ConcreteFinderGet Quotes

Concrete bag calculator

Last updated: By ConcreteFinder Research Team

Skip to the calculator

Bag counts go wrong in two places: people forget that the post displaces a third of the hole, and they read a yield off an American bag that Canadian stores do not sell. This calculator does the displacement, and it uses the yields the manufacturers publish for Canadian 25 kg and 30 kg bags rather than the 60 lb and 80 lb figures that dominate search results.

How many bags of concrete do you need?

Bags needed, including 10 percent waste

16 to 17 bags of 30 kg

Volume to fill, before waste
7.37 cu ft
Volume to buy, after waste
8.10 cu ft / 0.229 m³
30 kg bags
16 to 17 bags
25 kg bags
19 to 20 bags
30 kg material cost at $6.20 to $7.58 a bag
$99 to $129

This is comfortably bag territory. Builders Ontario puts the point where ready-mix overtakes bags at roughly 0.75 cubic metres, and this pour is under it.

A 25 kg bag count is shown without a price because none of the Canadian retailers that publish a readable price list quote the 25 kg concrete mix; the three that do quote the 30 kg bag. Prices exclude 13 percent HST.

How the bag count is worked out

A post hole is a cylinder with a post standing in it. The cylinder is pi times the radius in feet squared times the depth in feet, where the radius in feet is the hole diameter in inches divided by 24. The post is its actual side, not its nominal one: a 4x4 measures 3.5 inches, a 6x6 measures 5.5. Subtract one from the other and that is what you pour per hole.

Bag counts always round up, because you cannot buy 0.4 of a bag, and the count is given as a range because the two manufacturers state slightly different yields. The fewest bags assumes the higher yield, the most assumes the lower. Pads and footings use the same length by width by depth arithmetic as the concrete calculator.

Bags or a truck?

Mixing by hand is cheap up to a point and then it is punishing. A cubic metre is roughly 70 bags of 30 kg, which is over two tonnes to carry, tip, mix and place before the first batch starts setting. Builders Ontario puts the crossover at 0.75 to 1 cubic metre, and that is about right for one person with a mixer.

The reason a truck is not automatically cheaper below that line is the partial-load charge. Canada Building Materials lists $300.00 for a load between 1.2 and 3 cubic metres and $400.00 for a single cubic metre, on top of the rate itself. 2AZ Group puts the same idea differently: $50 to $80 per cubic metre under the 3 cubic metre minimum. On a four-post fence, bags win easily.

One thing bags cannot fix is depth. Builders Ontario states the minimum footing depth is the greater of 1.2 m or the local frost depth and puts the GTA at about 1.2 m, so a 48 inch hole is the Toronto default. Digging shallower to save two bags buys a heaved fence in the first hard winter.

Frequently asked questions

How many bags of concrete do I need for a fence post?

A 10 inch hole dug 48 inches deep holds 2.18 cubic feet, and a 4x4 post fills 0.34 of that, so each hole takes about 1.84 cubic feet. At the 0.50 to 0.52 cubic foot yield Sakrete Canada and Target Products publish for a 30 kg bag, that is 4 bags a hole.

How deep should a fence or deck post hole be in Toronto?

Builders Ontario states the minimum is the greater of 1.2 m (4 ft) or the local frost depth, and puts the GTA and southern Ontario at about 1.2 m. A hole that stops short of frost depth will heave in a Toronto winter, which is why the calculator defaults to 48 inches.

How many bags of concrete are in a cubic metre?

Basalite states 86 of its 25 kg sacks fill a cubic metre and 66 fill a cubic yard. For the 30 kg bag, a cubic metre is 35.31 cubic feet and each bag yields 0.50 to 0.52, so it takes roughly 68 to 71 bags.

When should I stop buying bags and order a truck?

Builders Ontario puts the break-even at roughly 0.75 to 1 cubic metre. Past that you are mixing 60 or more bags by hand, and a partial ready-mix load costs $300.00 for 1.2 to 3 cubic metres on the Canada Building Materials GTA list.

What does a bag of concrete cost in Ontario?

A 30 kg bag ran $6.20 at King Construction Supplies in Brampton, $7.29 at Chamberlain Timber Mart in Gravenhurst and $7.58 at Housing Guards in Scarborough when this page was checked in August 2026, before HST.

Sources

Yields are the manufacturers' own published figures and prices were read off the retailers' own listings on 2026-08-25. Canadian dollars, before HST. Dataset July 2026.