Slabs and flatwork are the flat, poured surfaces of a property, such as driveways, patios, sidewalks, garage floors, shed bases and pool decks. GeniesMag groups the calculators for these jobs here, so you can size the concrete, plan the joints and reinforcement, work out the base underneath, and price or finish the surface in one place.
Flatwork is thin and wide, so a small change in thickness changes the order by cubic yards. Pick the tool that matches your slab, then check the joints, mesh, base and finish below.
Slab and Pad Calculators
Start with the shape and the use. Thickened edges, ramps and pool cut-outs each change the geometry, so these tools match the cubic yards you order to the slab you pour.
Joints, Reinforcement and Load
Joints decide where a drying slab cracks, reinforcement holds cracks tight, and a load check shows whether the slab suits what will sit on it.
Base and Paving
A compacted base gives even support, and pavers are the main alternative to a poured surface.
Surface Treatments
Once the slab is poured, the surface decides how it looks and how long it lasts. These tools cover protecting, decorating and repairing the top of a slab.
How Flatwork Is Estimated and Built
The general volume formula is covered on the main concrete calculators page. For flatwork, the useful question is how each layer of the slab changes the estimate, because the concrete is only one of several quantities you will buy.
Choose the tool by the slab you are pouring
Use the concrete slab calculator for any plain rectangle. Switch to a specific tool when the shape or the edge changes the volume: the garage floor and monolithic slab tools add a thickened perimeter, the pool deck tool removes the pool, and the ramp tool treats the pour as a wedge. Using the plain slab tool for those jobs under-orders, because it ignores the extra concrete at the edges.
Thickness by use
Four inches is commonly used for patios and walkways, and 5 to 6 inches for driveways and garage floors. That is common practice, not a specification. Your plans, local code or an engineer decide the thickness of any slab that carries real load.
Base, joints and reinforcement
A compacted gravel or sand base keeps the slab evenly supported, and the gravel base calculator turns its area and depth into tons. For joints, a common rule is spacing in feet of 2 to 3 times the slab thickness in inches, so a 4 inch slab gets joints every 8 to 12 feet. Saw-cut depth is about one quarter of the thickness, and a common guideline keeps panels roughly square and under about 225 square feet. Welded wire mesh is ordered by area with an allowance for laps, and the wire mesh calculator counts the sheets.
A worked example: a 20 by 20 foot, 5 inch slab
Convert the thickness first: 5 divided by 12 is 0.4167 feet. The volume is 20 x 20 x 0.4167, which is 166.67 cubic feet, or 6.17 cubic yards. Adding a 10 percent waste allowance gives about 6.79 cubic yards to order. In 80 pound bags at 0.60 cubic feet each, 166.67 divided by 0.60 is 278 bags before waste, which is why a slab this size is poured as ready-mix.
The same slab has 400 square feet of surface. At 5 inches, joints can be 10 to 15 feet apart, and 10 foot panels are 100 square feet each, well under the 225 square foot guide. The saw cut is about 1.25 inches deep. Mesh at 400 square feet plus 10 percent is 440 square feet, and with 5 by 10 foot sheets covering 50 square feet each, that is 8.8, so you buy 9 sheets.
The base is the third quantity. A 4 inch compacted base under 400 square feet is 400 x 4 / 12 = 133.33 cubic feet, or 4.94 cubic yards. Granite or limestone base compacts to about 3,060 to 3,300 pounds per cubic yard, so that is roughly 15,100 to 16,300 pounds, or about 7.6 to 8.1 tons before waste. The 4 inch depth here is an example only; the base depth comes from your plans, engineer or supplier.
Surface finish and pavers versus a poured slab
A poured slab is one continuous surface, and it can be sealed, stamped or resurfaced later. Pavers are a different build: a compacted base, a nominal 1 inch bedding sand layer (about 2 inches for permeable pavements), the pavers, and edge restraint. A 12 by 10 foot patio of 4 by 8 inch pavers needs 120 / 0.222 = 540 pavers, or 594 with 10 percent waste, and 10 cubic feet of bedding sand. A settled paver can be lifted and reset, which a cracked slab cannot.
Common Mistakes
Skipping the base and joints in the estimate
The gravel base, mesh and saw cuts are real costs and real work, so add them early.
Using one tool for every shape
A plain slab tool ignores thickened edges, pool cut-outs and ramp rise. Choose the tool that matches the shape, and order from its result.
Spacing joints too far apart
Joints that are too far apart let the slab pick its own crack lines. Use the 2 to 3 times thickness rule and cut early, then check the layout with the control joint spacing calculator.
Treating a load result as a safe load
A slab load calculation is a learning estimate for one interior load, not a safe working load. Heavy or repeated loads need a licensed engineer.
Frequently Asked Questions
Which calculator should I use for a driveway?
Use the concrete slab calculator for the volume, then the driveway cost calculator to add the gravel base, labor and reinforcement. Driveways are commonly poured 5 to 6 inches thick, but your plans or code decide.
How far apart should control joints be?
A common rule is 2 to 3 times the slab thickness in inches, expressed in feet. A 4 inch slab gets joints every 8 to 12 feet. The saw cut is about a quarter of the thickness deep.
Do I need wire mesh in a slab?
Mesh is common in flatwork, but whether your slab needs it comes from your plans, local code or an engineer. If it does, the wire mesh calculator counts sheets with an allowance for laps and waste.
How much gravel goes under a slab?
The depth comes from your plans or supplier. Once you know it, the gravel base calculator converts area and depth to tons, compacted yards and loose yards to haul. A 400 square foot base 4 inches deep is about 4.94 cubic yards compacted.
Can a slab hold a hot tub?
It depends on the filled weight and the footprint. A filled six-person spa can weigh 4,500 to 6,000 pounds, which loads a pad at roughly 80 to 130 pounds per square foot depending on its footprint. The hot tub pad calculator checks the load, but have an engineer confirm an existing slab or soft ground.
Are pavers cheaper than a poured slab?
There is no single answer, since it depends on region, labor and the paver you choose. Price both for your own area before deciding.
How much sealer do I need?
It depends on the coverage rate printed on the product label, the number of coats and some waste. The concrete sealer calculator takes those three inputs and returns the gallons to buy.
More Concrete Calculators
Flatwork is one stage of a project. These pages cover the rest.
Ready to start? Pick your slab calculator, then work through the joints, base and finish before you order.