Concrete calculator: how much do I need?
Free concrete calculator: cubic metres, cubic yards, bags or a truck order. No sign-up, nothing stored.
CONCRETE TO ORDER
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Turn on JavaScript and this works out your concrete as you type. The formulas are below if you'd rather do it by hand.
Your sections, read back
Two ways to buy the same concrete
Bags
Ready-mix
Still to confirm before you buy
This sheet adds up the sizes you gave it. It has not checked whether they are right for the job.
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How the concrete calculator works
You type the sizes the way you measured them. Feet and inches, millimetres, a tape that reads both: every field keeps its own unit, so a 12 by 16 foot shed pad at 100 millimetres goes in exactly as written. Before anything gets added up, you see each section read back in both systems. That's where you catch a thickness typed in feet instead of inches, an easy slip that buys twelve times the concrete.
The arithmetic is short. A slab, pad, footing or wall is length times width times thickness. A form tube, pier or post hole is a circle times its depth: pi times half the diameter, squared, times the depth. Anything inside that concrete won't fill, like the post standing in each hole, comes off first, and only then do we multiply by how many you have. Every conversion uses the exact definitions in Canada's Weights and Measures Act, where a yard is 0.9144 metres and the foot, the inch and the cubic yard all follow from it.
You don't buy the net volume, though. Your allowance goes on top of the net volume, and the total gets rounded up once, to something you can actually buy: whole bags, counted from the yield printed on your own bag, or a ready-mix order in your supplier's steps. Say your plans call for a 10 by 10 foot pad, 4 inches thick. That's 0.94 m³, or 33.3 cubic feet. Add 10% and you're at 1.04 m³. At 14 litres a bag, it takes 75 bags. From a plant that sells in half-cubic-metre steps, it's 1.50 m³, which is 58.9% more than the pad itself holds. Both of those show up as their own lines, so you can see what the rounding cost before you pay for it.
A total that lands exactly on a step stays there, with no extra step on top. And we never count bags from the weight printed on the front: 30 kg tells you what you're lifting, not what it makes.
Who this is for
Homeowners with a shed pad, a walkway, deck footings or a row of fence posts marked out, and one purchase ahead: a skid of bags from the building centre or a call to a ready-mix plant.
Small contractors with the numbers on site and a dispatcher to call before the cut-off.
And anyone holding plans in feet while the supplier sells in cubic metres.
Homeowner questions about concrete and bags
How many 30 kg bags of concrete do I need?
Divide the volume by what one bag makes, then round up. The makers of the three Canadian 30 kg premixes we checked list about 14 litres a bag (Sakrete and Bomix) or 0.52 cubic feet, about 14.7 litres (Quikrete). So a full cubic metre takes 72 bags at 14 litres, or 68 at 0.52 cubic feet, before anything for spills. We took those from the makers' web sheets on October 8, 2026, not off a bag, so check yours. If it prints two figures, go with the smaller. That's the safer count.
How many bags of concrete for a 10x10 slab?
That depends on the thickness, which is your plans' call, not ours. If they say 4 inches, the slab holds 33.3 cubic feet, or 0.94 m³. At 14 litres a bag that's 67.4 bags' worth, so 68 bags. At 0.52 cubic feet a bag it's 65. Both are before any allowance, and an uneven base eats into them faster than you'd think.
How much concrete do I need for a Sonotube?
Work out the circle, then the depth: pi times half the diameter squared, times the depth. A 10-inch form tube filled 4 feet deep holds 61.8 litres, or 2.18 cubic feet. That's 5 bags at 14 litres a bag, and still 5 at 0.52 cubic feet. Sonotube is a brand name; any round cardboard form works the same way. Pick Round tube or pier and set how many you're filling.
How much concrete per fence post?
The hole, minus the post. A hole 10 inches across and 36 inches deep holds 46.3 litres. If your post measures 3.5 by 3.5 inches, it takes 7.2 litres of that. Measure yours: the 4x4 name is a nominal size. Each hole in this example needs about 39.1 litres: 3 bags at 14 litres. Choose Post holes and the calculator takes the post out of every hole for you. How deep the hole goes is for your fence plans and local rules to say.
How do I convert cubic metres to cubic yards?
A cubic yard is exactly 0.764554857984 m³, because the Weights and Measures Act fixes the yard at 0.9144 metres. Turned around, one cubic metre is about 1.308 cubic yards. The calculator prints both side by side for every total, so you can talk to a plant in metres and still read a bag chart in yards without doing it by hand.
How much extra concrete should I order?
We don't pick that number for you. Concrete goes missing in spills, in the mixer and the wheelbarrow, and in a base that isn't perfectly flat. Even the bag sheets say "approximately". What we do show is what your choice costs: the allowance you type, then what rounding adds on top. On a 10 by 10 foot pad at 4 inches, a 10% allowance turns into a 58.9% surplus once a plant rounds the order up to its next half cubic metre.
How thick should a concrete slab be?
That's not a number we'll give you, and we'd be wary of any calculator that hands one out from a dropdown. The right thickness depends on what sits on the slab, your soil and your local rules. It comes from your plans, the kit instructions or your contractor. Footings, foundations, retaining walls and anything that carries a structure fall under your provincial building code and local permit rules. We add up the sizes you give us. We don't check any of that.
Is bagged concrete or ready-mix cheaper?
That depends on your quotes, so the calculator puts both side by side and picks neither. Plants sell by the cubic metre in steps above a minimum: Miller Concrete, for one, takes residential orders from 1 m³ in 0.25 m³ steps and adds a minimum load charge below 4 m³. Dufferin Concrete's 2025 Ontario list has small-load charges in three bands up to 5 m³. Bags cost you lifting, mixing and time instead. Put both quotes into Add your own prices and compare the totals yourself.
How much concrete do I need?
Measure, convert, multiply, then buy in whole units. For a slab, pad or footing it's length times width times thickness. For a tube, pier or post hole it's pi times the radius squared times the depth. Keep every measurement in one unit before you multiply, or let the calculator convert each field for you. A cubic metre is 1,000 litres, about 35.3 cubic feet and 1.308 cubic yards.
That gives you the net volume. What you buy is bigger. Bagged concrete comes in whole bags, and of the three Canadian 30 kg mixes we checked, two makers list 14 litres a bag and one lists 0.52 cubic feet. Ready-mix comes in a plant's steps, above its minimum. Add your allowance, round up once at the very end, and you have a number you can say on the phone. Whether you came for a concrete slab calculator, a bag count or cubic yards, it's the same tape measurements underneath.
Sources and last checked
Checked October 8, 2026. Data version concrete-ca-2026-10-07.
- Unit definitions: Weights and Measures Act, R.S.C. 1985, c. W-6, Schedules I and II (Justice Laws website, current to 2026-09-21).
- Bag yields quoted in the answers above: Sika Canada, SAKRETE Concrete Mix product page; Les Produits Daubois, Bomix Concrete Mix technical data sheet (version V260724); Target Products, QUIKRETE Concrete Mix product page. These are the makers' web sheets, not the printed bag.
- Ready-mix ordering terms: Miller Concrete, residential concrete page; Dufferin Concrete, 2025 Ontario Price List.
How we made it: the calculator is arithmetic over the numbers you type. It holds no bag yields, no thicknesses and no prices of its own, and the makers and plants above are named for what they publish, not recommended.
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