By Mike Richardson, construction estimator with 10+ years of experience.
Concrete Fill Calculator
Post holes, sonotubes, block cores & trenches
Table of Contents
What Is a Concrete Fill Calculator and Why Do You Need One?
A concrete fill calculator estimates how much concrete you need to fill enclosed voids — post holes, sonotubes, CMU block cores, and trenches. Unlike slab or footing calculators that estimate flat pours, a fill calculator handles the irregular, contained spaces where concrete is poured into an existing form or cavity.
Getting fill volumes right matters because:
- Post holes waste concrete fast. A 12-inch hole that is 36 inches deep holds only 0.087 cubic yards — but a 4×4 post inside it displaces part of that volume. Without subtracting the post, you will overbuy.
- Sonotube projects add up quickly. Ten 12-inch sonotubes at 48 inches deep need 2.6 cubic yards — enough to justify a ready-mix delivery instead of mixing 88+ bags by hand.
- CMU block fill is deceptive. A standard 8-inch CMU wall looks solid, but its hollow cores consume about 1.12 cubic yards of concrete per 100 square feet of wall face.
- Over-ordering means waste, under-ordering means cold joints. A cold joint from running out mid-pour weakens the structure permanently.
How Concrete Fill Calculations Work
Every fill calculation follows the same core formula: calculate the void volume, subtract any displacement (like a post inside a hole), multiply by the number of units, then convert to cubic yards and add waste.
Step 1: Determine the Void Shape
Post holes and sonotubes are cylinders: Volume = pi x radius squared x depth. Trenches are rectangular: Volume = length x width x depth. CMU block cores use a fixed volume per block based on the block width.
Step 2: Subtract Any Displacement
When setting a post in concrete, the post itself takes up space inside the hole. A 4×4 post (actual 3.5 x 3.5 inches) displaces about 0.0025 cubic yards in a 36-inch deep hole. A 6×6 post (actual 5.5 x 5.5 inches) displaces almost twice that. Always subtract the post volume from the hole volume.
Step 3: Multiply by Quantity
Most fill projects involve multiple identical voids — 8 post holes for a deck, 12 sonotubes for a pier foundation, or 200 blocks in a CMU wall. Multiply the single-void volume by the total count.
Step 4: Add a Waste Factor
Add 10% for post holes and trenches (spillage and uneven holes), and 5-10% for sonotubes (minimal waste since the tube is a precise form). CMU block fills need 5-10% extra for mortar voids and uneven core depths.
Step 5: Choose Bags or Ready-Mix
For projects under 1 cubic yard (about 45 bags of 80-lb mix), hand-mixing bags is practical. Between 1 and 3 cubic yards, consider renting a mixer trailer. Over 3 cubic yards, ready-mix delivery ($130-$170 per cubic yard) saves hours of labor and produces more consistent concrete.
Concrete Fill Volume by Application
| Application | Typical Dimensions | Volume per Unit | Concrete per 10 Units |
|---|---|---|---|
| Post Hole (10″ dia x 36″ deep, 4×4 post) | 10″ diameter, 36″ depth | 0.055 yd³ net | ~0.51 yd³ |
| Post Hole (12″ dia x 42″ deep, 6×6 post) | 12″ diameter, 42″ depth | 0.076 yd³ net | ~0.74 yd³ |
| Sonotube (12″ dia x 48″ deep) | 12″ diameter, 48″ depth | 0.116 yd³ | 1.16 yd³ |
| Sonotube (18″ dia x 48″ deep) | 18″ diameter, 48″ depth | 0.261 yd³ | 2.61 yd³ |
| 8″ CMU Block Core | 8″ x 8″ x 16″ block | 0.167 ft³ per block | 0.062 yd³ |
| Trench (6″ wide x 12″ deep x 10 ft long) | 6″ x 12″ x 10 ft | 0.185 yd³ | 1.85 yd³ |
Post Hole Concrete Fill Guide
Setting fence posts and deck posts in concrete is one of the most common DIY concrete projects. The rule of thumb is simple: dig the hole 3 times the post diameter and one-third of the total post length deep.
