Concrete Footing Calculator: Post Holes & Sonotube Yardage

Use this concrete footing calculator to estimate how much concrete you need for cylindrical pours like deck footings, Sonotubes, and post holes. Enter the diameter, depth, and quantity to get results in cubic yards (for ready-mix orders), cubic feet, plus optional estimates for bags of concrete and concrete weight. This is ideal for deep, round holes where slab math doesn’t apply.

This section functions as a sonotube concrete calculator for round forms used in deck footings, porch supports, pergola posts, and small foundation piers. Sonotubes are sized by diameter (like 8 in, 10 in, 12 in, 14 in, 16 in). Measure the actual hole depth to match frost depth requirements in your area and avoid under-ordering.

Use this post hole concrete calculator for fence posts, gate posts, mailbox posts, and similar supports. Many users search for a fence post concrete calculator—the same cylinder math applies: measure hole diameter and depth, then multiply by the number of holes. If you have varying sizes, calculate each size separately and add the totals.

For round holes and Sonotubes, cylinder volume is the most accurate approach. This is the core math behind any cylindrical footing estimate (and why it differs from slab calculators).

Step 1 (convert to feet): radius(ft) = (diameter(in) ÷ 2) ÷ 12, depth(ft) = depth(in) ÷ 12

Step 2 (cubic feet): ft³ = π × r² × depth(ft) where π ≈ 3.1416

Step 3 (cubic yards): yd³ = ft³ ÷ 27

Note: Some people call this a concrete footer calculator—it’s the same footing yardage estimate.

If your footing has a wider “bell” base, estimate it by splitting the shape into two parts: a main cylinder (shaft) plus a second cylinder for the base. Then add the volumes together. This approach is simple and accurate enough for ordering concrete.

• Shaft ft³ = π × r(shaft)² × shaftDepth(ft)
• Base ft³ = π × r(base)² × baseHeight(ft)
• Total ft³ = (Shaft + Base) × Quantity
• Total yd³ = Total ft³ ÷ 27 (then add waste factor)

Example A (Sonotube footings): 12 in diameter × 48 in deep × 6 holes.

• Radius = (12 ÷ 2) ÷ 12 = 0.5 ft
• Depth = 48 ÷ 12 = 4.0 ft
• Per-hole ft³ = π × 0.5² × 4.0 ≈ 3.1416 ft³
• Total ft³ = 3.1416 × 6 ≈ 18.85 ft³
• Total yd³ = 18.85 ÷ 27 ≈ 0.70 yd³

Example B (Fence post holes): 10 in diameter × 30 in deep × 20 holes.

• Radius = (10 ÷ 2) ÷ 12 ≈ 0.4167 ft
• Depth = 30 ÷ 12 = 2.5 ft
• Per-hole ft³ ≈ π × 0.4167² × 2.5 ≈ 1.36 ft³
• Total ft³ ≈ 1.36 × 20 ≈ 27.2 ft³
• Total yd³ ≈ 27.2 ÷ 27 ≈ 1.01 yd³

Tip: Add 5–10% waste if holes are uneven, over-dug, or if you expect spillage.

If you’re mixing concrete yourself, estimate bags of concrete using total volume in cubic feet and bag yield. Planning yields are:

• 60 lb bag: ~0.45 ft³
• 80 lb bag: ~0.60 ft³

Bag formula: Bags = Total ft³ ÷ Bag Yield (ft³ per bag)

Best practice: Use the exact yield printed on your bag for the most accurate count.

Concrete is heavy, so weight estimates can help with hauling, staging, and DIY mixing plans. Normal-weight concrete is often estimated around 145–150 lb per cubic foot.

• Weight (lb) = Volume (ft³) × 150
• Weight (lb) = Volume (yd³) × 4,050

• Deck footings: Sonotubes for posts and beams
• Fence posts: post holes for straight runs and corners
• Gate posts: deeper / larger holes for heavier loads
• Pergolas & mailboxes: small post supports
• Small piers: basic round foundations for light structures

• Measure real hole size: hand-dug holes are rarely perfect cylinders—consider a 5–10% waste factor.
• Account for over-dig: deeper or wider holes increase volume quickly.
• Group by size: if you have mixed diameters/depths, calculate each group and add totals.
• Round up: it’s safer to have extra than to run short mid-pour.