Concrete Calculator - Slabs, Footings, Columns & Steps

How to use this tool

1. Select the shape of your concrete pour: slab, footing, round column, or steps.
2. Enter the dimensions in feet/inches (imperial) or meters/centimeters (metric).
3. Enter the quantity if you are pouring multiple identical units.
4. Compare clean volume with waste-adjusted order volume and whole-bag counts; optional material prices show cost estimates.

How to calculate concrete volume

For a rectangular slab or footing, multiply length × width × thickness (converted to the same unit). For round columns, use V = π r² h. The result in cubic feet can be converted to cubic yards (divide by 27) for ordering ready-mix.

The calculator keeps the measured clean volume separate from the order volume. Set a waste factor from 0% to 25% (10% by default); order volume is clean volume × (1 + waste percentage ÷ 100). The per-unit result stays clean, while the selected waste factor is applied to the order volume and bag counts.

Concrete for solid rectangular steps

The steps option treats the staircase as solid rectangular blocks, not as a single sloped wedge. For N identical rises and runs, the stacked cross-section contains 1 + 2 + ... + N rectangles. Its volume is rise × run × width × N(N + 1) ÷ 2, after converting all dimensions to the same unit. This is appropriate for solid poured steps; enter a separate design estimate for a sloped fill, hollow treads, or a base slab.

How many bags do I need?

A standard 80-lb bag of pre-mixed concrete yields approximately 0.60 cubic feet when mixed. A 60-lb bag yields 0.45 ft³ and a 40-lb bag yields 0.30 ft³. The calculator divides the waste-adjusted volume by the selected bag's approximate yield and rounds up to whole bags. Choose a waste percentage that fits the project; the default is 10% and the control allows 0–25%.

When to use ready-mix vs. bagged concrete

Ready-mix delivered by truck is cost-effective for pours larger than about 1 cubic yard. For smaller projects (footings, fence posts, or repair work), bagged concrete is more practical. Use the waste-adjusted order volume when requesting a ready-mix quote, and confirm order amounts and minimums with the supplier.

Optional material cost estimates

Enter a local ready-mix price per cubic yard in imperial mode or per cubic meter in metric mode to multiply that rate by the waste-adjusted order volume. For bagged concrete, choose a 40-, 60-, or 80-lb bag size and enter its price; the estimate multiplies that price by the whole-bag count. Leave either price blank to hide that estimate. These figures cover concrete material only and exclude delivery, short-load fees, labor, and forms. Prices vary by supplier and location, so use a current local quote.

Slab thickness guide

Reinforcement

Unreinforced concrete is strong in compression but brittle in tension. For most applications:

• Wire mesh (6×6 W1.4×W1.4): standard for residential slabs and walkways; controls cracking from shrinkage.
• Rebar: required for structural slabs, driveways subject to heavy loads, and any slab with significant tensile demands. Typically #3 (3/8”) or #4 (1/2”) rebar on a 12”×18” grid.
• Fiber reinforcement: polypropylene or steel fibers mixed into the concrete control plastic shrinkage cracking during curing. Often used instead of wire mesh for residential work.

Mixing ratios

Standard concrete mixing ratios by volume (cement : sand : coarse aggregate):

• 1:2:4 - general-purpose concrete, moderate strength (~20 MPa / 3,000 psi)
• 1:1.5:3 - stronger mix (~25 MPa / 3,600 psi), used for reinforced slabs and beams
• 1:2:3 - standard residential slab mix (roughly M20 grade)

For most DIY projects, pre-mixed bags specify the ratio on the packaging. Achieving consistency by weight (not volume) produces more reliable results.

Curing time

Keep concrete moist during the first 7 days. Drying too fast causes surface cracking and reduces strength.

Common failure modes

• Plastic shrinkage cracking: occurs in the first few hours as water evaporates from the surface faster than it bleeds from below. Prevent with evaporation retarder, shade, or wind breaks.
• Crazing: a network of fine surface cracks caused by premature surface drying or over-troweling. Cosmetic but affects appearance.
• Spalling and heaving from freeze-thaw: water that seeps into cured concrete expands when it freezes, breaking off the surface layer. Use air-entrained concrete (with chemical admixture) for freeze-thaw climates and apply a penetrating sealer.