CalcaTools

Concrete Calculator

A Concrete Calculator is a free online tool that computes how much concrete do I need using how much concrete do I need, cubic yards, concrete bags. Tradespeople, planners, and curious owners use it to sanity-check estimates, plan projects, and answer everyday questions against published guidance from Portland Cement Association.

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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Everyday calculators are the workshop drawer of the site — quick utilities for the moments when you need a number now and a phone calculator won't cut it. Age, date difference, GPA, fuel cost, time-to-decimal, password strength — pick a tool and you're one click from your answer.

speed Loads in under a second

Every tool is plain HTML + JavaScript with the calculation logic compiled in. No spinner, no "verifying browser" splash, no five-second wait while ads load. Hit the page, fill the form, get the answer. Slow tools waste your time and ours.

devices Works on every device

Responsive design tested down to a 320px-wide phone screen. The input fields use native number/date keyboards on iOS and Android, the calculate button is thumb-sized, and the result still reads big and bold on the smallest screen. No app to download, ever.

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No newsletter pop-ups, no "create an account to see the answer", no fake countdown, no fake stock counter, no upsell. Two ad slots per page (clearly labelled as Advertisement) keep the site free — that's the whole business model.

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

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 requested values, review the instant result, then compare the reference table and interpretation guide below the calculator. Adjust one input at a time to see which assumption changes the result most.
What formula does the concrete calculator use?
The calculator uses Cubic yards = length(ft) × width(ft) × depth(ft) ÷ 27 and standard unit conversions or finance/statistics conventions where applicable.
Is the concrete calculator accurate?
Results are estimates based on the values entered; check source data and assumptions before making decisions.
Why do my results differ from another calculator?
Different calculators may round differently, include extra assumptions, or use a different input frequency. Match the units, dates, tax assumptions, and rounding method before comparing outputs.
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.

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