Concrete & Rebar: Calculators, Diagrams & Guides
2 calculators · 6 diagrams · 1 guides
Pouring concrete is one of the least forgiving jobs in construction: once the truck arrives or the mixer starts turning, you either ordered enough material or you did not. The calculators in this hub take the guesswork out of that moment. Enter your slab, footing, or column dimensions and get cubic yards, ready-mix volume with a proper over-order allowance, bag counts for every common bag size, and the rebar grid — bar count, total linear feet, and lap-splice allowances — needed to reinforce the pour correctly.
The diagrams show you what the numbers mean in the ground: slab-on-grade cross-sections with gravel base and vapor barrier, control-joint spacing rules, rebar cover distances, and grid layout. They are drawn the way an engineering manual would draw them, so you can hand a figure to a helper or an inspector and be talking about the same thing.
Everything follows published standards rather than rules of thumb we invented: ACI 318 for reinforcement cover and development length, ASTM A615 for bar grades, and the bag-yield math printed on the products themselves. Every tool here is free, runs in your browser, and requires no signup — calculate the pour, print the materials list, and go build with numbers you can defend to an inspector.
Concrete & Rebar calculators
- Concrete CalculatorInstantly calculate cubic yards, 60lb/80lb bags & cost for slabs, footings & posts. Free concrete calculator, results in seconds—no signup needed.
- Rebar CalculatorFree rebar calculator: exact bar count, linear feet, weight, and stock sticks to buy for slabs, driveways, footings and round pads. ASTM A615 & ACI 318.
Guides & references
Concrete & Rebar · 6 diagrams
- Concrete & Rebar
How many bags of concrete are in a cubic yard — 45 eighty-pound bags or 60 sixty-pound bags
A cubic yard is 27 ft³, so it takes 45 × 80-lb bags (yield 0.60 ft³ each) or 60 × 60-lb bags (0.45 ft³ each). Past about ½ yard — roughly 25 bags — ordering a ready-mix truck beats mixing by hand.
- Concrete & Rebar
What goes under a concrete slab — gravel sub-base, vapor barrier, reinforcement, and slab thickness
A slab is more than concrete: a compacted gravel sub-base (~4″), a 6-mil vapor barrier, then the slab (4″ standard, 3.5″ IRC minimum) with the steel held at mid-depth on chairs — not on the ground, where it does nothing.
- Concrete & Rebar
Concrete control joint spacing — cut joints so the slab cracks where you want it to
Concrete shrinks and will crack — control joints decide where. Space them (in feet) at about 2–3× the slab thickness (in inches): a 4″ slab gets joints every ~8–12 ft, cut ¼ of the depth within 6–12 hours.
- Concrete & Rebar
Rebar size chart — the bar number is its diameter in eighths of an inch (ASTM A615)
The rebar number is the diameter in eighths of an inch: #4 = 4⁄8″ = ½″ (0.668 lb/ft), #8 = 1″ (2.670 lb/ft). Weight per foot is identical across Grade 40/60/80 — grade is yield strength, not size.
- Concrete & Rebar
How many rebar bars — the “+1” grid formula (subtract cover, divide by spacing, add one)
Count bars with the “+1” rule: subtract cover from both edges, divide the clear span by the spacing, then add one for the far-edge bar. A 20×20 ft slab, #4 @ 12″, 3″ cover → ⌊(240−6)÷12⌋+1 = 20 each way. Keep spacing ≤ 18″.
- Concrete & Rebar
Rebar cover and lap splice — keep bar off the dirt and lap 40× the bar diameter
Two placement details: keep bar up on chairs (3″ cover cast against earth, ¾″ formed — ACI §20.6); and where a run needs splicing, lap 40 × the bar diameter (min 12″), stagger the laps, and tie every one (ACI §25.5).
Frequently Asked Questions
How do I figure out how many bags of concrete I need?
Calculate the volume in cubic feet (length × width × depth, all in feet), then divide by the yield of your bag size — an 80 lb bag yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft. The concrete calculator does this for every common bag size at once and rounds up to whole bags, which is how they are sold.
When should I order ready-mix instead of mixing bags?
The usual crossover is around one cubic yard — roughly 45 bags at 80 lb each. Beyond that, hand-mixing risks cold joints between batches and is brutally slow. Ready-mix also gives you a consistent water-cement ratio across the whole pour, which matters for slabs that need to finish evenly.
What size rebar do I need for a slab or footing?
Residential slabs typically use #3 or #4 bar on 12 to 18 inch centers; footings commonly run two or three #4 bars. The controlling rules are cover (3 inches when cast against earth per ACI 318) and lap splice length (about 40 bar diameters). The rebar calculator applies both automatically.
How much extra concrete should I order?
Plan on 5 to 10 percent over the computed volume. Subgrade is never perfectly flat, forms deflect slightly, and spillage is real — running short mid-pour creates a cold joint that permanently weakens the slab. Our calculators show both the exact math and the padded order quantity so you can see what the buffer costs you in material.
Do I need control joints in my slab, and how far apart?
Yes — concrete shrinks as it cures and will crack somewhere; joints decide where. The standard rule is joint spacing in feet at 2 to 3 times the slab thickness in inches (a 4-inch slab gets joints every 8 to 12 feet), cut to a quarter of the slab depth. The slab diagrams in this hub illustrate the layout.