Concrete Calculator

Q: How many cubic yards of concrete do I need for a 10×10 slab?

A 10×10 slab at 4 inches thick = 33.33 cubic feet ÷ 27 = 1.23 cubic yards. Order 1.35 yards with 10% waste.

Q: How many bags of concrete do I need for a 10×10 slab?

For a 10×10 slab at 4 inches thick: 56 bags of 80-lb, 74 bags of 60-lb, or 112 bags of 40-lb concrete mix.

Q: How thick should a concrete slab be?

4 inches for patios, sidewalks and residential driveways. 6 inches for driveways carrying heavy vehicles. Foundation footings are typically 8–12 inches per engineer specification.

Q: How do I convert cubic feet to cubic yards?

Divide cubic feet by 27. One cubic yard = 27 cubic feet (3 ft × 3 ft × 3 ft).

Q: What PSI concrete should I use for a driveway?

Use 3,500–4,000 PSI for residential driveways. In freeze-thaw climates, specify air-entrained concrete with 3.5–6% air.

Q: How many bags of concrete do I need for a fence post?

A 10-inch diameter, 3-foot deep tube footing needs about one 60-lb bag per post. A 12-inch diameter, 4-foot deep footing needs two 60-lb bags or one 80-lb bag.

Q: How long does concrete take to cure?

Concrete reaches ~70% strength in 7 days and full 28-day strength in 28 days. Walk on it after 24–48 hours; drive on a residential slab after 7 days.

Q: What is the difference between concrete and cement?

Cement is an ingredient (binding powder) in concrete. Concrete is the finished material made from cement, water, sand and aggregate. Pre-mixed bags contain all ingredients — just add water.

Cubic yards, cubic feet and bags of concrete for slabs, columns and walls — with a live cross-section diagram that shows sub-base, vapor barrier, rebar and control joints to scale.

Updated 17 May 2026 ACI 318 + PCA bag yields Methodology

Quick presets — click to load typical project values:

Shape / Project Type

Length (ft)

Width (ft)

Thickness (inches)

Cubic Feet

—

Cubic Yards

—

Cubic Yards + 10% Waste

—

40-lb Bags Needed

—

60-lb Bags Needed

—

80-lb Bags Needed

—

For estimating only. Foundation / structural concrete requires an engineer's specification. Bag yields per Portland Cement Association (40-lb=0.30 ft³, 60-lb=0.45 ft³, 80-lb=0.60 ft³).

Live cross-section

Updates as you change inputs. Shows sub-base, vapor barrier, rebar and control joints (slab mode).

Gray = concrete · Orange dots = #4 rebar · Tan dotted = compacted gravel sub-base · Red line = 6-mil vapor barrier · cj = control joint (saw cut at 1/4 slab depth)

📐 Fraction → decimal helper (tape measure conversions)

1/16 = 0.0625
1/8 = 0.125
3/16 = 0.1875
1/4 = 0.25
5/16 = 0.3125
3/8 = 0.375
7/16 = 0.4375
1/2 = 0.5
9/16 = 0.5625
5/8 = 0.625
11/16 = 0.6875
3/4 = 0.75
13/16 = 0.8125
7/8 = 0.875
15/16 = 0.9375
1 = 1.0

Type the decimal value into any input above. Example: 4-1/2″ slab = 4.5 ; 12-3/4″ tube diameter = 12.75.

Anatomy of a Concrete Slab

A residential slab is more than just poured concrete — proper preparation determines whether it lasts 30 years or cracks in 3.

Every layer has a job. Skipping the vapor barrier on an interior slab = moisture wicking up; skipping the sub-base = uneven settlement; skipping rebar = cracks at the first freeze.

Best Practices — Thickness, Cover, Joints, Cure

Four reference scales every concrete project uses. Mismatching any of these is what causes the cracks people blame on "bad concrete."

Standards per ACI 318-19 and Portland Cement Association Design and Control of Concrete Mixtures.

Concrete Project Types — Cross-Sections

Six common concrete project shapes, each with its own purpose, reinforcement and cure considerations.

Slab on grade. Floor slab cast directly on prepared ground. Patios, driveways, garage floors. 4–6″ thick with #4 rebar or wire mesh.

Continuous footing. Below-frost foundation that supports a wall. Width ≥ 2× wall thickness, depth varies by frost line.

Pad footing. Square pad under a single point load — porch column, deck post, signpost. Reinforced both directions.

Pier / tube footing. Round form filled with concrete. Fence posts, deck piers, sign bases. Goes below local frost depth.

Foundation wall. Sits on a continuous footing. 8″ typical residential, 10–12″ for basement walls. Vertical rebar + horizontal rebar.

Monolithic slab. Slab + footing poured in one operation. Common for warm-climate single-story homes. No cold joint between footing and slab.

Joint & Edge Details

Two joint types and one prep detail — get these right and your slab won't crack randomly.

Control joint. Saw-cut groove at 1/4 slab thickness, made 12–24 hours after pour. Tells the slab where to crack so it doesn't crack randomly. Spacing ≤ 24× thickness.

Expansion joint. 1/2″ fiber strip (asphalt-impregnated felt or cork) at every wall, column, or existing slab. Full slab depth. Stops thermal expansion from cracking adjacent structures.

Sub-base prep. Compacted gravel ≥ 4″ for drainage and uniform support, then 6-mil vapor barrier directly under interior slabs. Skipping the gravel is the #1 cause of slab failure.

