Concrete Calculator

What this concrete calculator does

This calculator works out the volume of concrete you need for a slab, footing, column, or wall, and tells you how many pre-mixed bags of concrete that translates to. It outputs the answer in cubic yards (the unit ready-mix companies sell in), cubic feet (the underlying geometry unit), and cubic metres (for international projects). It also breaks the volume down into 40 lb, 60 lb, and 80 lb bag counts so you can compare what to buy at the home centre versus what to order from a ready-mix truck. A waste-factor slider applies the safety margin contractors actually use when ordering — the calculator does not let you forget that real pours need a cushion.

How to use the concrete calculator

1. Pick a project shape — Slab (driveways, patios, basement floors), Footing (the trench under a wall or column), Column (round piers, deck posts, mailbox posts), or Wall (poured-in- place foundation walls).
2. Enter the dimensions. Length and width are in feet; thickness or depth is in inches because that’s how most US specs are quoted. Columns use diameter in inches and height in feet.
3. Adjust the waste factor. 10 % is the default; nudge to 5 % for tightly-formed columns or up to 15 % for sloppy ground or thick foundations.
4. Read the result. The big number is cubic yards — that’s what you tell a ready-mix dispatcher. The bag-count breakdown helps decide if it’s worth a truck delivery or a Home Depot run.
5. Tap Copy summary to put the headline numbers on your clipboard, ready to paste into a text to your concrete supplier.

How concrete is measured

Concrete is sold by volume, not weight. The standard US unit is the cubic yard — a cube three feet on a side, equal to 27 cubic feet. A short-haul ready-mix truck typically carries up to 10 cubic yards; a full mixer can carry 11–12 yd³. Below 1 cubic yard, most companies charge a short-load fee of $50–$200 per yd³ short of their minimum, so very small jobs are usually cheaper done with bagged concrete from a home centre. Above about 1 yd³, ready-mix becomes the better economic option — and you get a much higher-strength, better-batched mix.

The cubic foot is the underlying unit the geometry happens in. Multiply length × width × depth (all in feet) and you get cubic feet directly. Internationally, cubic metres are the norm; one cubic metre is about 1.31 cubic yards.

Concrete coverage by bag size

The yields above come from Sakrete and Quikrete spec sheets and apply to the standard mix lines (Sakrete Concrete Mix, Quikrete Concrete Mix). Specialty mixes — high-early-strength, fibre-reinforced, crack-resistant — yield slightly less because the additives displace some of the cement. For jobs over about 1 cubic yard, ordering ready- mix from a truck is almost always cheaper per cubic yard than buying bags, and the mix quality is more consistent.

Common shapes and how to estimate them

Slabs are the most common pour and the simplest geometry: length × width × thickness. A typical 20 × 10 ft patio at 4 inches thick needs 2.5 cubic yards. A 24 × 24 ft garage floor at 4 inches needs 7.1 yd³. Always verify the slab thickness against your local building code — patios can sometimes be 3 inches but slabs that bear vehicle loads must be 4 inches minimum, and many jurisdictions require 5–6 inches plus reinforcement for driveways.

Footings are the underground trenches that support walls and columns. They follow the same length × width × depth math as slabs. For a typical residential frost-protected footing in the US northeast, allow at least 8 inches of depth below the frost line — often 36–48 inches in total depth depending on the climate zone. The calculator treats footings as rectangular trenches; for stepped or sloped footings, sum each segment individually.

Columns (round piers, deck-post footings, mailbox bases) use the cylinder formula: π × radius² × height. The most common residential sizes are 8”, 10”, 12”, and 16” diameter sonotubes 3–6 ft deep. Add a small belled bottom in heavy-load applications.

Walls are length × height × thickness. A 30 ft long, 8 ft tall, 8” thick foundation wall needs 5.9 yd³ — a full truck plus a bit. ICF (insulated concrete form) walls follow the same math but you’ll want to check the manufacturer’s spec for the actual cavity dimensions.

Why a waste factor matters

The geometric volume is the minimum you’d need under ideal conditions. Real conditions never quite match the spec:

• Subgrade variation. A 4” target slab can become 4.5” in spots where the compaction dipped.
• Form spillage and over-pours. Workers pour to the form edge plus a few inches; some lands outside.
• Truck dump and wheelbarrow loss. Ready-mix dumped from a chute loses a measurable amount to splash and base build-up; bag mixes lose volume to mixing-tub residue.
• Pre-set vibration consolidation. Properly vibrated concrete occupies slightly less volume than the loose dump that was placed.

10 % is the industry default for residential slabs and footings. Bump it to 12–15 % for thick foundations, irregular shapes, or terrain where the sub-base isn’t perfectly level. Drop it to 5 % only for clean column pours into sonotubes. Under-ordering forces a short-load truck delivery at premium rates; over-ordering by a tiny amount is forgivable because the leftover can be tipped into a fence-post hole or used to weight down forms.

Privacy

This calculator does its arithmetic in JavaScript on your device. There is no fetch call, no analytics on the values you enter, no server-side logging. The page works the same way offline once loaded.