Concrete Slab Calculator
Calculate the exact amount of concrete needed for any slab - in yards, bags, and cost.
Free Concrete Slab Calculator - Estimate Yards, Bags & Cost
Quick Summary
The Concrete Slab Calculator helps homeowners, DIYers, and contractors calculate exactly how much concrete is needed for any slab - in cubic yards, cubic feet, and pre-mixed bags (40lb, 60lb, 80lb). Enter slab dimensions, choose rectangle, circle, or triangle shapes, add a waste factor, and instantly get yardage, bag count, weight, rebar estimate, and cost. No sign-up required. Perfect for planning patios, driveways, garage floors, shed bases, and concrete foundations.How to Calculate Concrete for a Slab - Step by Step
A concrete slab calculator takes three numbers you can measure with a tape - length, width, and thickness - and turns them into the only number a supplier actually cares about: cubic yards. Everything else (bag counts, weight, cost) is derived from that single figure. Once you understand the short formula behind it, you can sanity-check any quote or estimate in your head.
The reason thickness matters so much is that concrete is sold by volume, not by area. A 10 x 12 ft patio sounds small, but at 4 inches thick it swallows 40 cubic feet of material. Double the thickness and you double the bill. That is why the thickness guide further down this page is worth reading before you order anything.
The formula. Convert thickness from inches to feet by dividing by 12, multiply the three dimensions together, then divide by 27 (the number of cubic feet in a cubic yard):
Length (ft) x Width (ft) x (Thickness / 12) = Cubic Feet
Cubic Feet / 27 = Cubic Yards
Worked example. A 10 ft x 12 ft shed slab, 4 inches thick:
- 10 x 12 = 120 square feet
- 4 / 12 = 0.333 ft thick
- 120 x 0.333 = 40 cubic feet
- 40 / 27 = 1.48 cubic yards
At 0.6 cubic feet per 80 lb bag, that is 40 / 0.6 = 66.7, which rounds up to 67 bags before waste. Add 5% for spillage and uneven subgrade and you are at roughly 70 bags.
If your slab is an odd shape, break it into rectangles first and add the results. For irregular footprints, measure the total area with our Square Footage Calculator, then multiply by the thickness.
Concrete Slab Shapes - Rectangular, Circular & Triangular
Most slabs are rectangles, but not all. Round patios, hot tub pads, and triangular corner pours all need a different area formula before the thickness multiplier is applied.
| Shape | Area Formula | Example |
|---|---|---|
| Rectangle / Square | Length x Width | 10 x 12 = 120 sq ft |
| Circle | π x r² | r = 6 ft = 113.1 sq ft |
| Triangle | 0.5 x Base x Height | 8 x 10 = 40 sq ft |
Use the rectangle formula for driveways, garage floors, patios, and shed bases. Switch to the circle formula for round fire pit pads, hot tub bases, and decorative landings. The triangle formula handles corner infill pours and angled walkway sections. When a pour combines shapes, calculate each one separately and sum the cubic yards at the end - never mix area units mid-calculation.
Concrete Slab Thickness Guide
Thickness is where most DIY estimates go wrong. A slab that is too thin for its load will crack within a season, and one that is thicker than necessary wastes money on material you will never see.
| Thickness | Recommended Use | Notes |
|---|---|---|
| 4 in | Patios, walkways, shed bases | Foot traffic and light storage only |
| 5 in | Residential driveways (cars) | Common in mild climates |
| 6 in | Driveways, garage floors | Standard for vehicles and RVs |
| 8-12 in | Foundations, heavy equipment pads | Engineered; local code applies |
How Many Bags of Concrete for a Slab?
Pre-mixed bags are convenient for small pours because you buy exactly what you need, with no short-load fees and no leftover concrete to dispose of. The trade-off is labor: a 70-bag pour means 70 trips with a wheelbarrow and a lot of mixing.
Every bag size yields a fixed volume once mixed. Divide your total cubic feet by that yield and round up.
| Bag Size | Yield per Bag | Bags per Cubic Yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Bag formula: Total cubic feet / yield per bag = number of bags. Always round up, and always buy one or two extra bags. An opened bag keeps for months in a dry place; a short pour does not.
