How many bags of concrete do I need for a sonotube footing in NB?

The number of bags depends on the sonotube diameter and the depth of the footing — but for a typical NB deck post footing, plan on 3–6 bags of 30 kg premixed concrete per tube. Here is how to calculate it precisely for your project.

In New Brunswick, sonotube (cardboard tube) footings for decks, fences, and post structures must extend below the frost line — a minimum of 4 feet (1.2 metres) below grade in southern NB (Moncton, Fredericton, Saint John, Riverview), and 4.5–5 feet (1.4–1.5 metres) in northern NB (Bathurst, Miramichi, Campbellton). This frost depth requirement is more substantial than in most of Canada, and it means your sonotubes need to be long.

Volume calculation: The volume of a cylinder is π × r² × h, where r is the radius and h is the height.

• 8-inch diameter sonotube, 4 feet deep: π × (0.33 ft)² × 4 ft = 1.37 cubic feet ÷ 0.5 (cubic feet per 30 kg bag) = 2.7 bags → order 3 bags
• 10-inch diameter sonotube, 4 feet deep: π × (0.42 ft)² × 4 ft = 2.18 cubic feet → 4.4 bags → order 5 bags
• 12-inch diameter sonotube, 4 feet deep: π × (0.5 ft)² × 4 ft = 3.14 cubic feet → 6.3 bags → order 7 bags
• 12-inch diameter sonotube, 4.5 feet deep (northern NB): → 7 bags

Always round up and buy an extra bag — you do not want to run short when the tube is half-filled.

Bagged concrete for sonotubes is appropriate because the volume per footing is small, the pouring is intermittent (one tube at a time), and ready-mix truck minimums (typically 0.5–1 cubic metre) far exceed the quantity needed for a few footings. At $6–$8 per 30 kg bag, a 6-post deck project using 5 bags per tube runs $180–$240 in materials — straightforward DIY territory.

NB-specific note: tube diameter must be sized to support the load. For most residential decks, 10-inch or 12-inch tubes are typical; a structural engineer or your deck contractor can confirm the required diameter for your specific span and load. The bottom of the tube should sit on undisturbed native soil or properly compacted granular fill — never on organic soil, topsoil, or disturbed fill that could settle. Some NB contractors add a belled bottom (wider footing at the base) using a flared tube section for better load distribution.