Concrete Calculator
Calculate everything a slab needs. Enter the dimensions for concrete volume and bag counts, then add a ready-mix price, rebar spacing, and gravel base depth for a complete materials estimate. Need a different shape? Use the Shape selector for footings, walls, columns, curb & gutter, or stairs.
Example: with Shape Slab · Length 10 ft · Width 10 ft · Thickness 4 in → Concrete (cubic yards): 1.36 cu yd.
- 80 lb bags62
- 60 lb bags82
- Ready-mix cost$217
Computed by the calculator below using its default values. Change any input to see your own numbers.
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Beyond concrete volume, most slabs need a compacted gravel base and a rebar or wire grid. This estimates rebar as a grid at your chosen spacing (total linear feet to buy) and the gravel base as its own volume. Past about a cubic yard, ready-mix delivery usually beats bagging it by hand — enter a price per yard to compare. The waste allowance covers uneven subgrade and spillage.
Beyond a flat slab, this calculator also covers five other common pours: footings (a continuous strip under a wall or foundation), walls (a vertical concrete wall of a given thickness), columns or round footings (a cylinder, using the diameter and height you enter), curb & gutter (a linear run at a fixed cross-section), and stairs (a set of steps, approximated as a solid wedge). Switching shapes swaps the input fields but keeps the same waste allowance, bag-count math, and cost estimate below.
How it’s calculated
Slab (default): volume = length × width × thickness ÷ 27 cubic yards, plus waste. Bags = cubic feet ÷ yield (0.6 cu ft for an 80 lb bag, 0.45 cu ft for a 60 lb bag — standard Quikrete-style figures). Rebar is a grid at your spacing; the gravel base is its own volume.
Footing and wall use the same length × width × thickness volume formula as a slab, just with labels matching how those shapes are usually measured (a footing's "width" and "depth" being its cross-section, a wall's "thickness" being how thick the poured wall is).
Column / round footing: volume = π × (diameter ÷ 2)² × height × quantity, the standard cylinder-volume formula, converted to cubic yards.
Curb & gutter: volume = (gutter width + curb depth) × curb-and-gutter depth × length, approximating the curb-and-gutter cross-section as a simple rectangle at your entered width and depth, then multiplied along the run length. Real curb-and-gutter profiles are shaped (not a plain rectangle), so treat this as a reasonable estimate and confirm against your project's actual cross-section drawing for bid-critical orders.
Stairs: volume is approximated as a solid triangular wedge — 0.5 × total rise × total run × stair width, where total rise = steps × rise-per-step and total run = steps × run-per-step. This ignores any hollow space under the stairs, which is standard practice since concrete stairs are usually poured solid or backfilled.
All shapes then apply the same waste allowance (final volume = raw volume × (1 + waste% ÷ 100)), the same 60 lb / 80 lb bag-yield conversion, and the same ready-mix cost = cubic yards × price per cubic yard.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Common mistakes
- Mixing up inches and feet for thickness.
- Ordering bags for a job where ready-mix is cheaper.
Frequently asked questions
How accurate is the curb and gutter shape?
It models the cross-section as a simple rectangle multiplied by curb length. Real profiles are shaped rather than rectangular, so confirm against your project's actual cross-section for bid-critical orders.
How is rebar length estimated?
As a grid at your spacing: bars run both directions across the slab, and the calculator totals their linear feet. Add overlap for splices on big pours.
Bags or ready-mix?
An 80 lb bag yields ~0.6 cu ft. Above roughly one cubic yard, ready-mix is usually cheaper and far less work.
Why a gravel base?
A compacted gravel base improves drainage and load support, reducing cracking. Four inches is common under slabs.
What shapes can this calculator handle?
Slab, footing, wall, column or round footing, curb & gutter, and stairs. Pick one from the Shape selector — the input fields change to match, but the waste allowance, bag counts, and cost estimate work the same way across every shape.
How is stair volume estimated?
As a solid triangular wedge: 0.5 × total rise × total run × stair width. This is a standard simplification for concrete stairs, which are usually poured solid, and ignores any hollow space that might exist underneath.
How accurate is the curb & gutter shape?
It models the cross-section as a simple rectangle (gutter width plus curb depth, times your entered depth), multiplied by the curb length. Real curb-and-gutter profiles are shaped rather than rectangular, so for a bid-critical order, confirm against your project's actual cross-section.