Concrete Calculator — Bags, Yards & Cost

Quick answer: a 10 × 10 ft slab at 4 in thick is about 1.23 yd³ of concrete — add 10% for waste and it's ~62 × 80-lb bags, or order 1.5 yd³ of ready-mix.

Concrete is sold two ways: bagged mix for small pours and ready-mix delivered by the cubic yard for anything bigger. This calculator gives you both — total volume in cubic feet, cubic yards and cubic metres, exact bag counts for every common bag size, and a straight cost comparison so you can see when calling the truck is cheaper.

It handles slabs, footings, round and square columns, and steps — with a waste factor you control and results rounded the way suppliers actually sell (whole bags, quarter-yard ready-mix increments).

How to use it

Pick what you're pouring — slab, footing, column or steps — and enter the dimensions from your plan or tape measure.
Set the waste / overage percentage: 5–10% is typical; add more for uneven subgrade or a first pour.
Choose the bag size you'll buy and your local prices (bag price and ready-mix price per yard).
Read the order-ready result: volume, bags by size, the ready-mix equivalent, weight and total cost — then take the material list to the store or supplier.

How it works

Volume = length × width × thickness (all converted to feet). Cubic yards = cubic feet ÷ 27. Bags = cubic feet ÷ bag yield (80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³). Round columns: volume = π × r² × height.

Bag yields per manufacturer labels: 40 lb = 0.30 ft³, 50 lb = 0.375 ft³, 60 lb = 0.45 ft³, 80 lb = 0.60 ft³, 90 lb = 0.68 ft³.
Waste defaults to 10% — covers spillage, uneven subgrade and form bowing.
Ready-mix orders are rounded up to the next ¼ cubic yard; many suppliers add a short-load fee below ~4 yd³.
Cured concrete weight ≈ 150 lb/ft³ (about 4,050 lb/yd³).

Worked example

A 10 × 10 ft slab at 4 in thick: 10 × 10 × (4 ÷ 12) = 33.3 ft³ = 1.23 yd³. With 10% waste: 1.36 yd³ → order 1.5 yd³ ready-mix (≈ $225), or 62 × 80-lb bags (33.3 ÷ 0.6 = 56, +10% → 62).

Quick reference

Bags per cubic yard

Bag size	Yield per bag	Bags per yd³
40 lb	0.30 ft³	90
50 lb	0.375 ft³	72
60 lb	0.45 ft³	60
80 lb	0.60 ft³	45
90 lb	0.68 ft³	40

Common slab sizes (4 in thick)

Slab	Cubic yards	80-lb bags (+10%)
8 × 8 ft	0.79	40
10 × 10 ft	1.23	62
10 × 12 ft	1.48	74
12 × 12 ft	1.78	88
12 × 16 ft	2.37	118
16 × 20 ft	3.95	196
20 × 20 ft	4.94	245

FAQ

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

A 10 × 10 ft slab at 4 in thick is 1.23 yd³ (33.3 ft³). At 0.60 ft³ per 80-lb bag that's 56 bags — buy 62 with a 10% waste allowance. In 60-lb bags it's 74, or 82 with waste.

How many 80-lb bags of concrete are in a cubic yard?

About 45. A cubic yard is 27 ft³ and an 80-lb bag yields ~0.60 ft³, so 27 ÷ 0.6 = 45 bags. With 60-lb bags it's 60; with 40-lb bags, 90.

Should I use bags or order ready-mix?

As a rule of thumb, bags make sense under about 1 cubic yard (fewer than ~45 × 80-lb bags). Above 1.5–2 yd³ ready-mix is usually cheaper and far less work — but check the short-load fee, which suppliers often charge below ~4 yd³ and can add $50–150.

How thick should a concrete slab be?

4 in is standard for patios, walkways and shed floors; 5–6 in for driveways and garage floors that see vehicles; 6–8 in for heavy loads. Footings are sized by code and soil — this calculator estimates volume, not structural design.

How much does a yard of concrete cost?

Ready-mix typically runs $125–180 per cubic yard in the US (2026), plus fees for small loads and short pours. Bagged mix works out to roughly $290–360 per yard of equivalent volume — convenient for small pours, expensive at scale.

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Sources & verification

Data verified: October 11, 2026.

Bag yields: manufacturer bag labels (Quikrete, Sakrete) — typical published values.
Ready-mix ordering practice: National Ready Mixed Concrete Association (NRMCA) guidance.
Concrete weight: 150 lb/ft³ cured (standard reference value).

Estimates are for planning only — always confirm quantities with your supplier and check local building codes before you buy or build.