Concrete slab calculator (US)
Calculate concrete volume, bag counts, ready-mix orders, sub-base and mesh for slabs, bases and pads in cubic metres or cubic yards.
Set up for United States: USD prices, Sales tax, and imperial units with local product sizes.
Calculates in cubic yards and converts to the bag sizes US home centers sell: 40, 50, 60 and 80 lb ready-mix.
Ready-mix is rounded to the 0.25 cubic yard increments US plants supply in, and the base course is estimated as dense graded gravel base.
Your project
Showing United States units, product sizes and Sales tax.
Quick start
Slab dimensions
4 in suits patios and shed bases; 5–6 in for driveways.
Use this for repeated pads such as fence-post bases or pier footings.
How the concrete is supplied
Practical up to roughly 1 m³ (1.3 yd³) before mixing becomes the job.
5–10% covers spillage, an uneven excavation base and over-dig. Running out mid-pour causes a cold joint.
Build-up beneath the slab
Estimate updated. 49 80 lb bags of Ready-to-use concrete — 80 lb bags required.
Concrete slab calculator
United States · Imperial units · 30 July 2026
Shopping list
2 lines- 4980 lb bagsReady-to-use concrete — 80 lb bags0.99 yd³ (0.76 m³) of placed concrete
- 1.5cubic yardsRoad base / Class II base4.00 in compacted beneath the slab
Estimate results
What you measured
- Slab area
- 80.00 ft²
- 7.43 m²
- Thickness
- 4.00 in
- Concrete volume
- 0.99 yd³ (0.76 m³)
- Concrete weight
- 2.00 tons (1.81 tonnes)
What to buy
10% waste included| Material | Buy | Weight |
|---|---|---|
Ready-to-use concrete — 80 lb bags 0.99 yd³ (0.76 m³) of placed concrete Rounding: The calculation requires 44.44 80 lb bags. Adding 10% waste gives 48.89. Rounded up to 49 80 lb bags. Main material | 4980 lb bags | 1,998 kg |
Road base / Class II base 4.00 in compacted beneath the slab Rounding: The calculation requires 1.28 cubic yards. Adding 10% waste gives 1.41. Rounded up to the next 0.5 cubic yards to give 1.5 cubic yards. Base and membrane | 1.5cubic yards | 2,408 kg |
Assumptions behind each line
Ready-to-use concrete — 80 lb bags
- Bag yield derived from the published rate of 0.00047 m³ per kg of dry mix — a 80 lb bag yields about 17 L.
- Yields vary between manufacturers. Check the bag before ordering a large quantity.
Road base / Class II base
- Includes a 1.3× compaction allowance.
- Density taken as 2,100 kg/m³.
Slab cross-section
Plan size: 10.00 ft × 8.00 ft
- Formwork should be set to the finished slab level and braced before the pour.
How this was calculated
Slab area
10.00 ft × 8.00 ft
= 80.00 ft² (7.43 m²)
Concrete volume
80.00 ft² (7.43 m²) × 4.00 in thickness
= 0.99 yd³ (0.76 m³)
Bags required
The calculation requires 44.44 80 lb bags. Adding 10% waste gives 48.89. Rounded up to 49 80 lb bags.
= 49 80 lb bags
Concrete weight
0.99 yd³ (0.76 m³) × 2,400 kg/m³
= 2.00 tons (1.81 tonnes)
- Wet, normal-weight concrete. Useful for checking access and barrow runs.
Sub-base volume
80.00 ft² (7.43 m²) × 4.00 in × 1.3 compaction
= 1.28 yd³ (0.98 m³)
Weight and delivery
Approximate weight
4.86 tons
Likely delivery
dump truck
- 4.86 tons needs a dump truck. Confirm access width, overhead clearance and where the driver can tip.
- Check that the delivery route can take the weight — block paving, drainage covers and soft verges are easily damaged.
Safety and compliance
Material planning only
This estimate is for material planning only. Structural dimensions, foundations and compliance requirements should be checked by a qualified professional.
This is not a structural design
Slab thickness, reinforcement, joint layout and ground-bearing capacity are design decisions. A slab supporting a building, a retaining structure or a vehicle load must be designed by a qualified engineer.
Check for buried services before digging
Call 811 before you dig. Utility locating is free in every US state and is required by law in most of them.
Check whether approval is required
Slabs forming part of a building, garage base or extension are usually notifiable. Confirm the requirements with your local building department before you start.
