Fence post concrete bags and volume
Fence Post Concrete Calculator
Answer first: a 10-inch diameter, 3-foot deep fence post hole needs about 2 bags of 80 lb concrete. This calculator gives you the exact bag count per hole and total bags for your fence project, with cost estimate, so you can buy the right amount at the hardware store.
10-in hole, 3 ft deep (standard fence post)
2 bags of 80 lb concrete per hole
Most common residential fence post. Volume: 1.64 ft³. At 0.6 ft³ per 80 lb bag, that is 2.7 bags — round up to 3 for a little extra.
12-in hole, 36 in deep (deck post)
3–4 bags of 80 lb concrete per hole
Deck posts typically use larger holes and go deeper for stability. Add 1 bag to your calculation as a buffer.
20 fence posts (typical backyard fence)
40–60 bags of 80 lb concrete
10-in holes, 3 ft deep = 2 bags each = 40 bags minimum. Add 10–15% for holes that run a little bigger than planned.
Fast-setting concrete (Quikrete Fast-Setting)
Same bag count, no mixing required
Pour dry into the hole, add water. Sets in 20–40 minutes. Same volume per bag as standard mix. Costs $1–$2 more per bag but saves mixing time.
Interactive workflow
Run the numbers, then save or export the project
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How many bags of concrete do I need for fence posts?
Quick reference: 10-in hole, 2.5 ft deep = 1–2 bags of 80 lb. 10-in hole, 3 ft deep = 2–3 bags. 12-in hole, 3 ft deep = 3–4 bags. For frost-line depth (4 ft) at 8-in hole = 3 bags of 80 lb (Quikrete QuikTube spec). The Quikrete website says 4 bags for most holes — real DIYers say 1–2 is enough for shorter holes. The right answer depends on hole depth, diameter, and your frost line.
What is the difference between fast-setting (red bag) and regular (yellow bag) concrete for fence posts?
Fast-setting (Quikrete Fast-Set, red bag): pours dry into hole, add water on top — no mixing. Sets in 20–40 minutes, hard enough to build on same day. Costs $1–$2 more per bag. Regular concrete (Quikrete 5000, yellow bag): mix with water or add water in hole. Sets in 24–48 hours. Cheaper. Reddit contractors: "80 posts in one day with yellow bags — add water going into the hole dry, rock hard tomorrow." Use red if building same day, yellow if returning next day.
What size hole should I dig for a 4×4 fence post?
The rule of thumb: hole diameter = 3× the post diameter. A 4×4 post is actually 3.5 inches wide, so dig a 10–11 inch diameter hole. A 6×6 post: 16–18 inch diameter. The hole should be ⅓ of the total post length deep — for a 6 ft fence with 8 ft posts, dig 30–32 inches deep. Add 6 inches of gravel at the bottom for drainage to prevent post rot.
Should I use concrete or expandable foam for fence posts?
TikTok's most-watched fence test (I set a 4×4 the right way): tested concrete, expandable foam, and no-mix dry pour side by side. Results: concrete is the strongest and cheapest. Expandable foam ($6–$10/can per hole) is fast but weaker. Foam is good for fixing leaning posts without digging. For new fence construction, concrete is the standard. Foam is a repair tool, not a foundation.
How deep should fence posts be in concrete?
Fence posts should be buried ⅓ to ½ of their total length. For a 6 ft fence: use 8 ft posts, buried 2–2.5 ft minimum (deeper in cold climates). In frost-prone areas, go below the frost line: SE Wisconsin = 4 ft, Minnesota = 5+ ft, Texas = 12–18 in, Florida = 12 in. Shallow posts above the frost line will heave in winter. The concrete should extend to the soil surface (or slightly above) to shed water.
Is the dry pour (no-mix) method for fence posts reliable?
Yes — pouring dry concrete into the hole and adding water on top works well. The concrete pulls moisture from the soil and water poured in. Reddit fence builders: "Yellow bag poured dry with water added in hole — set overnight, rock hard." Quikrete's Fast-Setting product is specifically designed for dry pour. Regular Quikrete can also be dry poured but takes longer to set. Make sure the hole is damp first and add water slowly on top.
How many bags of concrete for a 6-foot privacy fence?
A 6-ft privacy fence calculation: 100 ft of fence = about 25 posts (8 ft spacing). Each 10-in, 30-in deep hole needs 2–3 bags of 80 lb concrete. Total: 50–75 bags. With 8-ft spacing, 10-in holes at 30 in deep: 2 bags each = 50 bags total. If you use 6-ft spacing (more posts): 17–18 posts = 34–50 bags. Buy 10–15% extra for holes that go slightly bigger than planned.
What is the cubic feet per bag of concrete for fence posts?
Bag yields: 40 lb bag = 0.30 ft³. 50 lb bag = 0.37 ft³. 60 lb bag = 0.45 ft³. 80 lb bag = 0.60 ft³. Formula: hole volume (ft³) = π × (radius in ft)² × depth (ft). Then subtract the post volume and divide by bag yield. Example: 10-in hole, 30-in deep = π × 0.42² × 2.5 = 1.37 ft³ - 0.03 ft³ post ÷ 0.60 (80-lb bag) = 2.2 bags. Round up to 3.
Should I put gravel at the bottom of a fence post hole?
Yes — always put 4–6 inches of crushed stone or gravel at the bottom of the hole before setting the post. This allows water to drain away from the post base, preventing rot and extending the life of wood posts dramatically. Without drainage, water pools at the bottom, the wood stays wet, and rot starts within 5–10 years. Gravel drainage can double the life of a pressure-treated post.
How many 80 lb bags of concrete for a fence post?
At 0.60 ft³ per 80 lb bag: for a 10-in hole at 2.5 ft = 1.14 ft³ ÷ 0.60 = 1.9 bags → use 2. For 10-in hole at 3 ft = 1.37 ft³ ÷ 0.60 = 2.3 bags → use 3. For 12-in hole at 3 ft = 1.97 ft³ ÷ 0.60 = 3.3 bags → use 4. For frost-depth holes (4 ft) at 8-in = 1.26 ft³ ÷ 0.60 = 2.1 bags → use 3 (Quikrete QuikTube spec confirms). 80 lb bags are the most economical per cubic foot.
What concrete should I use for fence posts — Quikrete or Sakrete?
Both Quikrete and Sakrete are equivalent in strength and price. For fence posts, choose by set time: Quikrete FastSet (red bag) or Sakrete FastSet — sets in 20–40 min, same-day building. Quikrete 5000 (yellow bag) or Sakrete 5000 — standard, overnight set. Both brands yield similar ft³ per bag. Quikrete is slightly more available at Home Depot, Sakrete at Lowe's. The brand matters less than the product type (fast-set vs standard).
How long does concrete take to set around fence posts?
Fast-setting concrete (red bag): walkable in 20–40 minutes, can build fence in 4 hours, fully cured in 24–48 hours. Standard concrete (yellow bag): hard enough to build on in 24 hours, fully cured in 7 days (28-day full strength). For fence building, you need posts to be stable enough to attach fence boards — fast-set lets you do this same day. Do not stress posts (hang fence boards) for at least 24 hours even with fast-set.
Planning disclaimer
Bag counts assume standard hole dimensions and 10% waste. Actual usage varies with soil conditions, post size, and hole irregularity. Always buy 10–15% extra.