Example. The 10 x 12 shed slab at 4 inches needs 40 cubic feet. With a 5% waste factor that becomes 42 cubic feet. Divided by 0.60, that is 70 bags of 80 lb mix.
Practical rule: under about 2 cubic yards, bagged mix usually wins on total cost and avoids minimum-load charges. Above 2 cubic yards, ready-mix delivery is faster, cheaper per yard, and far less physical work. A 4 x 4 ft pad at 4 inches needs only 9 bags - clearly a bag job. A 24 x 24 ft driveway at 7.1 cubic yards is a ready-mix job, no question.
Concrete Slab Cost Estimator
Material cost is the easy part. Ready-mix concrete runs roughly $120 to $150 per cubic yard delivered, depending on your region, the mix design, and fuel surcharges. Bagged mix runs about $3.50 to $5.00 per 80 lb bag, which works out to a higher cost per yard - but with no delivery minimum.
Three other factors move the final number:
- Short-load fees. Many ready-mix suppliers charge $100 to $300 extra for loads under 3 to 5 cubic yards. This is what flips the economics in favor of bags for small pours.
- Labor. If you hire out the pour, finishing, and cleanup, expect $3 to $8 per square foot on top of material - more for stamped or broom-finished work.
- Reinforcement and base. Rebar, wire mesh, gravel base, and form lumber add roughly 10% to 20% to the total project cost.
| Slab Size (4 in thick) | Cubic Yards | Ready-Mix Material | Bagged Material |
|---|---|---|---|
| 4 x 4 ft pad | 0.20 | Short-load fee applies | ~$50 (9 bags) |
| 10 x 12 ft shed | 1.48 | ~$200 + fees | ~$270 (67 bags) |
| 20 x 20 ft garage | 4.94 | ~$650 | ~$890 (222 bags) |
| 24 x 24 ft driveway | 7.11 | ~$920 | Not practical |
For pours larger than a single slab - footings, stem walls, and combined flatwork - use the broader Concrete Calculator to total everything at once, and the Concrete Footing Calculator if your slab sits on a thickened edge.
How to Estimate Concrete for a Slab - Quick Reference Table
These are the four most common residential slabs at the thickness most homeowners actually use. Numbers include a 5% waste factor.
| Slab Size | Area | Cubic Feet | Cubic Yards | 80 lb Bags |
|---|---|---|---|---|
| 4 x 4 ft patio (4 in) | 16 sq ft | 5.3 | 0.20 | 9 |
| 10 x 12 ft shed (4 in) | 120 sq ft | 40.0 | 1.48 | 67 |
| 20 x 20 ft garage (4 in) | 400 sq ft | 133.0 | 4.93 | 222 |
| 24 x 24 ft driveway (4 in) | 576 sq ft | 192.0 | 7.11 | 320 |
Rebar & Reinforcement for Concrete Slabs
Concrete is strong in compression and weak in tension. A slab on grade mostly experiences tension from shrinking, temperature swings, and uneven soil settlement - which is exactly what reinforcement resists. Thin patios and walkways can survive on welded wire mesh, but driveways, garage floors, and foundations should get rebar.
Estimating rebar. For a grid, divide the slab length by the spacing to get the number of bars running the other direction, then repeat and add 10% for overlaps and waste:
Bars (one direction) = (Slab Length / Spacing) + 1
Total Length = Bars x Slab Width, then repeat for the other axis
A 20 x 20 ft garage with #4 rebar at 24 inches on center needs 11 bars each way, 20 ft long - about 440 linear feet, or roughly 7.5 pieces of 60 ft stock rebar. Add 10% and buy 9.
Place the grid in the middle or lower third of the slab thickness. Rebar lying directly on the subgrade does nothing for cracking resistance. For engineered structures, always follow the stamped plans over any rule of thumb. The Concrete Network publishes detailed reinforcement guides, and the Portland Cement Association maintains the technical references most engineers cite.
Do not forget the base. A 4-inch compacted gravel layer under the slab improves drainage and reduces settlement. If you are ordering base material, the Gravel Calculator will size it for you.