Over-excavation, spillage and uneven subgrade. 5-10% for formed work, 10-15% on rough ground.
Yields are the figures printed on the bag. Speciality mixes differ — check yours.
Optional. Leave at zero to skip the cost estimate.
Material quantity estimate for planning and purchasing. Not structural engineering advice, not a bid, and not a code determination.
How much concrete for a 10x10 slab?
A 10 x 10 ft slab at 4 inches thick needs 1.23 cubic yards of concrete (33.3 cubic feet), or 1.36 cubic yards with a 10% waste allowance.
In bags that is 62 bags at 80 lb, or 82 at 60 lb. At 6 inches thick the same slab takes 1.85 cubic yards, or 2.04 with waste.
At this size you are right on the bag-versus-ready-mix line. Sixty-two bags is four to six hours of mixing and a slab has to be poured in one go, so if a plant will deliver 1.5 yards without a punitive short-load fee, take it.
How thick should the slab be?
Thickness drives both the concrete volume and whether the slab survives. These are the standard residential figures from the IRC and common practice:
| Application | Thickness | Reinforcement |
|---|---|---|
| Garden shed floor, walkway | 4 in (100 mm) | Optional wire mesh |
| Patio, sidewalk | 4 in (100 mm) | 6x6 W1.4 mesh or #3 bar |
| Car driveway, garage floor | 5 in (125 mm) | #3 bar at 18 in on centre |
| Pickup, RV, boat trailer | 6 in (150 mm) | #4 bar at 16-18 in on centre |
| Hot tub pad | 6 in (150 mm) | #4 bar at 12 in each way |
Bags or ready-mix?
This is the decision that actually costs money, and it turns on volume rather than preference.
Bagged mix runs roughly $290-330 per cubic yard once you account for how many bags a yard takes (45 at 80 lb, 60 at 60 lb). Ready-mix delivered is typically $140-190 per cubic yard — but almost every plant charges a short-load fee of $60-150 under about 3-5 yards, and many have a 1-yard minimum.
The practical rule: under 1 cubic yard, use bags. Over 2 cubic yards, call a plant. In between, price the short-load fee against the bag total and factor in labour, because 45 bags is four to six hours of mixing and you cannot stop halfway through a slab.
What this calculator assumes
- A rectangular slab of uniform thickness. For an L-shape, split it into rectangles and add the results.
- Bag yields are the published figures for standard pre-mixed concrete: 0.30 ft³ per 40 lb bag, 0.45 ft³ per 60 lb, 0.60 ft³ per 80 lb. Fast-setting and high-strength mixes yield slightly less — check your bag.
- Waste allowance covers spillage and an uneven subgrade. It does not cover a form that bulges, which is the usual reason a pour runs short.
- Volume is for the slab only. The compacted gravel base underneath is separate.
Common questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet, so it takes 45 bags at 80 lb (0.60 ft³ each), 60 bags at 60 lb (0.45 ft³), 72 bags at 50 lb, or 90 bags at 40 lb.
How much concrete do I need for a 10x10 patio?
A 10 x 10 ft slab at 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. With 10% waste that is 1.36 cubic yards — about 62 bags at 80 lb, or a 1.5-yard ready-mix load.
Do I need gravel under a concrete slab?
Yes for anything structural or exposed to frost. Four inches of compacted crushed stone gives you drainage and a stable, level bearing surface. It is not optional on a driveway or garage floor.
How far apart should control joints be?
Space joints in feet at roughly two to three times the slab thickness in inches — so 8 to 12 feet for a 4 inch slab. Cut them a quarter of the slab depth. Concrete cracks; joints decide where.