Calculate Yards of Cement for Fence Posts, Mailbox & Deck Anchors (2026 Guide)
Published on 2026-06-26
Calculate Yards of Cement for Fence Posts, Mailbox & Deck Anchors
When you need to calculate yards of cement for fence posts, mailbox anchors, or deck footings, the math is different from a flat slab. You are filling cylindrical holes, not rectangles. This guide gives you the exact formulas, conversion tables, and pro tips so you order the right amount - without wasting money on extra material or making multiple hardware store runs.
Quick Answer
For a standard 4×4 fence post in a 10" diameter hole that is 3 feet deep, you need approximately 0.064 cubic yards of cement per post. That means:
- 16 fence posts: ~1 cubic yard
- 10 fence posts: ~0.64 yards (order 0.75 yards or use bags)
- 6 mailbox/deck posts: ~0.38 yards (4–6 bags each depending on hole size)
For anything over 15 posts, ordering a single cubic yard from a ready-mix supplier usually saves time and money compared to bags.
Step-by-Step Volume Calculation for Cylindrical Post Holes
The formula for a cylinder is:
Volume = π × radius² × depth
Step 1: Measure the hole diameter. Standard post holes are 8" (for 4×4 posts) or 12" (for 6×6 posts or heavy-duty applications).
Step 2: Convert radius to feet.
- 8" hole: radius = 4" = 0.33 feet
- 10" hole: radius = 5" = 0.42 feet
- 12" hole: radius = 6" = 0.50 feet
Step 3: Convert depth to feet. Standard frost-line depths vary:
- Shallow (24"): Non-load-bearing fences in warm climates
- Standard (36"): Most residential fences, code in 30+ states
- Deep (42"–48"): Frost-line regions, load-bearing deck footings
Step 4: Apply the formula. Example for a 10" hole, 36" deep:
- Volume = 3.1416 × (0.42 ft)² × 3 ft
- Volume = 3.1416 × 0.176 × 3
- Volume = 1.66 cubic feet
- Convert to cubic yards: 1.66 / 27 = 0.061 cubic yards per post
Cement Yield Per Bag - What You Actually Get
Understanding bag yield is critical when you calculate yards of cement for a project:
- 40-lb bag: 0.30 cubic feet (0.011 yd³)
- 60-lb bag: 0.45 cubic feet (0.017 yd³)
- 80-lb bag: 0.60 cubic feet (0.022 yd³)
To fill one 10" × 36" post hole (0.061 yd³):
- 80-lb bags: 0.061 / 0.022 = 2.8 bags → 3 bags per post
- 60-lb bags: 0.061 / 0.017 = 3.6 bags → 4 bags per post
- 40-lb bags: 0.061 / 0.011 = 5.5 bags → 6 bags per post
Calculate Yards of Cement by Post Type and Size
| Post Type | Hole Diameter | Depth | Yards Per Post | Bags (80-lb) |
|---|---|---|---|---|
| 4×4 Fence Post | 8" | 30" | 0.043 | 2 |
| 4×4 Fence Post | 10" | 36" | 0.061 | 3 |
| 4×4 Fence Post | 10" | 42" | 0.072 | 4 |
| 4×4 Fence Post | 12" | 36" | 0.087 | 4 |
| 6×6 Deck Anchor | 12" | 42" | 0.116 | 6 |
| 6×6 Deck Anchor | 12" | 48" | 0.132 | 6 |
| 6×6 Deck Anchor | 14" | 42" | 0.158 | 8 |
| Mailbox Post (4×4) | 10" | 30" | 0.051 | 3 |
| Mailbox Post (4×4) | 12" | 36" | 0.087 | 4 |
Note: Add 10% waste factor for rocky soil, over-digging, or bottom gravel pads.
How Many Posts Per Cubic Yard?
This is the most practical question when you calculate yards of cement for a fence project:
| Post Configuration | Posts Per Cubic Yard | Recommended Order for 20 Posts |
|---|---|---|
| 4×4 post, 8" hole, 30" deep | ~23 per yard | 1 yard (slightly over, good for waste) |
| 4×4 post, 10" hole, 36" deep | ~16 per yard | 1.25 yards or 60 bags |
| 4×4 post, 10" hole, 42" deep | ~14 per yard | 1.5 yards or 70 bags |
| 4×4 post, 12" hole, 36" deep | ~11 per yard | 2 yards or 82 bags |
| 6×6 post, 12" hole, 42" deep | ~8.6 per yards | 2.5 yards or 108 bags |
| 6×6 post, 14" hole, 42" deep | ~6.3 per yard | 3 yards or 130 bags |
Fence Line Calculation: Full Project Example
You are building a 150-linear-foot privacy fence at 6-foot sections. That is 27 post locations (25 sections + 2 end posts). Using 10" holes at 36" deep:
- 27 posts × 0.061 yd³ = 1.65 cubic yards
- Add 10% waste: 1.65 × 1.10 = 1.81 yards → order 2 yards
- Cost at $180/yard (2026 national average): 2 × $180 = $360 for concrete alone
Without waste factor: $297. With waste factor: $360. That $63 difference buys you peace of mind against the alternative - a $200 short-load fee plus a second delivery.
