How much concrete do I need for footings?

1. The formula, step by step

Footing volume is pure geometry. Choose the shape that matches your hole, apply the formula, then divide by the yield per bag and round up to the next whole bag.

Round (cylindrical) post holes

The standard shape for deck and fence posts. The formula is:

Worked example (250 mm diameter, 1,200 mm deep): V = π × (0.125 m)² × 1.2 m = π × 0.015625 × 1.2 = 0.0589 m³ = 58.9 litres. At 0.0112 m³ per 25 kg bag: 0.0589 ÷ 0.0112 = 5.26 → 6 bags.

Square footings

Used under fence posts where a spade or post-hole auger cuts a square hole, or for small pad footings under pergola posts.

Example (300 mm × 300 mm × 900 mm): V = 0.3 × 0.3 × 0.9 = 0.081 m³ (81 L) — about 8 bags of 25 kg mix.

Rectangular footings

Rectangular pad footings are common under columns, stair stringers and load-bearing beams.

Example (450 mm × 450 mm × 600 mm): V = 0.45 × 0.45 × 0.6 = 0.1215 m³ — about 11 bags of 25 kg mix.

2. Post hole concrete reference table

Round holes — the most common shape for structural post footings. Bag counts use an 80 lb / 25 kg bag, rounded up. Add 5–10% extra for spillage on larger jobs.

V = π × (d/2)² × h. Bag counts rounded up. 80 lb bag yield = 0.60 ft³; 25 kg bag yield ≈ 0.0112 m³ (well-established manufacturer figures). For exact dimensions use the calculator above.

These figures are for the hole itself — not the actual post. A 6×6 timber post sitting in a 10-inch hole displaces a small amount of concrete. For most DIY jobs the difference is less than a bag, so rounding up already covers it.

3. How deep should concrete footings be?

Footing depth is the most important decision in the whole job — and the one that varies most by location. The rule is simple: the bottom of the footing must sit below your local frost line, the depth at which soil freezes in winter.

When saturated soil freezes it expands — a phenomenon called frost heave — and anything anchored in it gets lifted. A post footing that ends above the frost line will work its way out of the ground over a few winters, causing decks and fences to tilt and pull apart at the connections.

Typical frost line depths by climate

These are general guidance ranges, not code minimums. Always confirm the required footing depth with your local building department before breaking ground on any permitted structure.

Deck and structural footings

A deck is a structural element attached to your home, so footing requirements typically go beyond just the frost line. Most jurisdictions also specify a minimum diameter (often 10–12 inches / 250–300 mm for a standard deck post), a minimum bearing area relative to the load, and sometimes a soils report or engineer stamp for larger structures. Pull the permit — the inspector will check footing depth before concrete is poured.

Fence posts

Fence posts carry lateral (wind) load rather than vertical gravity load, so their footing requirements differ. A widely used rule of thumb is that one-third of the total post length should be in the ground — a 6 ft fence post needs 2 ft below grade. In frost-prone areas, some builders pour a slightly flared or belled footing at the base to resist heaving.

4. Choosing the right bag size

Pre-mixed bags of concrete come in several sizes. For post holes and small footings, the choice comes down to how much you can carry and how many holes you have to fill.

• 20 kg bag (≈ 0.009 m³ / 9 L): the lightest and most manageable option. Takes about 7 bags to fill a 250 mm × 1,200 mm post hole. Good when access is tight or you are working alone.
• 25 kg bag (≈ 0.0112 m³ / 11.2 L):the most common size at builder's merchants. About 6 bags per standard deck post hole. The best all-round choice for most jobs.
• 40 kg bag (≈ 0.018 m³ / 18 L): heavier to handle but fewer bags to carry. Best when you have many footings to fill and a wheelbarrow handy.

For jobs with six or more deep footings, consider fast-set mixes (such as Quikrete Fast Setting or equivalent) that harden in 20–40 minutes. You can pour the dry mix directly into the hole, add water, and brace the post while it sets — no mixing required.

For very large pours — a deep garage slab or a large spread footing — a ready-mix truck is far cheaper per cubic yard than bagged concrete. Most ready-mix suppliers have a minimum order of 1 cubic yard (about 0.76 m³), so small post-hole jobs still favour bagged mix.

5. Practical tips for pouring footings

• Check for underground utilities first. Before digging, call your local utility-notification service (811 in the US, Call Before You Dig in Canada and Australia) to locate buried pipes and cables.
• Buy 10% more bags than calculated. Soil walls are rarely perfectly smooth, so the actual volume is always a little more than the geometric formula predicts. An extra bag or two costs less than a return trip to the hardware store.
• Work in stages on large jobs. Concrete sets faster in hot weather. If you have more than 8–10 holes to fill, consider mixing in batches of 3–4 holes so no mix hardens in the bag or bucket.
• Tamp and rod the concrete as you pour. A piece of rebar or a long stick pushed up and down several times removes air pockets and ensures the concrete contacts the post and hole walls completely.
• Slope the top away from the post. Finish the top of the footing so it slopes away from the post and sits slightly above grade. This sheds water and reduces the chance of rot at the ground line.
• Allow adequate curing time. Standard premix concrete reaches about 80% strength in 7 days and full design strength at 28 days. In cold weather (below 5 °C), protect the fresh concrete from freezing for at least 24 hours.

Common questions

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

A 10-inch (254 mm) hole dug to 48 inches (1,219 mm) holds about 2.18 ft³ of concrete — four 80 lb bags or six 25 kg metric bags. A narrower 8-inch hole at 36 inches deep takes about 1.05 ft³ — two 80 lb bags. Use the formula V = π × (diameter ÷ 2)² × depth, divide by the bag yield, and round up.

How do you calculate concrete for footings?

For a round hole: Volume = π × (diameter ÷ 2)² × depth. For a square or rectangular footing: Volume = length × width × depth. Convert the result to cubic feet (or cubic metres), then divide by the bag yield — an 80 lb premix bag yields about 0.60 ft³; a 25 kg bag yields about 0.0112 m³. Always round up to the next whole bag.

How deep should concrete footings be?

Footing depth must extend below your local frost line so freeze–thaw movement cannot heave the post. Frost line depth ranges from about 12 inches (305 mm) in mild climates to 48 inches (1,219 mm) or more in cold regions like Canada and the northern US. Structural footings for decks and buildings must also meet your local building code. Check with your local building department for the minimum required in your area.

How much concrete do I need for 6 post holes?

Multiply the single-hole volume by 6. A typical 10-inch-diameter, 48-inch-deep deck post hole uses about 2.18 ft³, so 6 holes need 13.1 ft³ (about 0.48 yd³). At 0.60 ft³ per 80 lb bag, that rounds up to 22 bags. Enter 6 in the 'number of footings' field in the calculator for an instant result.