For a standard 6-foot privacy fence using 4×4 posts:
- Hole diameter: 10-12 inches (3x the 3.5-inch post width)
- Hole depth: 24-30 inches (one-third of 8-foot post, with 6 feet above ground)
- Concrete per hole: About 1.5–2 bags of 80-lb mix (after subtracting the post)
- Post spacing: 6-8 feet on center
For deck posts using 6×6 posts, increase the hole to 12-14 inches diameter and 36-48 inches deep (below frost line in cold climates). Each hole needs 1.5-3 bags of 80-lb mix depending on depth.
Sonotube and Round Column Fill Guide
Sonotubes (cardboard concrete forms) create smooth, round concrete piers for decks, porches, additions, and elevated structures. They come in diameters from 6 inches to 60 inches, though 8, 10, 12, and 16-inch tubes handle most residential projects.
Volume increases dramatically with diameter. A 12-inch sonotube at 48 inches deep needs 3.14 cubic feet (0.116 cubic yards) of concrete. Double the diameter to 24 inches and the volume quadruples to 12.57 cubic feet (0.465 cubic yards) — because volume scales with the square of the radius.
Key sonotube tips:
- Brace tubes plumb before pouring — once concrete is in, you cannot adjust.
- Pour in lifts for tubes over 4 feet deep to prevent air pockets.
- Vibrate or rod the concrete every 12 inches of pour depth.
- Set anchor bolts within 30 minutes of the final pour while concrete is still workable.
CMU Block Core Fill Guide
Filling CMU (concrete masonry unit) block cores with concrete and rebar is called grouting. Building codes require it for load-bearing walls, retaining walls, and walls in seismic zones. Even non-structural garden walls benefit from filled cores for wind resistance.
Core volume depends on block width:
| Block Width | Core Volume per Block | Blocks per 100 ft² Wall | Concrete per 100 ft² Wall |
|---|---|---|---|
| 6-inch CMU | 0.125 ft³ | 112.5 | 0.52 yd³ |
| 8-inch CMU | 0.167 ft³ | 112.5 | 0.70 yd³ |
| 10-inch CMU | 0.208 ft³ | 112.5 | 0.87 yd³ |
| 12-inch CMU | 0.250 ft³ | 112.5 | 1.04 yd³ |
You do not always fill every core. Partial grouting (filling only the cores with rebar) uses 30-50% less concrete. Our calculator lets you set a fill percentage to estimate partial grouting accurately.
Trench and Rectangular Void Fill Guide
Trenches filled with concrete serve as strip footings, grade beams, foundation stems, and utility encasements. The calculation is straightforward — length times width times depth — but trenches dug by machine are rarely perfectly rectangular, so always add 10-15% waste.
Common trench dimensions for residential foundations:
- Standard footing trench: 16-24 inches wide, 8-12 inches deep
- Frost-protected footing: 16-20 inches wide, 36-48 inches deep (below frost line)
- Grade beam: 12-16 inches wide, 18-24 inches deep
- Utility trench (encasing pipe): 6-8 inches wide, varies by depth
Bag Yield Reference: How Much Concrete per Bag?
| Bag Size | Weight | Yield (ft³) | Yield (yd³) | Bags per Cubic Yard |
|---|---|---|---|---|
| Large | 80 lb | 0.60 ft³ | 0.022 yd³ | 45 |
| Medium | 60 lb | 0.45 ft³ | 0.017 yd³ | 60 |
| Standard | 50 lb | 0.375 ft³ | 0.014 yd³ | 72 |
| Small | 40 lb | 0.30 ft³ | 0.011 yd³ | 90 |
One cubic yard of concrete weighs approximately 4,000 pounds. At $5-$7 per 80-lb bag, mixing one cubic yard from bags costs $225-$315 in material alone — compared to $130-$170 per yard for ready-mix delivery. Bags make economic sense only below about 0.75 cubic yards.
