Concrete Calculator

Choose the shape, type the measurements in feet and inches or metres and centimetres, and read the volume in cubic yards, cubic feet and cubic metres with your waste allowance added. The result shows the ready-mix quantity to order, rounded to the quarter yard, and how many bags of every common size you would need instead, with the cost if you type a price. Slabs, strip footings, round columns and post holes, formed walls and solid steps are covered, and the working is written out under the numbers.

How it works

Concrete Calculator is a web page that does its work on your own device. Everything you type or choose is worked out by your browser and never sent to a server, so nothing is stored and there is nothing to delete. There is no account, no sign-up and no limit on how often you use it.

Straight to one job

Concrete Slab Calculator · Concrete Footing Calculator

Volume first, then yards or bags

Concrete is ordered by volume: cubic yards in the US (a yard is 27 cubic feet) and cubic metres elsewhere. Whatever the shape, the calculator multiplies the dimensions to get the volume of one piece, multiplies by how many you are pouring, then adds the waste allowance. Slabs, footings and walls are plain boxes; a column is a cylinder; solid steps are a stack of boxes, one tread deeper than the last, which is where the "steps × (steps + 1) ÷ 2" comes from. Mixed units are handled for you: a slab is typed in feet with its thickness in inches, or in metres with centimetres.

Ready-mix or bags

Ready-mix trucks deliver by the quarter yard or tenth of a cubic metre, usually with a minimum load and a short-load charge below it, so the "to order" figure rounds the waste-inclusive volume up to that step. Bagged mix is sold by weight, and each bag makes a fixed volume once mixed: the figures printed on common bags are 0.30, 0.375, 0.45 and 0.60 cubic feet for 40, 50, 60 and 80 lb bags, and about 0.0094, 0.0117, 0.014 and 0.0187 cubic metres for 20, 25, 30 and 40 kg bags. Divide the volume by the yield and round up. Around one cubic yard, about 45 bags of 80 lb, is where most people switch from bags to a truck; the table shows every size so you can compare prices.

Why add waste

A sub-base is never perfectly flat, forms bow, some mix stays in the mixer and the barrow, and a footing trench is always dug a little wide. Ten percent covers a tidy job on prepared ground; rough excavation or a thin slab over uneven ground can need 15 to 20 percent. Running out mid-pour costs far more than the extra bags, since a cold joint weakens the slab.

What this calculator does not do

• It does not design the slab. Thickness, reinforcement, sub-base and expansion joints come from the use and your local code; a driveway and a shed base are not the same slab.
• It does not account for rebar, mesh or embedded pipes, which displace a little concrete, nor for forms that are not square.
• Weight is for normal-weight concrete at about 150 lb per cubic foot (2,400 kg per cubic metre); lightweight and heavy mixes differ.

What it does

• Slab, footing, round column or post hole, wall, and solid steps
• Cubic yards, cubic feet and cubic metres, with a waste allowance
• Ready-mix quantity rounded up to the quarter yard or tenth of a cubic metre
• Bags of 40, 50, 60 and 80 lb, or 20, 25, 30 and 40 kg, with the count for each
• Several pieces at once, and a cost estimate per yard or per bag
• Feet and inches with fractions, or metres and centimetres
• No upload, no account, no sign-up

Questions

How do I calculate how much concrete I need?

Multiply length by width by thickness in the same unit to get the volume, then convert: cubic feet divided by 27 is cubic yards. A 10 by 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 cubic yards, and about 1.36 yards with 10% for waste. The calculator does the unit conversion and the rounding for you.

How many bags of concrete make a yard?

A cubic yard is 27 cubic feet, so it takes 45 bags of 80 lb (0.60 cu ft each), 60 bags of 60 lb or 90 bags of 40 lb. In metric, about 107 bags of 20 kg make a cubic metre. Check the yield on your bag, since mixes differ a little.

How much concrete is in a post hole?

A round hole is a cylinder: π times the radius squared times the depth. A 12 inch hole 4 ft deep holds 3.14 cu ft, about 0.12 yards, which is six 80 lb bags before waste. Choose Column and type the diameter and depth; use How many for several posts.

Why do I need to add extra?

Ground is never flat, forms spread, some mix stays in the barrow and trenches are dug a little wide. 10% covers a prepared base; rough ground can need 15 to 20%. Running short mid-pour leaves a cold joint, which is worse than a few spare bags.

How much does a yard of concrete weigh?

Normal-weight concrete is about 150 lb per cubic foot, so a cubic yard weighs roughly 4,050 lb, a little over two US tons, and a cubic metre about 2,400 kg. The weight shown is for ordering and for checking what a trailer or barrow can carry.

Should I order ready-mix or buy bags?

Below about half a yard, bags are cheaper and simpler. Around a yard it is about 45 bags of 80 lb to mix by hand, which is a long day; a short-load ready-mix service or a towable mixer is usually worth pricing. Type a price per yard and per bag to compare.

Is this calculator free?

Yes, completely free, with no sign-up. The numbers are worked out in your browser and never sent to us.

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