CalcaTools

Concrete Calculator | Estimate Volume and Number of Bags Needed

Estimates how much concrete a project needs. Enter the slab, footing, or column dimensions — length, width, and depth — and the tool computes the volume in cubic yards and cubic metres, then converts it to the number of 40 lb, 60 lb, or 80 lb bags, with an allowance for waste.

Last updated: July 2026 · Free · No sign-up required

Results

Enter values above and click Calculate to see your result instantly.

Quick reference

Bag size (Quikrete/Sakrete)Yield per bagBags per yd³
40 lb0.30 cu ft90
60 lb0.45 cu ft60
80 lb0.60 cu ft45
90 lb0.68 cu ft40

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Interpretation guide

Planning factorRecommendation
Waste allowanceAdd 5–10% for spills/uneven base
DepthConvert inches to feet before volume
Ready-mixOrder cubic yards, not bags

lightbulb Concrete for a 10 ft × 10 ft patio slab

You are pouring a 10 ft × 10 ft patio slab, 4 inches thick. How much concrete is that?

Result: 1.23 cubic yards (about 84 bags of 60 lb). Volume = 10 × 10 × (4/12) = 33.3 cubic feet; ÷ 27 = 1.23 cubic yards.

What this means: Add 5–10% for spillage and uneven sub-base. Above ~1 cubic yard it is usually cheaper to order ready-mix than to buy bags.

Formula & methodology

Formula: Cubic yards = length(ft) × width(ft) × depth(ft) ÷ 27

The Concrete Calculator is built on a well-established calculation method. It uses the formula Cubic yards = length(ft) × width(ft) × depth(ft) ÷ 27 to turn your inputs into a reliable result. Estimates how much concrete a project needs. Enter the slab, footing, or column dimensions — length, width, and depth — and the tool computes the volume in cubic yards and cubic metres, then converts it to the number of The steps are shown on the page so you can follow the reasoning from input to output.

Concrete volume is slab area multiplied by depth. Convert all dimensions to feet, divide cubic feet by 27 for cubic yards, then add a small waste allowance.

Authoritative source: Portland Cement Association

Frequently asked questions

How do I use the concrete calculator?
Enter the dimensions of the slab, footing, or column — length, width, and depth — choosing the unit that matches your tape measure. The tool converts everything to cubic yards with Cubic yards = length(ft) × width(ft) × depth(ft) ÷ 27 and suggests the number of ready-mix bags needed. A 10 ft × 10 ft slab 4 inches thick needs about 1.23 cubic yards of concrete.
What formula does the concrete calculator use?
The Concrete Calculator uses the standard Cubic yards = length(ft) × width(ft) × depth(ft) ÷ 27. You enter your values, and the calculator applies the formula step by step, showing the working so you can verify the math and understand how the result is derived rather than trusting a black box.
Is the concrete calculator accurate?
The Concrete Calculator applies the standard formula: Cubic yards = length(ft) × width(ft) × depth(ft) ÷ 27. It is tested against published worked examples before launch and uses native double-precision arithmetic, so results are accurate to typical precision for the values you enter — with no intermediate rounding that could distort the answer.
Why do my results differ from another calculator?
The Concrete Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Estimates how much concrete a project needs. Enter the slab, footing, or column dimensions — length, width, and depth — and the tool computes the volume in cubic yards and cubic metres, then converts it to the number of 40 lb, 60 lb, or 80 lb bags, with an There is no limit on usage, and it works on any device.
How many 80 lb bags of Quikrete or Sakrete do I need?
An 80 lb bag of any standard concrete mix — Quikrete, Sakrete, or a store brand — yields 0.60 cubic feet, so a full cubic yard takes 45 bags. For a 4-inch slab, figure one 80 lb bag per 1.8 square feet. Above roughly 25–30 bags, ready-mix delivery usually beats bagged concrete on both price and your back.
When should I order ready-mix instead of bags?
The usual cutoff is one cubic yard (about 45 80-lb bags). Ready-mix trucks charge a short-load fee under 3–4 yards, but bagged concrete needs mixing water, a mixer rental, and continuous pours are hard to keep wet-on-wet beyond a yard. For slabs bigger than a small shed pad, get a ready-mix quote first.
How much concrete does a slab need?
Multiply length × width × thickness. A 10 × 10 ft patio at 4 inches is 10 × 10 × 0.333 = 33.3 cubic feet, which is 1.23 cubic yards — order 1.5 yards to cover grading imperfections. Standard slab thickness is 4 inches for patios and walkways, 5–6 inches for driveways that carry vehicles.
What is the right sand-to-cement ratio for mortar or a mix?
Classic site-mix concrete is 1 part Portland cement, 2 parts sand, 3 parts gravel. Bricklaying mortar drops the gravel and uses 1 part cement to 3 parts sand. A thinner cement-water slurry (no aggregate) is only for filling voids and bonding layers — it shrinks too much to be structural on its own.
What does the Concrete Calculator do?
Estimates how much concrete a project needs. Enter the slab, footing, or column dimensions — length, width, and depth — and the tool computes the volume in cubic yards and cubic metres, then converts it to the number of 40 lb, 60 lb, or 80 lb bags, with an. The calculator takes your inputs, applies the standard calculation, and returns a clear result so you can make an informed decision without doing the math by hand.
Is the Concrete Calculator free and private?
Yes — the Concrete Calculator. There is no sign-up, no paywall, no trial, and no limit on how many calculations you can run. CalcaTools covers its costs with non-intrusive display advertising, so the calculator itself never asks for payment or restricts any feature.
How accurate is the Concrete Calculator?
The volume is exact for the dimensions entered: cubic yards = length × width × depth ÷ 27 is a precise conversion. Practical accuracy depends on your measurements and site conditions — forms can bow, slabs can be thicker in places, and the ground may absorb water. Ordering 5–10% extra is the standard hedge against these real-world variations.

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