Estimate how much concrete you need for a slab, footing, or column, in cubic yards, cubic meters, and bags.
For a slab or rectangular pad, volume is length × width × thickness. For a round column or footing, volume is π × (diameter ÷ 2)² × height. This calculator converts your inputs into cubic feet first, then reports the total in cubic yards and cubic meters, since ready-mix concrete is usually ordered by the cubic yard (or cubic meter outside the US).
For small jobs — fence posts, a shed pad, a few stepping stones — pre-mixed bags from a hardware store are usually more practical than ordering a truck. This calculator estimates bag counts assuming a 60 lb bag yields about 0.45 cubic feet of cured concrete and an 80 lb bag yields about 0.6 cubic feet, which are standard manufacturer figures.
Concrete calculations should always be padded, not rounded down. Uneven ground, formwork gaps, and spillage typically use 5–10% more material than the "on paper" volume, and running short mid-pour creates a cold joint that weakens the slab. Most contractors order at least 10% extra for exactly this reason.
| Project | Typical thickness |
|---|---|
| Shed / patio slab | 4 in |
| Driveway | 5–6 in |
| Sidewalk | 4 in |
| Fence post footing | Full post depth |
This tool gives a planning estimate only — always confirm quantities with your supplier or contractor before ordering.
Choose the shape you're pouring — a rectangular slab, or a round column or footing. For a slab, enter the length, width, and thickness. For a round shape, enter the diameter and height. Click Calculate, and the tool returns the total volume needed in cubic yards and cubic meters, along with an estimated bag count if you're planning to use pre-mixed bags instead of ordering ready-mix concrete by truck.
Ready-mix concrete trucks measure and deliver concrete by volume, specifically cubic yards in the US, because that's the practical unit for large-batch mixing and delivery. Most residential ready-mix orders have a minimum order size — often around 1 cubic yard — since dispatching a truck for a very small amount isn't cost-effective for suppliers. This is exactly why small projects (fence posts, stepping stones, a small patch) typically make more sense with bagged concrete rather than ordering a truck, while larger projects (driveways, foundations, large patios) become more economical with ready-mix delivery once the volume clears that minimum order threshold.
A rectangular slab's volume is simply length times width times thickness — straightforward multiplication once all three measurements are in the same unit. A cylindrical shape, like a round fence post footing or column, uses the formula π × radius² × height, where radius is half the diameter. It's worth double-checking that thickness or height is measured in the same unit as the other dimensions before calculating — a common mistake is entering length and width in feet but thickness in inches, which throws off the result by a large margin unless properly converted first.
Experienced contractors consistently order more concrete than the calculated volume suggests, typically adding 5-10% extra, for several practical reasons. Ground surfaces are rarely perfectly level or perfectly smooth, which means actual poured volume often exceeds a theoretical flat-surface calculation. Formwork can have small gaps or minor settling that consumes extra material. Spillage and waste during mixing, pouring, and finishing is essentially unavoidable on any real job site. Most importantly, running short mid-pour creates a "cold joint" — a weak seam where an interrupted pour meets a later addition — which can meaningfully compromise the finished slab's structural integrity, making it far better to over-order slightly than to risk running out.
Pre-mixed bags are labeled by dry weight (commonly 60 lb or 80 lb bags), but what actually matters for planning is the cured, mixed volume each bag produces — approximately 0.45 cubic feet for a 60 lb bag and about 0.6 cubic feet for an 80 lb bag, based on standard manufacturer specifications. For small jobs, this makes bagged concrete convenient since you can precisely calculate bag count needed rather than dealing with a large minimum truck order. For anything beyond a modest number of bags, however, the labor of manually mixing and pouring multiple batches becomes a real consideration — many contractors switch to ready-mix once a project exceeds roughly 1 cubic yard, purely due to the physical effort involved in bag mixing at that scale.
How thick should a concrete slab be for a shed or patio? 4 inches is a common standard for light-duty slabs like sheds and patios, though local building codes and the specific load the slab needs to support should always be the final reference.
Do I need rebar or wire mesh reinforcement? For most standard slabs, especially anything supporting vehicle weight (like a driveway), reinforcement is typically recommended or required by code — check local building requirements for your specific project.
How long does concrete need to cure before it can be used? Concrete typically reaches enough strength to walk on within 24-48 hours, but full curing strength usually takes around 28 days, which is why heavier use (like driving on a new driveway) is often delayed until curing is more complete.
Should I round up to the nearest whole cubic yard when ordering ready-mix? Yes — combined with the standard extra 5-10% buffer, rounding up to the nearest whole or half cubic yard is standard practice, since most suppliers price and deliver in those increments anyway.
Does temperature affect how concrete cures? Yes — very cold or very hot weather can significantly affect curing time and final strength, which is why many contractors avoid pouring concrete in extreme temperatures without special precautions.
What's the difference between concrete and cement? Cement is one ingredient within concrete — concrete is the finished mixture of cement, water, sand, and aggregate (gravel or crushed stone) that hardens into the solid material used for slabs and structures.
Last reviewed by Mehmed on July 11, 2026.