Concrete Fill Cost Estimator: Bags vs. Ready-Mix in 2026
| Project Size | Bag Cost (80-lb @ $6) | Ready-Mix Cost ($150/yd³) | Best Option |
|---|---|---|---|
| 0.25 yd³ (11 bags) | $66 | $150 + delivery | Bags |
| 0.50 yd³ (23 bags) | $138 | $150 + delivery | Bags (marginal) |
| 1.0 yd³ (45 bags) | $270 | $150 + delivery | Ready-mix |
| 2.0 yd³ (90 bags) | $540 | $300 + delivery | Ready-mix |
| 5.0 yd³ (225 bags) | $1,350 | $750 + delivery | Ready-mix |
Ready-mix delivery fees typically run $50-$150 depending on distance from the batch plant. Most plants have a minimum order of 1 cubic yard, and some charge a short-load fee for orders under 3-5 cubic yards ($50-$100 extra). Factor this into your decision.
10 Tips for Filling Concrete Voids Like a Pro
Check the frost line before digging post holes. Your local building department publishes the frost depth — footings and post bases must extend below it to prevent frost heave. In northern states, that means 36-48 inches deep.
Use high-early-strength mix for post holes. Products like Quikrete Fast-Setting Concrete set in 20-40 minutes, letting you brace posts and continue working the same day instead of waiting 24-48 hours.
Never dry-pack sonotubes. Some tutorials suggest pouring dry mix into sonotubes and adding water on top. This produces weak, honeycombed concrete. Always pre-mix to proper consistency before pouring.
Overfill sonotubes by 1 inch and crown the top. Mound concrete slightly above the tube rim and trowel a dome shape so water runs off the pier instead of pooling on top.
Fill CMU cores in lifts, not all at once. For walls taller than 4 feet, fill cores in 4-foot lifts and vibrate or rod each lift before adding the next. This prevents air pockets and ensures full consolidation around rebar.
Use grout, not standard concrete, for CMU cores. CMU grout (a fluid mix with smaller aggregate) flows into cores better than standard concrete mix. If using bagged concrete, add extra water beyond the normal ratio to improve flow — the block will absorb excess moisture.
Compact the bottom of post holes before pouring. Loose soil at the bottom compresses under load, causing the post to settle. Tamp the bottom with a 2×4 or tamping bar, then add 2-3 inches of gravel for drainage.
Slope concrete away from wood posts. Shape the top of the concrete so it crowns around the post, directing water away from the wood-to-concrete joint — the most rot-prone area.
Buy 10% more bags than calculated. Bag yields assume perfect mixing. Real-world conditions — spillage, irregular hole shapes, mixing losses — always consume more than the theoretical amount.
Time your pours for mild weather. Concrete sets too fast above 90 degrees F and too slow below 50 degrees F. Early morning pours in summer and midday pours in spring/fall give you the best working window.
Choosing the Right Concrete Mix for Fill Applications
| Mix Type | Strength (PSI) | Set Time | Best For | Price per 80-lb Bag |
|---|---|---|---|---|
| Standard Mix | 4,000 | 24-48 hrs | Sonotubes, footings, trenches | $5-$6 |
| Fast-Setting | 4,000 | 20-40 min | Post holes, fence posts | $6-$8 |
| High-Strength | 5,000+ | 24-48 hrs | Structural piers, load-bearing columns | $7-$9 |
| CMU Grout | 2,000-3,000 | 24-48 hrs | Block core fill | $5-$7 |
| Ready-Mix (delivered) | 3,000-5,000 | 24-48 hrs | Any project over 1 yd³ | $130-$170/yd³ |
For a standard 10-inch diameter hole that is 36 inches deep with a 4×4 post, you need about 0.055 cubic yards of concrete — roughly 3 bags of 80-lb fast-setting mix. A 12-inch hole at 42 inches deep with a 6×6 post needs about 2 bags. Our calculator subtracts the post volume automatically for an accurate estimate.
A typical 4×4 fence post set in a 10-inch hole at 24-30 inches deep needs 1 bag of 80-lb concrete mix. A 6×6 post in a 12-inch hole at 36 inches deep needs 2-3 bags. For a 50-foot fence with posts every 8 feet, that is 7 posts requiring 7-21 bags total depending on post size and hole depth.