Data behind this estimate
| Assumption | Value | Source |
|---|---|---|
Wet concrete Normal-weight concrete. Lightweight mixes are considerably lighter. | 2,400 kg/m³range 2,300–2,500 | British Standards InstitutionReviewed 2026-07-01 |
Bagged ready-to-use concrete yield Derived from the published 80 lb (36.3 kg) ≈ 0.60 ft³ (0.0170 m³) yield. | 0 m³ per kg of dry mixrange 0–0.001 | Bagged concrete manufacturers (Quikrete, Sakrete)Reviewed 2026-07-01 |
Crushed aggregate base / road base Dense-graded base material, the US equivalent of MOT Type 1. | 2,100 kg/m³range 1,950–2,200 | Mineral Products AssociationReviewed 2026-07-01 |
Compaction allowance for granular sub-base Order about 30% more loose material than the finished compacted depth. | 1.3 × loose volumerange 1.2–1.4 | National Highways (UK)Reviewed 2026-07-01 |
Default waste allowance Covers cutting, breakage, setting-out error and the odd unusable unit. | 10 %range 5–15 | How Much NeededReviewed 2026-07-01 |
Cost
Enter the prices your supplier quoted. This site does not hold or estimate supplier prices, so totals appear only once you fill these in.
Sales tax varies by state and locality, so it starts at 0%. Enter your own combined rate to include it.
No prices entered yet. Everything else in this estimate — quantities, weight, delivery and the shopping list — works without them.
Save, print or share
Create a share link
Generates a private link with a random, non-guessable address. Shared estimates are not indexed by search engines and expire automatically.
Looking for concrete suppliers?
No supplier network is connected to this site yet, so there is nothing to show here. When one is, quote requests will appear in this space — and it will always be obvious which listings are paid placements.
How this calculator works
Every figure below is derived rather than looked up, so you can check it against your own numbers. Nothing is hidden behind a black box.
Volume
Volume is length × width × thickness, multiplied by the number of identical slabs. Thickness is entered in millimetres or inches because that is how it is specified, and converted internally to metres.
volume = length × width × thickness × number of slabs
Bag counts
Bagged concrete is sold by dry weight but used by volume. The engine uses a yield of 0.00047 m³ per kilogram of dry mix, derived from the published figure of 0.60 ft³ from an 80 lb bag.
That gives 0.0170 m³ for an 80 lb bag, 0.0127 m³ for a 60 lb bag, 0.0118 m³ for a 25 kg bag and 0.0094 m³ for a 20 kg bag. Manufacturers vary slightly, so check the bag before placing a large order.
bags = volume with waste ÷ yield per bag, rounded up
Ready-mix
Ready-mix is rounded to the increment the plant sells in — 0.1 m³ in the UK and 0.25 yd³ in the US — with a supplier minimum applied. Short loads normally attract a surcharge.
Sub-base
Sub-base volume is the slab area multiplied by the sub-base depth and by a 1.3 compaction factor, because granular material loses volume as it is whacked down.
sub-base = area × depth × 1.3
Reinforcement
Mesh is estimated by effective coverage after lapping, not by sheet size. A UK A142 sheet is 11.52 m² gross but only about 10.13 m² once a 300 mm lap is allowed on two edges.
This is a quantity estimate only. Mesh grade, position and cover are design decisions.
Common questions
- How many bags of concrete do I need for a cubic metre?
- About 85 bags of 25 kg ready-to-use concrete, or 108 bags of 20 kg. In US terms a cubic yard needs roughly 45 bags of 80 lb. This is why anything over about a cubic metre is usually ordered as ready-mix.
- How thick should a concrete slab be?
- 75 mm (3 in) for footpaths and light shed bases, 100 mm (4 in) for patios and general slabs, and 125–150 mm (5–6 in) for driveways and garages, usually with reinforcement. Anything carrying a building load needs a designed foundation.
- Is ready-mix or bagged concrete cheaper?
- Bagged concrete is cheaper for very small pours because there is no delivery minimum. Ready-mix becomes cheaper per cubic metre well before 1 m³, and it saves hours of mixing. The crossover point depends on your local delivery charges.
US product sizes and conventions
US bag yields
- An 80 lb bag yields 0.60 ft³ — 45 bags per cubic yard.
- A 60 lb bag yields 0.45 ft³ — 60 bags per cubic yard.
- A 50 lb bag yields 0.375 ft³, and a 40 lb bag 0.30 ft³.
- One cubic yard is 27 cubic feet, which is the conversion behind every figure above.
Ready-mix in the US
- Ordered in cubic yards, usually in 0.25 yd³ steps.
- A short-load fee normally applies below about 4 cubic yards.
- Have the forms braced, rebar or mesh set on chairs, and enough hands on site before the truck arrives.
Base and reinforcement
- Dense graded aggregate base, compacted in lifts of 4 in or less.
- Order about 30% more loose material than the compacted depth.
- 6 × 6 W1.4/W1.4 welded wire sheets are 5 × 10 ft; allow a 6 in lap, giving about 43.6 ft² of effective coverage per sheet.
UK version: Concrete calculator — metric units and UK product sizes →