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Concrete

What it's for

Figures how much concrete a pour takes: a slab, a footing, a wall, round columns or a set of formed steps. You get cubic yards, the bag count for a small pour, rebar footage or mesh sheets if you ask for them, a section drawing and a material list. It is a quantity takeoff. It does not check anything against code.

Step by step

The example is the screen as it opens: a 10' 0" by 10' 0" slab, 4" thick, 10% waste.

Concrete inputs at their defaults: POUR TYPE Slab, LENGTH 10' 0", WIDTH 10' 0", DEPTH 4", REBAR off, WELDED-WIRE MESH off, WASTE FACTOR 10%. The answer bar reads ≈ 1.36 yd³ over 62 bags @ 80lb · 82 bags @ 60lb. RESULTS repeats the bags line, with the BASIS & WHAT WAS ASSUMED row and the link Check before you cut · Measure twice under it. 1 2 3 4 5
Pour type first. It decides which dimensions the screen asks for.
  1. Tap POUR TYPE and pick Slab, Footing, Wall, Column or Steps. The rows under it change to suit. The list for each type is below.
  2. Enter the dimensions. For a slab that is LENGTH and WIDTH measured inside the forms, and DEPTH, the slab thickness. Tap a value and type it in feet and inches.
  3. Turn on REBAR if you want bar footage. Two more rows appear: REBAR SIZE (#3 to #6) and REBAR SPACING, the on-center spacing of the grid. On a slab you can turn on WELDED-WIRE MESH instead, or as well. Mesh is offered for slabs only.
  4. Set WASTE FACTOR with the − and + buttons. It moves in 5% steps and is added to the concrete volume before the yards and bags are figured.
  5. Read the answer. The answer bar under the title reads ≈ 1.36 yd³, then 62 bags @ 80lb · 82 bags @ 60lb, and RESULTS repeats the bag counts.

What each pour type asks for

Reading the results

Concrete results: the answer bar keeps ≈ 1.36 yd³ and 62 bags @ 80lb · 82 bags @ 60lb. Under the BASIS & WHAT WAS ASSUMED row, the section drawing of a 10' 0" slab 4" thick is marked SECTION — 10' 0" LONG, with the title strip CONCRETE — SLAB ≈ 1.36 yd³ NOT TO SCALE. MATERIALS lists Concrete — 2,500 psi; garage or exterior slab 3,000–3,500 + 5–7% air (IRC Table R402.2) 1.50 yd³, 80-lb bags (alt) 62 bags and Form boards — slab perimeter, depth 4" — 2x stock 40 lin ft, over a dimmed SHARE PDF with a list icon at its right end, a lit SAVE and the line Enter your own measurements to export. 6 7 9 11 12
The headline and bag counts, the basis row, the section drawing and the top of the material list.
  1. The headline is cubic yards, waste included. The ≈ in front of it means the two decimals shown are rounded — a pour is seldom an exact hundredth of a yard. While every input is still a default the card is dimmed and tagged sample. It clears when you change an input.
  2. Bags or truck. Up to 3 yd³ the line under the headline gives bag counts for 80 lb and 60 lb bags. Over that it reads truck territory — order ready-mix and the bag row drops out of the material list.
  3. Rebar. With REBAR on, a second line gives total feet of bar. It is bar in the pour only. The material list row says to add laps and round to stock lengths when you order. For a wall it is a single curtain of bar. For a column it is a 4-bar cage with ties at your spacing, and the row says to verify per code.
  4. BASIS & WHAT WAS ASSUMED — tap it to open. On this screen it holds the ordering advice: order in 1/4-yd steps, small loads draw short-load fees, and steps poured against fill need less than the solid figure.
  5. On a footing, BASIS also says what is not checked: the loads are gravity only, so where IRC R301.2.1.1 requires wind design, or in Seismic Design Category D0 to E (R301.2.2; townhouses from C), the added rules are not applied.
  6. The drawing is a section of the pour with your dimensions on it. Pinch to zoom, double-tap to go back, and use the expand button at its top right for full screen. See Drawings.
  7. MATERIALS lists the concrete to order: the volume rounded up to the next 1/4 yard, the step ready-mix is sold in, so it is never under what the pour needs (≈ 6.52 yd³ orders 6.75 yd³; on the defaults ≈ 1.36 yd³ orders 1.50 yd³), with the strength to order from IRC Table R402.2 (2,500 psi at least; 3,000–3,500 psi with 5–7% air for steps, garage and exterior slabs and exposed walls where weathering is moderate or severe). Then the 80-lb bag count as an alternative, rebar in lineal feet with the number of 20' bars before laps (chairs and tie wire are not counted), form boards for a slab's edge or a formed footing's sides, dowels for a wall with rebar (one per vertical bar, footing to wall), and mesh sheets when mesh is on — lapped 8" both ways, one mesh square plus 2" (ACI 439.5R), so a sheet covers about 40.4 sq ft. A footing's bars are drawn 3" off the earth, the cover concrete cast against soil needs; a column's ties are no farther apart than 16 bar diameters (8" for #4).

Only a Footing carries a code line. Under 12" wide or 6" deep it reads red, FOOTING … IS UNDER THE 12" × 6" MINIMUM (IRC R403.1.1), and the drawing is flagged the same way. At or over 12" × 6" the line is amber, not green: the width your footing needs comes from IRC Table R403.1(1)–(3) for your soil and the number of stories, which this screen does not ask. BASIS & WHAT WAS ASSUMED adds that the bottom goes at least 12" below undisturbed ground and below your frost line (IRC R403.1.4). A slab, wall, column or steps pour has no pass or fail.

The Concrete calculator's results and materials are free. The drawing, SHARE PDF and SAVE are part of Pro, a one-time purchase — without Pro the drawing pane shows a Pro card instead. SHARE PDF and the list icon at its right end stay dim until you have entered your own measurements, so a sheet of sample numbers never reaches a job. SAVE stays lit, but until then a tap only answers Enter your job's numbers first and saves nothing.

Common mistakes