A 12-inch sonotube at 48 inches deep needs 3.14 cubic feet (0.116 cubic yards) — about 6 bags of 80-lb mix. An 18-inch tube at the same depth needs 7.07 cubic feet (0.261 cubic yards) — about 12 bags. Volume increases with the square of the diameter, so doubling the tube size quadruples the concrete needed.
For a standard 8-inch CMU wall, you need about 0.70 cubic yards (0.167 cubic feet per block x 112.5 blocks) per 100 square feet of wall face. A 12-inch CMU wall needs about 1.04 cubic yards per 100 square feet. Partial grouting (filling only rebar cores) reduces this by 50-70%.
The general rule is one-third of the total post length. For a 6-foot fence using 8-foot posts, dig 24-30 inches deep. For deck posts or structural posts, dig to below the frost line (36-48 inches in northern states). The hole diameter should be 3 times the post width — so 10-12 inches for a 4×4 post.
You need 45 bags of 80-lb concrete mix to make one cubic yard. Each 80-lb bag yields 0.60 cubic feet, and one cubic yard equals 27 cubic feet (27 / 0.60 = 45). For 60-lb bags you need 60 bags, for 50-lb bags you need 72 bags, and for 40-lb bags you need 90 bags per cubic yard.
For projects under 0.5 cubic yards (about 23 bags), mixing bags yourself is cheaper at $130-$140 total. Over 1 cubic yard, ready-mix delivery at $130-$170 per yard plus $50-$150 delivery is cheaper and saves hours of labor. The break-even point is around 0.75 cubic yards — above that, order ready-mix.
You can, but CMU grout is better. Standard concrete mix has large aggregate that can bridge across narrow cores and leave voids. CMU grout uses smaller aggregate and a wetter mix that flows into cores more completely. If using bagged concrete, add extra water and use a vibrator or rebar rod to consolidate the mix into every void.
Most residential decks use 10 or 12-inch diameter sonotubes. For a standard deck (up to 200 sq ft with normal snow loads), 10-inch tubes are sufficient. For larger decks, hot tubs, or heavy snow areas, use 12-inch tubes. Check your local building code — many jurisdictions specify minimum pier diameter and depth below frost line.
Use the cylinder volume formula: Volume = pi x (diameter / 2)² x depth. Convert all measurements to feet first (divide inches by 12). For a 12-inch diameter hole that is 36 inches deep: 3.1416 x (0.5)² x 3.0 = 2.36 cubic feet. Divide by 27 to get cubic yards: 0.087 yd³. Our calculator does this automatically.
Fast-setting concrete (like Quikrete Fast-Setting) is ideal for post holes. It sets in 20-40 minutes so you can brace and align posts quickly, then continue fencing the same day. Regular mix works too but requires 24-48 hours of bracing. Fast-setting costs $1-$2 more per bag but saves significant labor time on multi-post projects.
Add 10% for post holes and trenches (hand-dug holes are rarely perfectly shaped). Add 5-10% for sonotubes (minimal waste since the form is precise). Add 5-10% for CMU block cores (mortar joint voids and uneven cores). For projects with irregular or rocky soil, increase to 15%. Our calculator includes an adjustable waste factor from 0% to 20%.
Not always. Building codes require full grouting for load-bearing walls, retaining walls, and walls in seismic zones. For non-structural walls (garden walls, privacy walls), you can partially grout — filling only the cores containing rebar, typically every 32-48 inches. This uses 30-50% less concrete while still meeting structural requirements.
One cubic yard of standard concrete weighs approximately 4,000 pounds (2 tons). This includes the cement, sand, gravel aggregate, and water. Lightweight concrete mixes weigh 2,800-3,200 lbs per cubic yard. For comparison, one cubic foot of concrete weighs about 150 pounds.
Running out mid-pour creates a cold joint — a weak plane where old and new concrete meet. For post holes, this is usually acceptable if you resume within 2 hours. For structural piers and footings, a cold joint can compromise load capacity. Always order 10% more than calculated, and for ready-mix orders, have a plan for where to pour any excess.