Fence Post Concrete Calculator
Find how many bags of Quikrete (or any premix) you need per fence post hole. Subtracts post displacement and gravel base, then rounds bags up per hole so you leave the store with the right count — not a second trip mid-pour.
What Is a Fence Post Concrete Calculator?
A fence post concrete calculator (also called a post hole concrete calculator) tells you how many bags of premix — like Quikrete Fast-Setting or standard 60/80 lb bags — you need to set fence posts, gate posts, mailbox posts, and deck piers. It turns hole diameter, depth, post size, and bag yield into a store-ready bag count so you are not guessing at Home Depot or Lowe's.
Unlike a generic slab calculator, this tool models a post inside a round hole: it subtracts the wood or metal post, optional gravel drainage base, and rounds bags up per hole because leftover mix in an open bag rarely transfers cleanly to the next hole.
How to Use This Quikrete Fence Post Calculator
- Pick a preset (privacy fence, frost-line depth, gate post, mailbox) or enter custom hole and post sizes.
- Set gravel base if you add 4–6 inches of stone for drainage under wood posts.
- Choose bag size — 80 lb is most common; 60 lb bags are easier to lift solo.
- Enter post count and optional price to get total bags, haul weight, and estimated cost.
Post Hole Concrete Formula
Bags/hole = ceil( concrete × (1 + waste) ÷ bag yield )
r = hole radius in feet · h = concrete depth in feet (hole depth − gravel) · bag yield defaults: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 50 lb ≈ 0.375 ft³, 40 lb ≈ 0.30 ft³.
Always confirm the yield printed on your specific Quikrete or Sakrete bag — brands can vary slightly.
Worked Example: How Many Bags for a 4×4 Fence Post?
You are setting a 4×4 post (actual 3.5×3.5 in) in a 10-inch diameter hole, 30 inches deep, with 6 inches of gravel. Concrete depth = 24 in = 2 ft.
- Hole volume ≈ π × (5/12)² × 2 ≈ 1.09 ft³
- Post displacement ≈ (3.5/12)² × 2 ≈ 0.17 ft³
- Net concrete ≈ 0.92 ft³ (+10% waste ≈ 1.01 ft³)
- At 0.60 ft³ per 80 lb bag → 2 bags per hole after rounding up
For 12 posts, buy 24 bags of 80 lb Quikrete — not 22 from dividing the grand total, which would leave some holes short.
Quick Reference: Bags of Concrete Per Fence Post
| Setup | Hole | 80 lb bags* |
|---|---|---|
| 4×4 privacy | 10″ × 24″ | 1–2 |
| 4×4 standard | 10″ × 30″ | 2 |
| 4×4 frost line | 10″ × 36″ | 2–3 |
| 6×6 gate / corner | 12″ × 36″ | 3–4 |
| 2″ metal / chain link | 8″ × 24″ | 1 |
*Approximate with ~6″ gravel base and 10% waste. Run the calculator above for your exact numbers.
Common Use Cases
- How many bags of concrete per fence post for privacy or picket fencing
- Quikrete calculator for fence posts before a Home Depot run
- Gate post and corner post bag counts (larger holes, more bags)
- Mailbox, sign, and deck pier post hole concrete estimates
- Comparing 60 lb vs 80 lb bag counts and total lift weight
Frequently Asked Questions
How many bags of concrete per fence post?
A standard 4×4 in a 10-inch, 24–30 inch hole usually needs 1–2 bags of 80 lb Quikrete (or 2–3 of 60 lb). Deeper frost-line holes and 6×6 gate posts need more — use the calculator for your exact hole.
How much concrete for a 4x4 fence post?
For a 4×4 (actual 3.5×3.5 in) in a 10×30 inch hole with a 6-inch gravel base, expect about 2 bags of 80 lb mix after per-hole rounding.
How do I calculate post hole concrete volume?
Volume = π × r² × concrete depth − post volume. Concrete depth is hole depth minus gravel. Divide by bag yield and round up per hole.
How many 80 lb bags of Quikrete for fence posts?
Multiply bags per post by post count. Example: 2 bags/post × 20 posts = 40 bags of 80 lb Quikrete for the project.
Should I subtract the post volume from the hole?
Yes. Skipping post displacement over-orders by roughly 10–25%. This calculator subtracts actual post cross-section automatically.
Why round bags up per hole instead of the project total?
You cannot reliably save a partial bag for the next hole. Per-hole rounding matches what you actually open on site.
How deep should a fence post hole be?
About ⅓ of above-ground height, and below the local frost line in cold climates (often 30–42 inches). Confirm with local code for permanent fences.
Fast-setting Quikrete or regular mix for fence posts?
Fast-setting post mix sets in minutes and is popular for fence posts. Regular mix is cheaper on large jobs but needs longer cure time. Check the bag label for exact yield.
How much gravel under a fence post before concrete?
Many DIYers use 4–6 inches of gravel for drainage under wood posts. Subtract that depth before calculating concrete.
Is this fence post concrete calculator free?
Yes — free, no signup, 100% in your browser. Safe to use on a job site from your phone.