Post-Hole Concrete Calculator
Concrete bags per fence-post hole — diameter, depth, post displacement, and bulk-vs-bag recommendation.
Typical: 8 in for fence, 12 in for deck
Below frost line. 30 in standard fence; 42 in cold-climate deck.
4×4 actual: 3.5 in. 6×6 actual: 5.5 in.
Usually = hole depth minus 6 in gravel base
5% covers small over-dig + spillage
Result
- Volume per hole (net of post)
- 0.74 ft³
- Total volume
- 9.31 ft³ · 0.34 yd³
- 60 lb bags needed
- 21
- Bag total cost
- $116
- Bulk delivery costIncludes $150 short-load fee
- $240
- Recommendation
- Buy bags
This estimate is based on national average costs and may vary by region, project specifics, and market conditions. Use as a starting point for your bids.
How much concrete for post holes?
The math is volume of a cylinder: π × r² × depth (in feet) gives you cubic feet per hole. Multiply by post count for total volume. The calculator above subtracts the post's own volume from the hole so you only buy concrete that actually fills the void.
Standard hole dimensions
| Use | Diameter | Depth |
|---|---|---|
| 4×4 PT fence post (6 ft fence) | 8 in | 30 in |
| 4×4 PT gate post or corner | 10 in | 36 in |
| 6×6 deck post (under-5-ft deck) | 12 in | 36 in |
| 6×6 deck post (taller deck) | 12 in | 42 in |
| Mailbox post (4×4) | 8 in | 24 in |
Bag yields and pricing
- 60 lb bag: yields ~0.45 ft³. Retail $4.50–6.50.
- 80 lb bag: yields ~0.60 ft³. Retail $5.50–8.00.
- Bulk truck: $140–220 per cubic yard, with $150 short-load fee under 1.25 yd³.
Crossover point from bags to bulk is roughly 18 60-lb bags (1 yd³). Below that, bags are simpler. Above that, bulk wins on time and back strain.
The 6 in gravel base
Always put 6 in of compacted gravel under the post before setting in concrete. Two reasons: (1) it drains water away from the bottom of the post so the wood doesn't rot, and (2) it prevents the concrete from sealing the post into a frost-heave cup that lifts the post over winter. The calculator depth assumes you've already accounted for the gravel layer separately.
Frequently asked questions
How much concrete per fence post hole?
For a typical 8 in diameter × 30 in deep hole with a 4×4 post, you need about 1.5 60-lb bags (or 1 80-lb bag). A 10 in diameter hole takes ~2 60-lb bags. A 12 in diameter × 36 in deep hole takes 3 60-lb bags. The calculator above subtracts the post's volume from the hole so you only buy the concrete that actually fills the void.
How deep should a post hole be?
Standard residential rule: bury one-third of the post + 6 in of gravel below. For a 6 ft tall fence on an 8 ft post, that's 30 in deep + 6 in gravel. In freeze zones (Zone 5 and north), go below the frost line — often 36–42 in. Deck posts and gate posts go 6 in deeper than line posts.
60 lb vs 80 lb bags for setting posts?
80 lb bags are cheaper per cubic foot ($5.50 vs $4.50 — 80 lb wins by ~12%) but harder to carry to remote post locations. For 10 or fewer posts, the savings often don't justify the back strain. For 20+ posts on a clear site, 80 lb bags save meaningful money.
When does bulk concrete delivery make sense?
About 1 cubic yard (27 ft³) — roughly 18 60-lb bags or 50 post holes at standard dimensions. Below that, bags are simpler than coordinating a truck. Above 1 yd³, factor in: delivery is faster on site, you avoid carrying 50+ bags, and most ready-mix plants do small loads (1–3 yd³) for a short-load fee around $150.
Do I need gravel under the post?
Yes — 6 in of compacted gravel below the post serves two functions: it drains water away from the bottom of the post (preventing rot) and prevents the concrete from sealing the post into a frost-heave cup. Skipping gravel is one of the most common reasons fence posts rot from the bottom up.
Related calculators
Concrete Pour
NewEstimate concrete by cubic yard, bag count (60 lb + 80 lb), and total material + labor cost for slabs, footings, driveways, sidewalks, and pads.
Fence
NewFence material takeoff — posts, panels, footings, and gates by linear foot for wood, vinyl, chain-link, or aluminum.
Waste Factor
NewCalculate trade-specific waste percentages so you order enough material without over-buying.