Concrete Calculator

Order (yd³) = length × width × (thickness ÷ 12) × (1 + wastage ÷ 100) ÷ 27

L

Length of the pour, in feet

W

Width, in feet

T

Thickness of the slab or footing, in inches

w

Wastage allowance, as a percentage of the net volume

ρ

Density of the concrete, in kilograms per cubic metre — 2,400 for plain concrete

Use it as soon as you know the shape of the pour and before you book the truck or buy bags. Thickness is the field people get wrong: it is in inches, so a 4 in slab is a third of a foot, and doubling it doubles the order. If you are building a wall rather than a pour, the brick and block pages in this section do the same job with masonry units; if you only want to know what a cubic yard weighs, the concrete weight calculator answers that without asking for any dimensions.

1. A 10 ft by 10 ft slab, 4 inches thick, with 5% allowed

   1. Area: 10 × 10 = 100 square feet
   2. Thickness in feet: 4 ÷ 12 = 0.333 ft
   3. Net volume: 100 × 0.333 = 33.33 cubic feet
   4. With 5% allowed: 33.33 × 1.05 = 35.0 cubic feet
   5. In cubic yards: 35.0 ÷ 27 = 1.30 yd³ (0.99 m³)
   6. In 80 lb bags: 0.99 m³ ÷ 0.0170 m³ per bag = 58.3, rounded up to 59

   This is the default setting on the page, and 1.3 cubic yards is below most suppliers' minimum load — expect a short-load charge, or buy bags. The bag figure is the number to compare against: 59 bags is about two pallets.

2. A driveway slab 20 ft by 12 ft, 8 inches thick, 10% allowed

   1. Area: 20 × 12 = 240 square feet
   2. Thickness in feet: 8 ÷ 12 = 0.667 ft
   3. Net volume: 240 × 0.667 = 160 cubic feet
   4. With 10% allowed: 160 × 1.1 = 176 cubic feet
   5. In cubic yards: 176 ÷ 27 = 6.52 yd³ (4.98 m³)
   6. In 80 lb bags: 4.98 m³ ÷ 0.0170 = 293.2, rounded up to 294

   6.52 yards is a full truck load with room to spare, and it is where mixing by hand stops being realistic — 294 bags is over nine pallets. The 10% allowance is there because a driveway is usually poured against forms on ground that has been graded rather than shuttered, so more escapes under the edges.

3. A 5 m by 4 m pad, 100 mm thick, 5% allowed

   1. Switch the length and width fields to metres and the thickness field to centimetres or millimetres — the page converts before it computes
   2. Net volume: 5 × 4 × 0.1 = 2.0 m³
   3. With 5% allowed: 2.0 × 1.05 = 2.10 m³
   4. In cubic yards: 2.10 × 1.308 = 2.75 yd³
   5. In 80 lb bags: 2.10 ÷ 0.0170 = 123.6, rounded up to 124

   The same pad read in metric: 2.1 cubic metres is what a Chinese or European supplier would quote, and 2.75 cubic yards is the same concrete in the unit an American yard sells by. The two volume outputs are always the same pour — switching the input unit changes nothing but the reading.

The mix proportions in the table are a reference mix, not a site mix: they are typical figures for ordinary Portland cement at the 32.5 grade with 20 mm aggregate, and every real job needs a laboratory mix design that is then adjusted on site for the actual moisture in the sand and stone. The density behind the bag figures is likewise an order of magnitude rather than a specification — plain concrete sits near 2,400 kg per m³ and moves about ±5% with the aggregate, the reinforcement ratio and how wet it is. The wastage allowance is a judgement: nothing publishes a standard for it, and the right figure depends on the subgrade, the formwork and how the concrete is placed and screeded, so it is an input with a range from 0 to 50 rather than a fixed percentage. The geometry is treated as a rectangle of constant thickness — a slab with thickened edges, a footing with a keyway, a stepped pad or a sloped surface all take more than this returns. And this page prices nothing: it returns concrete, not concrete plus delivery, pump hire, a short-load fee or reinforcing mesh.

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

At 4 inches thick, 1.30 cubic yards with a 5% wastage allowance — 33.33 cubic feet of net volume, which is 1.23 yards before the allowance. The thickness is what moves the answer: the same slab at 6 inches is 2.0 yards, and at 2 inches it is 0.68. Enter your own thickness rather than reading a rule of thumb off a table.

How much wastage allowance should I add?

Five percent is a fair default for a slab formed on a flat, compacted subgrade. Go to 10% for a footing poured against rough trench walls or a pour over an uneven subgrade, and higher still for a small job where a single bag more or less is the difference between finishing and stopping. Nothing publishes a standard figure, which is why it is a field on this page rather than a hidden 5%.

How many bags of concrete is a cubic yard?

About 45 bags of 80 lb ready mix. One bag yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45. The 10 by 10 slab in the first example is 1.3 cubic yards, which is where the 59 bags come from. A 60 lb bag yields about 0.45 cubic feet instead, so that takes 60 bags. Past a yard or so the bags stop making sense and a ready mix delivery is cheaper and better mixed.

Why is the thickness in inches and not in feet?

Because that is how slabs and footings are specified — a 4 inch slab, a 12 inch footing, a 6 inch curb. The page divides by 12 for you. It matters more than it looks: entering 4 where the field wants feet would give a slab three times too thick, and the page would happily return 3.9 cubic yards for it.

What does C20 mean in the mix table?

It is a cube strength class: C20 is concrete whose characteristic 20 MPa cube strength is met, C25 and C30 being the next two steps up. The psi figure in brackets is a unit conversion, not a claim that C20 is the same concrete as an American 3,000 psi mix — the two are tested on different specimens, a cube against a cylinder, and the cylinder figure is roughly 80% of the cube figure.

Can I use this for a footing or a wall instead of a slab?

For a footing, yes — it is a rectangle of constant thickness for the purpose of ordering, which is what a strip footing is. For a masonry wall, no: bricks and blocks are counted rather than poured, so use the brick calculator or the concrete block calculator in this section. For filling the cores of a block wall, use the block fill calculator; for the concrete around fence or deck posts, use the post hole calculator.

• Approximate Conversions from U.S. Customary Measures to Metric — the chart behind the cubic yard, cubic foot and pound figures used here — National Institute of Standards and Technology
• The International System of Units (SI) brochure — the metre, the cubic metre and the kilogram, the units the volumes and the density on this page are defined in — Bureau International des Poids et Mesures (BIPM)