Bag vs. Ready-Mix for Post Hole Projects
The break-even point for post holes is different from flatwork:
- Fewer than 10 posts (10" holes, 36" deep): Bags are better. You need ~30 bags at $6 each = $180. A ready-mix truck charges $150–$200 minimum delivery for a partial yard anyway.
- 10–20 posts: Close call. Bags: ~$300–$400. Ready-mix: $250–$350 including delivery. Ready-mix wins on labor savings alone. More than 20 posts: Always go ready-mix. A single truck delivery saves you hours of mixing. Even at $200 minimum order, you come out ahead on labor value.
For post holes specifically, you can also rent a post hole auger ($50/day) and a mixer ($40/day) if you insist on bags for a large project. But most homeowners find that one ready-mix delivery for $200–$300 is significantly less work than mixing 60+ bags by hand.
Pro Tips to Calculate Yards of Cement Accurately for Post Holes
- The post displaces concrete: A 4×4 post in a 10" hole takes up 0.014 yd³ of space. This reduces your concrete needed by ~20% per hole. However, for simplicity, most contractors ignore this small offset because waste factor more than compensates.
- Gravel pad at the bottom: Always put 4–6" of gravel at the bottom of each hole for drainage. This reduces concrete volume by 0.01–0.02 yd³ per post - that is 2–4 fewer bags total. Adjust your calculation accordingly.
- Soil expansion matters: Clay soils create wider holes than the auger diameter due to soil bounce. Sandy soil creates narrower holes. If you are in heavy clay, add 10% more concrete to your yard calculation.
- Frost heave prevention: In frost regions, the bottom of the hole must be below the frost line. Some inspectors require the full depth below frost - meaning if your frost line is 42" and you need the post 36" below grade, the hole must be 42" + 6" gravel = 48" deep. Always check local code.
- Don't forget the concrete surround above grade: For deck footings, the concrete often rises 2–4" above ground level to create a pedestal. This adds ~0.005 yd³ per post - small but adds up across 12+ posts.
- Concrete set time is 30–40 minutes: With a full truck delivery for 20+ posts, you need to be ready to work fast. Have all posts set in holes, plumbed, and braced BEFORE the truck arrives. Any delay means the mix starts setting in the truck.
Using Our Concrete Calculator Yards Tool for Post Holes
Our free concrete yardage calculator includes a dedicated post-hole mode. Just enter the hole diameter, depth, and number of posts, and it outputs total cubic yards and bag counts - including the waste factor and gravel pad adjustment. No manual math needed.
For complex projects combining flatwork and posts (for example, a new patio with fence posts around the perimeter), calculate each section separately: rectangles in the slab mode, cylinders in the post mode, then sum the totals. Our tool does this automatically when you add multiple shapes.
FAQ: Common Questions When You Calculate Yards of Cement for Posts
How many 80-lb bags of concrete do I need for a fence post?
For a standard 4×4 fence post in a 10" diameter hole, 36" deep, you need 3 bags of 80-lb concrete mix per post. Some contractors use 2 bags and fill the rest with backfill, but for structural integrity (wind load, gate posts, corner posts), always use the full 3 bags.
Can I use fast-setting concrete for fence posts?
Yes. Fast-setting mixes (like Quikrete Fast-Setting) come in 50-lb bags and set in 20–40 minutes without mixing - just pour dry into the hole and add water. They cost 30% more per bag but eliminate the mixing step. For 20+ posts, the time savings is significant. Calculate yards of cement the same way; volume does not change, only bag weight differs.
How do I calculate yards of cement for mixed post sizes?
Calculate each post size separately:
- Count corner/end posts (typically 6×6, 12" holes) → calculate total yards
- Count line posts (typically 4×4, 8–10" holes) → calculate total yards
- Count gate posts (typically 6×6, 12" holes, deeper) → calculate total yards
- Sum the three subtotals, then add 10% waste
What happens if I order too little concrete for post holes?
Each incomplete post becomes a structural failure point. You must dig out and reset the post - that is hours of labor. Always round UP to the next half-yard when ordering ready-mix. For bags, always buy 2 extra per trip. Running short by one bag on post #15 of a 30-post fence is a 45-minute round trip to the hardware store that ruins your entire workday.
Should I dig holes deeper than the post?
No. The concrete should surround the post embedment zone, with the post buried 1/3 of its total length (e.g., a 9-foot post needs 3 feet in the ground). Going deeper wastes concrete and can push the post below the frost line depending on your local code. The exception: posts in high-wind zones or supporting heavy gates may need 40–50% embedment.
Order Your Concrete Now - Get the Exact Yardage
Ready to calculate yards of cement for your fence, mailbox, or deck project? Use our free concrete yardage calculator - plug in your hole dimensions and post count, and get your exact cubic yard total and bag count in seconds.
Explore more: Concrete cost calculator guide | Truck load calculator | Round slab calculator
Calculate Your Post Hole Concrete Now
Use our free concrete yardage calculator - select "Post Holes" mode, enter your hole dimensions and count, and get your total cubic yards instantly.