Concrete Block & Post Hole Calculator
Use our Concrete Block & Post Hole Calculator to estimate how much concrete you need for block walls, fence posts, deck posts, or other small foundation projects.
Table of Contents
How much concrete do I need to fill a block wall or a post hole?
To fill the cores of 100 standard 8×8×16 CMU blocks, you need about 0.93 cubic yards of concrete (25 cu ft, at 0.25 cu ft per block). For a 12-inch sonotube or post hole 4 feet deep, that’s roughly 0.12 cubic yards — about 6 bags of 80 lb mix. A set of 5 concrete steps (12″ run, 7″ rise, 36″ wide) runs close to 0.4 cubic yards. Pick your project below for an exact number.
You need
135 blocks
120 sq ft wall, 8″ CMU
- Core fill volume1.25 cu yd
- 80 lb bags for fill57 bags
- Mortar (approx.)5 bags
You need
0.12 cu yd
3.14 cu ft, per tube
- Total for all tubes/holes0.12 cu yd
- 80 lb bags needed6 bags
- 60 lb bags needed8 bags
You need
0.40 cu yd
10.7 cu ft, solid stacked steps
- 80 lb bags needed17 bags
- Total rise (top of stairs)35 in
In this guide
Why these projects trip people up
You may have been in this situation: you're setting fence posts, building a small retaining wall, or pouring a set of porch steps, and the concrete math suddenly isn't the simple "length × width × height" you remember from a slab. Block walls have hollow cores. Post holes are round, not square. Steps aren't one shape — they're a stack of shapes getting smaller as they go up.
So you either overbuy out of caution and end up with unused bags hardening in the garage, or you underbuy and make a second trip to the store mid-project. This calculator handles all three shapes with the actual formulas contractors use, so you order the right amount the first time.
How much concrete to fill a block wall?
Standard 8"×8"×16" concrete blocks (CMUs) have hollow cores that need filling with concrete for any load-bearing or retaining wall. Here's the method.
The block fill method
- Count your blocks. A standard 8×8×16 block covers about 0.89 sq ft of wall face, which works out to roughly 1.125 blocks per square foot.
- Find the core volume. A standard 8" block has about 0.25 cubic feet of hollow core (both cells combined). Smaller and larger blocks scale up or down from there.
- Decide how much to fill. Structural and retaining walls need every core filled. Some non-structural applications only fill every other core — check your local code before choosing.
- Multiply and add waste. Blocks × core volume × fill percentage gives your concrete volume. Add 5–10% for spillage and mortar joint absorption.
Switch to the Block Fill tab in the calculator above to run this automatically for your wall size and block thickness.
Sonotubes and post holes
What if your project is round instead of rectangular? Sonotubes, fence post holes, and deck footings are all cylinders, so they use one formula: volume = π × radius² × height. The trick is just keeping your units consistent — diameter is usually in inches, but concrete is ordered in cubic feet or yards, so convert to feet first.
What if the soil is soft or frost heave is a concern? Add a belled footing at the base — a wider flare that increases bearing area without widening the whole shaft. The calculator's bell footing option handles this using the frustum (truncated cone) formula automatically.
Concrete steps
First, count your steps and measure the rise (vertical height) and run (horizontal depth) of each one — most codes cap rise around 7¾" and require at least 10–11" of run. Then treat each step as a solid slice of concrete running all the way down to the ground, like a stack of shrinking rectangular blocks.
What if you don't want to do that math by hand? The pattern is: volume = width × run × rise × (1 + 2 + 3 + ... + number of steps). The Steps tab in the calculator does this instantly — just enter your step count, rise, run, and width.
A worked example
Let's run the numbers for a 12-inch sonotube, 4 feet deep, no bell footing.
Step-by-step
- Radius: 12 ÷ 12 ÷ 2 = 0.5 ft.
- Volume: π × 0.5² × 4 = 3.14 cubic feet.
- Convert to yards: 3.14 ÷ 27 ≈ 0.116 cu yd.
- With 10% waste: 0.116 × 1.10 ≈ 0.128 cu yd.
- In 80 lb bags (0.6 cu ft yield): 3.14 × 1.10 ÷ 0.6 ≈ 6 bags.
That's a manageable, single-bag-mixer job for one post. Multiply by however many posts your fence or deck needs, and the calculator's quantity field does that math for you automatically.
Building a fence, deck, or retaining wall?
Get your exact numbers in the calculator above →| Project | Shape | Key formula |
|---|---|---|
| Block wall fill | Hollow rectangular cores | Blocks × core volume × fill % |
| Sonotube / post hole | Cylinder | π × radius² × height |
| Belled footing | Frustum (truncated cone) | π × height ÷ 3 × (R² + Rr + r²) |
| Concrete steps | Stacked rectangular slices | Width × run × rise × step-sum |
Common mistakes to avoid
- ✕Using the hole diameter instead of the tube's inside diameter. The concrete only fills the tube's interior — measure that, not the excavated hole.
- ✕Forgetting the frost line. A footing that sits above the local frost depth can heave and crack over winter — always check your code before setting depth.
- ✕Guessing at block core volume. Core size varies by block thickness and manufacturer — a 6" block holds far less than a 12" block, so don't reuse one number for every wall.
- ✕Skipping the landing in a step calculation. If your top step doubles as a porch landing, it needs to be measured and poured as its own solid block, not just another step.
Why trust this guide
Formulas on this page follow standard industry references — NCMA and ASTM C90 for CMU core volumes, the standard cylinder and frustum formulas for round footings, and the stacked-rectangle method commonly used for solid concrete steps. Always confirm block core dimensions and frost depth requirements with your local supplier and building department before ordering.
Frequently Asked Questions
Related Article: Concrete Mix Ratio Calculator