How the Concrete Calculator Works

This concrete calculator converts your project dimensions into volume (cubic feet and cubic yards) and then tells you how many pre-mixed bags you need — or how much ready-mix to order. For a rectangular slab, the formula is straightforward: Length × Width × Thickness ÷ 12 (to convert thickness from inches to feet) = cubic feet. Divide cubic feet by 27 to get cubic yards (there are 27 cubic feet in one cubic yard, the standard unit for ready-mix concrete orders). For round columns and tube footings, the calculator uses π × radius² × height. For walls and footings, it is the same as a slab: length × height × thickness.

The "Cubic Yards + 10% Waste" figure is what to order from a ready-mix plant — overage accounts for subgrade variations, spillage and the fact that concrete must be placed continuously (you cannot order exactly the right amount and risk running short mid-pour). For bag work, the calculator already rounds up to whole bags.

Concrete Bag Coverage — Quick Reference

Bag Size	Coverage (cu ft)	Bags per Cu Yard	Best For
40-lb	0.30	90	Small repairs, steps, single posts
60-lb	0.45	60	Fence posts, small slabs, footings
80-lb	0.60	45	Larger slabs, driveways, walls

Recommended Slab Thickness by Application

Application	Thickness	Rebar / Mesh	PSI
Sidewalk / walkway	4 in	Wire mesh or none	3,000
Patio / pool deck	4 in	Wire mesh or #3 rebar	3,000–3,500
Residential driveway	4–6 in	#4 rebar @ 18 in OC	3,500–4,000
Garage floor	4–6 in	#4 rebar @ 12 in OC	4,000
Foundation / footing	8–12 in	#4–#5 rebar (per engineer)	3,000–4,000
Commercial floor	6 in+	#5 rebar @ 12 in OC	4,000–5,000

Bags vs. Ready-Mix: Which to Choose?

Use pre-mixed bags for projects under 1 cubic yard (27 cubic feet) — small slabs, footings, fence posts and repairs. For anything over 1 yard, ordering ready-mix concrete from a local plant is faster, physically easier and often less expensive per yard. Most ready-mix plants have a minimum order of 1 yard; some charge a short-load fee for orders under 3 yards.

Frequently Asked Questions

How many cubic yards of concrete do I need for a 10×10 slab?

A 10×10 slab at 4 inches thick = 10 × 10 × (4/12) = 33.33 cubic feet ÷ 27 = 1.23 cubic yards. Order 1.35 yards (1.23 + 10% waste). At 4 inches, you would need about 56 bags of 80-lb concrete mix or 111 bags of 40-lb.

How many bags of concrete do I need for a 10×10 slab?

For a 10×10 slab at 4 inches thick (33.33 cu ft): 56 bags of 80-lb, 74 bags of 60-lb, or 112 bags of 40-lb concrete mix. Using 80-lb bags is most cost-effective for slabs this size; for anything over 1 yard, consider ready-mix instead.

How thick should a concrete slab be?

4 inches is standard for patios, sidewalks and residential driveways. Use 6 inches for driveways that will carry heavy vehicles (RVs, heavy trucks). Garage floors are typically 4–6 inches. Foundation footings and structural slabs require an engineer's specification, typically 8–12 inches.

How do I convert cubic feet to cubic yards?

Divide cubic feet by 27. One cubic yard = 3 ft × 3 ft × 3 ft = 27 cubic feet. Example: a 4-inch thick 12×20 slab = 12 × 20 × (4/12) = 80 cu ft ÷ 27 = 2.96 cubic yards — order 3.26 yards with 10% waste.

What PSI concrete should I use for a driveway?

Use 3,500–4,000 PSI for residential driveways. Standard 3,000 PSI (the most common bag mix) is adequate for patios and walkways but may crack under vehicle loads over time. In freeze-thaw climates, specify air-entrained concrete (3.5–6% air) to improve durability.

How much does a yard of concrete weigh?

One cubic yard of standard concrete weighs approximately 3,900–4,050 pounds (about 2 tons), depending on aggregate type and mix design. This is important for delivery — most ready-mix trucks carry 8–10 yards and need a reinforced surface to drive on.

How many bags of concrete do I need for a fence post?

A typical 10-inch diameter, 3-foot deep tube footing needs about 0.17 cubic feet of concrete — roughly one 60-lb bag per post. For a 12-inch diameter, 4-foot deep footing, use two 60-lb bags or one 80-lb bag. Post diameter and depth depend on post height and local frost depth.

How long does concrete take to cure?

Concrete reaches approximately 70% of design strength in 7 days and full (28-day) strength in 28 days. You can walk on a slab after 24–48 hours and drive on a residential slab after 7 days. Keep the surface moist (curing) for the first 7 days — covering with plastic sheeting or applying curing compound dramatically improves final strength.

Should I add rebar to a concrete slab?

Yes for driveways, garage floors and any slab subject to vehicle traffic or uneven subgrade. Use #4 rebar at 18-inch spacing for a residential driveway or garage floor; #3 at 18 inches for a patio. Wire mesh is a lower-cost alternative for light residential patios and sidewalks but provides less structural reinforcement.

What is the difference between concrete and cement?

Cement is an ingredient in concrete — the binding powder (Portland cement) that reacts with water to harden. Concrete is the finished material made from cement, water, sand and aggregate (gravel). When people say "a bag of cement," they usually mean a bag of pre-mixed concrete mix, which already contains cement, sand and aggregate — just add water.