Fencing

How Many Bags of Concrete per Fence Post?

Fence length does not set bags—the hole does. Use the concrete section of the fence materials calculator for the whole post count, or the concrete bag calculator in post-hole mode for a one-off dug footing. Cardboard form tubes (Sonotube®-style) use the Sonotube concrete calculator. This guide is the annular math written out so you can audit a “two bags each” habit against the geometry you actually dug.

Annular volume

Convert diameters and depth to feet, then:

hole = π × depth_ft × (hole_radius_ft)²
post = (for square) depth_ft × (post_width_ft)²
net = max(0, hole − post) × post_count

Round posts use a circular post displacement instead. That is the same geometry the site’s concrete library uses for fence and standalone post-hole modes. If someone quotes bags from fence feet alone, they skipped this step.

Worked single-post example

Assumptions (visible, not universal): 4 in square post, 12 in hole (matches a 3× Quikrete-style baseline on a 4 in post), 36 in embedment, 10% concrete overage (trade convention for spill and uneven holes).

Yields are from Quikrete Concrete Mix data sheet No. 1101 (verified in this site’s materials JSON). Sakrete high-strength mix lists the same three yields for those bag weights—agreement is why those sizes ship on the tool. Where brands disagree on other bag weights, we leave them out rather than invent a blend.

Scaling to many posts

Ten identical holes are not ten times a rounded single-post bag count if you round each hole separately and then multiply. The honest approach is volume first, then ceil once:

10 posts → net ≈ 20.23 ft³ → +10% ≈ 22.25 ft³ → 38 × 80 lb bags

The fence calculator multiplies net volume by post count, applies waste, then converts to bags. That matches how you should check a pallet: total cubic feet, not a superstition about “always two bags.”

Deeper holes change the answer

Same 12 in hole and 4 in square post, but 42 in embedment (frost-driven deeper set) with 10% waste:

net ≈ 2.360 ft³ → +10% ≈ 2.596 ft³ → 5 × 80 lb

Determine the governing embedment first with the Fence Post Depth Calculator. Bag count then follows the depth you actually plan to pour. For the longer explanation, see the depth guide.

Gravel, gates, and local changes

Quikrete’s setting-posts guide adds 6 inches of gravel below the post. The fence calculator shows excavation including gravel but does not pour concrete into that gravel depth. Gate posts with larger holes need their own takeoff. Over-augered diameters raise volume even when depth stays fixed.

Slab and footing bag math (different geometry) is in how many bags of concrete and the footing companion guide when you need a continuous footing instead of post holes.

Hole diameter is the quiet multiplier

Quikrete’s general post guidance points at a hole about three times the post width. On a 4 in post that is the 12 in example above. Move to a 14 in or 16 in auger “because the soil was soft” and the annular volume jumps even when embedment never changed. Chain-link footings in our materials data follow a larger CLFMI-style diameter multiplier on round posts — again, diameter dominates bag count more than people expect when they only remember fence length. Measure the auger you will use, not the auger you meant to rent.

If you are checking a quote that says “two bags per post,” ask what hole and depth they assumed. Two bags of 80 lb mix is 1.2 ft³ of yield before waste — fine for some holes, short for deeper frost sets or wide gate footings. The worked numbers on this page exist so that conversation is about geometry, not superstition.

Mixing bag sizes on one fence

Crews sometimes start with 80 lb bags and finish with leftover 60 lb stock. That is fine if you recompute remaining volume with the correct yield. What fails is averaging yields or assuming “a bag is a bag.” Keep a running cubic-foot total from the holes still open, then divide by the yield of the bag in your hand. The fence calculator lets you pick one bag size for the whole takeoff; for mixed leftover stock, fall back to the standalone concrete tool’s post-hole mode for the remainder.

Ready-mix still wins when total volume climbs toward a yard — same planning threshold as the slab guide. A long privacy line with deep frost holes gets there faster than a short decorative run. When you are unsure, total the cubic feet from the fence calculator’s working, divide by twenty-seven for yards, and price both a pallet and a short load before the holes are open.