Concrete Calculator
Estimate concrete volume and bags needed.
Results update as you type.
How it works
A concrete calculator works out the volume of a slab and translates it into the units you actually order in: cubic yards, cubic feet, and the number of 60 or 80 pound bags. You supply length, width and thickness, and the tool multiplies them into a volume, converting the thickness from inches to feet so every dimension shares the same scale before dividing by 27 to reach cubic yards. Volume is the language of concrete because ready-mix trucks are billed by the cubic yard and bags are rated by the cubic feet they yield. Underestimating leaves you scrambling mid-pour, and a cold joint where fresh concrete meets partly set concrete is a permanent weak line. Overestimating wastes money on material that sets before it can be used. A concrete calculator lets you commit to a number with confidence and add a sensible margin on purpose rather than by accident. The length-times-width-times-thickness method is standard estimation practice because it is exactly how a rectangular pour occupies space. Contractors, landscapers, DIY builders and estimators use a concrete calculator for footings, driveways, patios and slabs, then order 5 to 10 percent extra to cover spillage, uneven subgrade and the odd overfilled form.
Volume (ft³) = length × width × (thickness ÷ 12); ÷ 27 for cubic yards
Length times width gives the slab's footprint in square feet, and multiplying by thickness turns that flat area into a three-dimensional volume. Thickness is divided by 12 first because it is entered in inches while length and width are in feet, so all three must share one unit. Dividing the resulting cubic feet by 27 converts to cubic yards, since one cubic yard contains 27 cubic feet.
Source:Standard construction estimation (volume method)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter length, width and thickness.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Concrete needed updates instantly as you type — no button to press.
Frequently asked questions
How much extra concrete should I order?
Add 5 to 10 percent to the calculated volume for most jobs. The extra absorbs spillage, uneven or over-dug subgrade, forms that bow slightly under the weight, and the material left clinging to tools and the chute. For a large or complex pour, or one you cannot easily top up later, lean toward the higher end. Running short mid-pour is far more costly than a small surplus, since a cold joint permanently weakens the slab.
Should I use bags or ready-mix concrete?
Bags suit small jobs — a few post holes, a compact pad or repairs — where mixing by hand or in a small mixer is manageable. Once you pass roughly one cubic yard, ready-mix delivery is usually cheaper per yard, far faster, and gives a consistent mix across the whole pour. Above a cubic yard, hand-mixing dozens of bags also risks the first section setting before the last is placed, creating weak joints.
How many bags of concrete make a cubic yard?
It depends on bag size, because larger bags yield more finished volume. A 60 pound bag produces about 0.45 cubic feet, so roughly 60 of them make one cubic yard. An 80 pound bag yields about 0.60 cubic feet, so about 45 make a cubic yard. That difference is why buying larger bags means handling fewer of them, though each one is heavier to lift and pour.
How thick should a concrete slab be?
Four inches is the standard minimum for patios, walkways and residential floors that carry only foot traffic. Step up to five or six inches for driveways and areas holding vehicles, and thicker still with steel reinforcement for heavy loads. Thickness drives volume directly in the formula above, so every extra inch noticeably increases the concrete you need. Always check local building codes, which set required minimums for your specific application.
Does this calculator include a margin for waste?
No. The result is the exact geometric volume of the slab you describe, with no allowance added, so the figure stays transparent and easy to check. Add your own waste margin on top — typically 5 to 10 percent — before placing the order. Keeping the raw volume separate lets you scale the margin to the job: a tidy indoor pad needs less cushion than a large outdoor slab poured over rough, uneven ground.