TOOL_03 · Concrete
Concrete calculator
Slabs, footings, post holes. Ready-mix yards with a 10% waste factor, or bag counts — the order-the-truck number, not the wishful one.
① Your pour
Ready-mix order
0.0cu yd
Includes 10% waste — always order a little extra
Bagged (80 lb)
0bags
89 × 60 lb or 134 × 40 lb
Material Cost Estimator
Estimated Total for 80 lb Concrete Bags
$435.50
67 units × $6.50 per 80 lb bag
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How to measure it on site
Concrete is ordered in cubic yards and poured in a finite window, so the measuring has to happen before the truck is called. Get length, width, and thickness in feet, then check the subgrade and the access.
Measure the slab, not the framing. Form boards sit outside the pour. Measure between the inside faces at several points — a 10 ft slab that is 10 ft 3 in at one end is a different order, and concrete cannot be trimmed to fit.
Pick the thickness the job needs. A footpath, shed floor, or patio is usually 4 inches. A driveway that takes vehicles is usually 6 inches. The thickness multiplies the whole order, so decide it before you calculate.
Convert thickness to feet. Divide inches by 12. Four inches is 0.333 ft; six inches is 0.5 ft. Forgetting this step inflates the volume twelvefold, which is the single most common input error.
Check the subgrade and the access. The 10 percent waste factor exists because dug subgrade is never perfectly flat. Walk the excavation and note any soft spots; also confirm a wheelbarrow can reach the pour from where a truck would park.
Common mistakes
Concrete errors are expensive because a short pour cannot wait. These are the five that show up most often.
- Using the slab's outside dimensions. A 10 × 10 slab framed with 2×4 lumber measures about 10.3 × 10.3 between the inside faces. The extra area is small, but it is the difference between finishing with a margin and running out at the last corner.
- Treating 4 inches and 6 inches as close enough. A 10 × 10 pour at 4 inches is about 1.23 cubic yards; at 6 inches it is about 1.85. That is half a yard more, and on a driveway the thickness is structural rather than cosmetic.
- Skipping the waste factor. Ten percent covers an uneven subgrade and slight over-excavation. Without it, a pour that measures exactly 1.23 yards arrives short because the ground took more concrete at the low corner.
- Counting the post as concrete in a post hole. A post displaces its own volume. A 12-inch hole 36 inches deep holds 2.36 cubic feet, but around a 4×4 post only about 2.10 cubic feet is poured — the difference is a bag on every post.
- Buying bags for a pour over one cubic yard. An 80 lb bag yields about 0.6 cubic feet, so a cubic yard takes 45 bags. Past roughly one yard, the mixing time and the risk of cold joints make a ready-mix delivery the practical choice.
Worked example
A 10 × 10 slab at 4 inches
A shed floor on level ground, formed with boards that sit outside the pour. The slab is a simple rectangle and no post holes are involved.
- Area
- 10 ft × 10 ft = 100 sq ft
- Thickness in feet
- 4 in ÷ 12 = 0.333 ft
- Volume
- 100 × 0.333 = 33.3 cu ft
- Cubic yards
- 33.3 ÷ 27 = 1.23 cu yd
- With 10% waste
- 1.23 × 1.1 = 1.36, rounded up to 1.5 cu yd
- 80 lb bags
- 33.3 ÷ 0.6 = 55.6, rounded up to 56 bags
Result: 1.5 cubic yards with the waste factor included, or 56 bags of 80 lb mix if you are mixing by hand. The raw volume before waste is 1.23 cubic yards.
What this calculator doesn't cover
This calculator sizes flat slabs, footings, and cylindrical post holes. It does not replace engineering or a site visit for these cases:
- Structural slabs and foundations — reinforcement, soil bearing, and thickness belong with an engineer or the local building department.
- Sloped or stepped pours — the calculator assumes one flat thickness; a sloped slab needs the average depth or a series of separate pours.
- Ready-mix scheduling — delivery minimums, pump access, and pour timing are supplier and site questions the volume math cannot answer.
- Mix design — bagged mix is a known blend; site-mixed concrete and fiber or admixture requirements need a specified mix.
Reviewed . Sources: Bagged concrete yield (about 0.6 cu ft per 80 lb bag) (Quikrete); Concrete volume and unit conversion reference (ASTM International). Full methodology.
How the math works
Slabs & footings: length × width × thickness (inches ÷ 12). Post holes: π × radius² × depth. Cubic feet ÷ 27 = cubic yards. The result card shows the raw yardage and the order number with 10% waste added — subgrade is never as flat as it looks.
Bag yields: an 80-lb bag yields about 0.6 cu ft, 60-lb about 0.45, 40-lb about 0.3. Bag counts are rounded up to the whole bag.
Rule of thumb: over ~1 cubic yard (45× 80-lb bags), schedule a ready-mix delivery. Your back will agree.
Common questions
How much concrete do I need for a 12x12 slab?
A 12×12 slab at 4 inches thick is 48 cubic feet — 1.78 cubic yards. Order 2 yards to cover waste and uneven subgrade. At 6 inches thick (driveway spec), it's 2.67 yards, so order 3.
How many 80-lb bags of concrete are in a yard?
One cubic yard is 27 cubic feet. An 80-lb bag yields about 0.6 cu ft, so a full yard takes 45 bags. That's why anything over roughly 1 yard is usually cheaper and far easier as a ready-mix truck delivery.
How deep should a post hole be for a fence?
Bury ⅓ of the post's total length: a 6-ft fence with 8-ft posts means a 2.5–3 ft hole. Diameter should be 3× the post width — 12 inches for a standard 4×4. This calculator's post-hole mode handles the cylinder math.
Should I order ready-mix or buy bags?
Bags make sense under about 1 cubic yard (45× 80-lb bags). Beyond that, a ready-mix truck is cheaper per yard, takes minutes instead of hours of mixing, and gives consistent strength. Most batch plants have a 1-yard minimum delivery — this calculator's waste-included yardage tells you exactly what to order.
Estimates are for planning purposes only. Structural pours require engineered specifications — consult a professional for load-bearing work.