 20 bags of 80 lb mix, which is 1,600 lb of material to haul home, mix, place, and finish in one continuous session with no do-overs. Pavers over a compacted base cover the same 32 square feet with no cure time, no forming, no finishing skill, and they can be lifted and reset if the ground moves.
We pour a slab this small when the surface needs to be monolithic, when something is going to be anchored to it, or when it is the landing for a door and needs to hold an exact elevation. We use pavers when the goal is just a flat, dry place to stand. Both are legitimate. Deciding before you dig saves the most money of anything in this article.
| Consideration | Poured slab | Pavers on compacted base |
|---|---|---|
| Material weight to move | ~1,600 lb of mix plus base | Similar, but in liftable units |
| Time pressure | One continuous pour, set clock running | None, stop and resume anytime |
| Skill required | Screeding, floating, timing the finish | Screeding sand, laying, tamping |
| Frost movement | Can crack or heave as one piece | Individual units move and can be reset |
| Repairable | Not really | Yes, unit by unit |
| Best for | Anchoring, door landings, clean single surface | Casual patios, curves, DIY first projects |
The formula, with the slope term people forget
Volume (cu ft) = length x width x (average thickness in inches / 12)
The word doing the work is average. A patio has to shed water, and how you build that slope decides your volume. If you slope the compacted subgrade so the slab is a constant thickness, the average equals the nominal thickness and nothing changes. If you leave the subgrade flat and build the slope into the concrete, the slab is thicker at one end and your average climbs.
A common drainage target is about 1/4 inch of fall per foot, away from the house. Across 8 feet that is 2 inches of fall. Build that into the top only and your 4 inch slab averages 5 inches, which is 25 percent more concrete. Sloping the subgrade instead is more shovel work and less money.
Worked example
4 x 8 = 32 square feet. At 4 inches, 32 x (4/12) = 10.67 cubic feet, which is 0.40 cubic yards.
Bags of 80 lb: 10.67 / 0.60 = 17.8, call it 18. Add 10 percent for waste and subgrade slop and you are at 20 bags. Bags of 60 lb: 10.67 / 0.45 = 23.7, plus waste, so 27 bags.
| Build | Avg thickness | Cubic feet | Cubic yards | 80 lb bags (+10%) | 60 lb bags (+10%) |
|---|---|---|---|---|---|
| Formed with 2×4 lumber | 3.5 in | 9.33 | 0.35 | 18 | 23 |
| Formed to a true 4 in | 4 in | 10.67 | 0.40 | 20 | 27 |
| 4 in with slope built into the top | 5 in | 13.33 | 0.49 | 25 | 33 |
| Heavier slab, true 5 in | 5 in | 13.33 | 0.49 | 25 | 33 |
That first row is worth a second look. A 2×4 is 3.5 inches, not 4. If you form with dimensional lumber and screed flush to the top of the form, you have poured a 3.5 inch slab and used about 12 percent less concrete than a calculator assuming 4 inches told you. That is the single most common reason bag estimates for small patios run high.
In the other direction, half an inch of over-excavation across 32 square feet is 1.33 cubic feet, which is more than two extra bags of 80 lb. On a job this small, sloppy grading is the entire waste allowance.
Control joints on a 4×8
The common rule of thumb is that control joint spacing in feet should be about two to three times the slab thickness in inches. A 4 inch slab therefore wants joints somewhere in the 8 to 12 foot range, and an 8 foot long slab technically needs none.
There is a second rule that pushes the other way. Panels should be roughly square, with the long side no more than about one and a half times the short side. A 4×8 panel is 2 to 1, which is outside that. That is why plenty of finishers put a single joint at the 4 foot mark and end up with two 4×4 panels. It costs nothing and it gives the slab a place to crack where you chose.
Joint depth should be about a quarter of the slab thickness, so 1 inch on a 4 inch slab. Tool it while the concrete is plastic, or saw cut it within several hours of finishing. Cut too late and the slab has already picked its own crack line.
Mix water is not a comfort setting
An 80 lb bag typically wants roughly 3 quarts of water, and the bag will state its own figure. Every extra quart you add past that makes the mix easier to place and meaningfully weaker, more prone to shrinkage cracking, and much more likely to dust or scale at the surface. On a small patio the temptation is enormous because soupy concrete self levels. Resist it.
The other water rule is about finishing. Never work bleed water back into the surface with a float or trowel. Wait until the sheen is gone, then finish. Broom the surface perpendicular to the direction of the slope so the grooves do not channel water sideways along the slab.
What is out of scope here
Base depth, frost protection, and whether a patio slab needs to be tied to or isolated from the house are all local questions. Frost depth in particular is jurisdictional, so check with your building department rather than copying a number. And if a hot tub, an outdoor kitchen, or anything with real weight is going on this pad, stop treating it as flatwork. That is a designed slab and it needs an engineer.
Related guides
- Driveway Slab Thickness and Volume
- How Much Concrete Per Fence Post
- Gravel Base Depth Under a Paver Patio
- Pea Gravel vs Crushed Stone
Sources and Further Reading
Run your own numbers with the Concrete Slab Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 23, 2026
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