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Fence Post Concrete Calculator

Calculate how much concrete you need for fence posts, sonotubes, deck piers, and mailbox posts. Enter the hole diameter, depth, and number of posts to get instant bag counts.

Concrete Calculator

Select your project type, enter dimensions, and get instant results with bag counts, weight, and cost.

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Total Volume

0.00 cu ft

0.00 cu yd

60 lb Bags

0

80 lb Bags

0

Total Weight

0 lbs

(0.00 tons)

Cost Estimate

How to Calculate Concrete for Post Holes & Sonotubes

Post holes and sonotubes are cylinders, so the formula is straightforward: Volume = π × r² × h, where r is the radius and h is the depth. When working in inches (the most common unit for post holes), convert to cubic feet with: Volume (ft³) = π × (Diameter ÷ 24)² × (Depth ÷ 12).

Use this cylinder formula whenever you're filling a round hole — whether it's a bare-earth post hole, a cardboard sonotube form, or a round pier footing. The calculator above handles the math automatically, but understanding the formula helps you double-check results and estimate on the fly. A 10-inch diameter hole at 30 inches deep yields roughly 0.45 ft³ per post. Multiply that by the number of posts and add 10% for waste, and you have your total concrete requirement.

Concrete Bags Per Post: Complete Reference Table

The table below covers the most common post hole sizes for residential projects. Bag counts assume filling the full cylinder volume without subtracting the post footprint, which provides a built-in safety margin.

Post TypeHole Size60lb Bags80lb Bags
4×4 fence post10" × 30"3–42–3
4×4 fence post (tall)10" × 36"4–53–4
6×6 fence/deck post16" × 36"10–117–8
6×6 post (deep)16" × 42"12–139–10
Mailbox post8" × 24"1–21–2
Deck pier12" × 42"6–74–5
8" sonotube (4 ft)8" × 48"3–42–3
10" sonotube (4 ft)10" × 48"5–64–5
12" sonotube (4 ft)12" × 48"7–85–6
16" sonotube (4 ft)16" × 48"13–1410–11
Basketball hoop18" × 48"17–1813–14
Flagpole (residential)14" × 36"7–85–6

Sonotube Concrete Calculator Guide

Sonotubes (also called concrete form tubes) are waxed cardboard cylinders you set into a hole to create a smooth, uniform concrete pier. They're commonly used for deck footings, porches, pergolas, and any structure that requires a clean, above-grade concrete column. Common sonotube diameters are 8", 10", 12", and 16", with lengths from 4 to 12 feet.

Use sonotubes instead of bare-earth holes when you need a consistent diameter (especially in loose or sandy soil), when the concrete must extend above grade, or when building codes require a formed footing. For simple fence posts in stable clay or loam soil, bare-earth holes are usually fine. However, if your soil crumbles when you dig or you're in a frost-heavy region, sonotubes give you a precise, inspectable footing that meets code requirements.

Fence Post Concrete Guide

4×4 vs 6×6 posts: Standard privacy fences and picket fences use 4×4 posts with 10-inch diameter holes. Gate posts, corner posts, and posts for tall (8-foot) fences should be 6×6 with 16-inch holes for extra rigidity.

Hole depth rule: Bury at least 1/3 of the total post length underground. For a 6-foot privacy fence, you'll use an 8-foot post with 24–30 inches in the ground. For a 4-foot fence (6-foot post), dig at least 24 inches deep.

Fast-setting vs standard mix: Fast-setting concrete (like Quikrete Fast-Setting) hardens in 20–40 minutes and lets you hang fence panels the same day. Standard concrete mix takes 24–48 hours to set and 3–5 days to reach working strength. Use standard mix for structural applications or when you need time to adjust multiple posts along a string line.

Drainage gravel: Always add 4–6 inches of compacted gravel at the bottom of the hole before setting the post. Gravel allows water to drain away from the post base, preventing rot and frost heaving. Reduce your calculated concrete depth by the gravel layer thickness.

Worked Example: 20 Fence Posts

Let's calculate concrete for a typical residential fence project — 20 posts using 4×4 lumber with 10-inch diameter holes dug to 30 inches deep.

  1. Convert diameter to feet: 10" ÷ 12 = 0.833 ft → radius = 0.417 ft
  2. Convert depth to feet: 30" ÷ 12 = 2.5 ft
  3. Calculate volume per hole: π × 0.417² × 2.5 = π × 0.174 × 2.5 = 1.36 ft³
  4. Total for 20 posts: 1.36 × 20 = 27.27 ft³
  5. Add 10% waste factor: 27.27 × 1.10 = 30.0 ft³
  6. Convert to bags: 80lb bags yield ~0.6 ft³ each → 30.0 ÷ 0.6 = 50 bags of 80lb concrete

At roughly $5–$6 per 80lb bag, that's $250–$300 in concrete for a 20-post fence. Buying in bulk pallets (56 bags per pallet) often saves 10–15% at home improvement stores.

Pro Tips for Setting Posts in Concrete

  1. Dig the hole 3× the post width and at least 1/3 the total post length deep
  2. Add 4–6 inches of compacted gravel at the bottom for drainage
  3. Use fast-setting concrete for fence posts — it sets in 20–40 minutes
  4. Crown the concrete surface away from the post to shed rainwater
  5. Brace posts plumb with 2×4 stakes until the concrete fully sets
  6. Stop concrete 2–3 inches below grade and backfill with soil to hide the footing
  7. In cold climates, dig below the frost line (check local codes) to prevent heaving
  8. Pre-soak the hole with water in dry conditions so the soil doesn't wick moisture from the concrete prematurely

Post Hole Calculator FAQ

How much concrete do I need per fence post?
A standard 4×4 fence post in a 10-inch hole at 30 inches deep requires 2–3 bags of 80lb concrete or 3–4 bags of 60lb. For 6×6 posts in 16-inch holes at 36 inches deep, plan on 7–8 bags of 80lb. Always add 10% extra for waste and uneven holes.
How deep should a fence post be in concrete?
Bury at least 1/3 of the total post length underground. For a 6-foot fence using an 8-foot post, that means 24–30 inches below grade. In cold climates, dig below the local frost line (typically 36–48 inches) to prevent frost heaving.
Can I use fast-setting concrete for fence posts?
Yes — fast-setting concrete is ideal for fence posts. It sets in 20–40 minutes and reaches working strength in 4 hours, so you can hang panels the same day. Pour the dry mix into the hole, add water, and let it expand and harden. Use standard mix only if you need extra working time to align multiple posts.
How many bags of concrete for a sonotube?
It depends on the diameter and depth. An 8-inch sonotube at 4 feet deep needs 2–3 bags of 80lb concrete. A 12-inch sonotube at the same depth requires 5–6 bags. A 16-inch sonotube at 4 feet deep takes 10–11 bags of 80lb. Use the calculator above with your exact dimensions for a precise count.
Should I put gravel at the bottom of a post hole?
Yes. Add 4–6 inches of compacted gravel (¾-inch crushed stone works well) at the bottom of every post hole. Gravel creates a drainage layer that prevents water from pooling around the post base, which reduces rot and minimizes frost heaving in cold climates.
How long before I can attach fence panels?
With fast-setting concrete, you can attach fence panels or rails in 4 hours. Standard concrete mix requires at least 24–48 hours before putting lateral load on the posts, and 3–5 days for full cure strength. Avoid pushing or pulling posts during the curing period.

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