Concrete slab calculator

How to calculate concrete for a concrete slab

A flat rectangular pour, patios, driveways, shed pads, garage floors and sidewalks all use this shape. The volume formula is Length × Width × Thickness, worked in feet and then converted. There are 27 cubic feet in a cubic yard, which is the unit every ready-mix plant quotes in. The default project on this page comes to 146.7 cubic feet, or 5.43 cubic yards.

The conversion that catches people is thickness. Slabs and footings are specified in inches and ordered in cubic yards, so a 4-inch slab is 4 ÷ 12 = 0.333 feet deep, not 4, an error that overstates the order twelve-fold. Working the other way, one cubic yard covers 81 square feet at 4 inches thick, 65 at 5 inches and 54 at 6 inches.

On top of the geometric volume the calculator adds a 10% waste allowance. That is not padding: subgrade is never perfectly level, forms bow outward under the weight of wet concrete, and some is always lost to the wheelbarrow. Ordering the exact calculated volume reliably leaves you short, and being short is the expensive direction. A partial second load carries the $75 short-load fee and leaves a cold joint where it meets concrete that has already begun to set.

How many bags of concrete for a concrete slab

Bag yields are the manufacturers' published figures: a 40 lb bag of QUIKRETE or Sakrete concrete mix yields 0.3 cubic feet, a 60 lb bag 0.45, and an 80 lb bag 0.6. That is 90, 60 or 45 bags to a cubic yard. For this project the concrete calculator returns 245 eighty-pound bags, 326 sixty-pound or 489 forty-pound.

All three counts are shown at once because the decision at the store is rarely about price. Larger bags are marginally cheaper per cubic foot and considerably heavier to move, 245 eighty-pound bags is 9.8 tons to load, carry and cut open, and 60 lb bags are what many people can actually handle repeatedly.

At 5.43 cubic yards this project is past the point where the bag count is a recommendation rather than a reference. Mixing that many bags to a consistent strength is a full day of work before any finishing happens.

Bags or ready-mix for a concrete slab?

Ready-mix is cheaper per yard almost every time: $155 delivered against $293 a cubic yard bought in 80 lb bags. What bags buy is scheduling, no delivery window, no minimum load, and the freedom to stop and start. That is worth real money below about 2 cubic yards and worth nothing above it.

For this project specifically, bags come to $1,593 and ready-mix about $842. Placing and finishing labor adds roughly $1,400 at national-average rates if you hire it out, which is the line that disappears on a DIY pour.

A truck also arrives uniformly mixed, which matters more than it sounds. Concrete mixed in batches at slightly different water ratios cures to different strengths, and a pour that stretches over several hours develops cold joints where each batch meets the last.

Base, forms and reinforcement under a concrete slab

A 4-inch compacted gravel base belongs under any slab you intend to keep. It gives uniform bearing so the slab does not crack over soft spots, breaks the capillary path that wicks groundwater up into the concrete, and drains the freeze-thaw cycle. On 400 square feet that is 4.94 cubic yards of crushed stone, or $200 to $600 installed. Order 10-15% over the calculated volume, because crushed stone loses that much bulk under a plate compactor.

Reinforcement does not stop concrete cracking. It holds the crack together once it happens. Welded wire mesh or fiber is enough for a patio or shed pad; rebar on 18-inch centres belongs in a driveway or anything carrying vehicle loads. Control joints matter as much: cut or tooled every 8 to 10 feet in each direction, they decide where the inevitable shrinkage crack goes instead of leaving it to chance.

Forms are the cheap part and the one that determines whether the finished slab looks square. Stake 2x lumber to the line, check the diagonals against each other before you pour, and set the forms with a slight fall, about an eighth of an inch per foot, so water runs off the finished surface rather than sitting on it.

How we calculated these estimates

Every figure on this page comes from the same open formula the calculator above uses. Volume is pure geometry, the shape's formula worked in feet, plus a 10% waste allowance. Prices start from 2026 national baselines: $155 per cubic yard delivered, a $75 short-load fee under 5 yards, retail bag prices against the yields QUIKRETE and Sakrete publish, and $3.50 per square foot for placing and finishing flatwork. Ready-mix and labor are then scaled by the construction earnings multiplier the Bureau of Labor Statistics publishes for your state; bagged concrete is not, because it is priced nationally. Nothing is hidden behind a lead form and no supplier pays to appear here. What the result is not is a quote: excavation, base preparation, forms, reinforcement, permits and demolition are excluded, and a plant will price your delivery distance and mix specification on top.