How Much Concrete Do I Need? A DIY Volume Guide

Whether you're pouring a patio, setting fence posts, or building a shed foundation, the first question is always the same: how much concrete do I need? Order too little and you're stuck with a half-finished pour that won't cure properly. Order too much from a ready-mix truck and you're paying to dispose of the excess. Neither is fun.

The math itself is simple — it's volume, and you probably haven't thought about volume since high school. But the difference between getting it right and getting it wrong can be hundreds of dollars and a wasted weekend. Let's walk through the formula, some common project sizes, and the practical stuff they don't print on the bag.

The Formula

Concrete volume is measured in cubic yards in the US (or cubic meters elsewhere). The formula for a rectangular slab is:

Volume (cubic yards) = Length (ft) × Width (ft) × Thickness (ft) ÷ 27

The ÷ 27 converts cubic feet to cubic yards (since 3 × 3 × 3 = 27 cubic feet per cubic yard).

The most common mistake here is forgetting to convert thickness from inches to feet. A 4-inch slab is 4 ÷ 12 = 0.333 feet, not 4 feet. That error would give you 12 times too much concrete.

Let's do a real example. You're pouring a 12 × 14-foot patio at 4 inches thick:

1. Convert thickness: 4 inches ÷ 12 = 0.333 feet
2. Calculate volume: 12 × 14 × 0.333 = 55.94 cubic feet
3. Convert to cubic yards: 55.94 ÷ 27 = 2.07 cubic yards

Add 10% for the extra rule (more on that below), and your order is about 2.3 cubic yards.

Quick Reference Table

Here are pre-calculated volumes for common patio and slab sizes at 4-inch thickness — the standard for patios and sidewalks. The bag count assumes 80-lb bags of pre-mixed concrete, each yielding about 0.6 cubic feet.

These numbers don't include the 5–10% extra buffer — so if the table says 56 bags, you'd actually want 60–62 bags on hand. For a 5-inch driveway slab, multiply the cubic-yards column by 1.25 to adjust for the added thickness.

Calculating for Footings and Post Holes

Not every concrete project is a flat slab. Footings and post holes are cylinders, so you need a different formula:

Volume (cubic yards) = π × r² × h ÷ 27

Where r is the radius in feet and h is the depth in feet. Remember: radius is half the diameter, and you need to convert inches to feet first.

Let's calculate for a common fence post hole — 10 inches in diameter, 36 inches deep:

1. Radius: 10 ÷ 2 = 5 inches = 0.417 feet
2. Depth: 36 inches = 3 feet
3. Volume: 3.14159 × 0.417² × 3 = 1.64 cubic feet
4. In cubic yards: 1.64 ÷ 27 = 0.061 cubic yards

That's roughly one 80-lb bag per hole (each bag yields 0.6 cu ft, and the post itself displaces some of that volume). Here's a quick reference for common post hole sizes:

These numbers include the full hole volume — the post itself will displace some concrete, so you'll use slightly less per hole in practice. But it's better to have an extra bag on the pile than to be one bag short on hole number 23 of 24.

For continuous strip footings (like a garage or shed perimeter), treat them as rectangular slabs: length × width × depth ÷ 27. A 40-foot perimeter footing that's 16 inches wide and 12 inches deep would be: 40 × 1.333 × 1.0 ÷ 27 = 1.98 cubic yards.

Bags vs Ready-Mix

You have two options for getting concrete: pre-mixed bags you hand-mix yourself, or ready-mix delivered by truck. The decision comes down to volume, time, and your back.

Pre-mixed bags

• Best for: Small projects under 1 cubic yard (roughly 45 bags or fewer).
• Cost: About $5–7 per 80-lb bag, or roughly $225–315 per cubic yard of material.
• Pros: Buy only what you need. Mix at your own pace. No delivery minimum or scheduling required.
• Cons: Physically demanding. Mixing 50+ bags by hand is a brutal day. Quality can vary if your water ratios aren't consistent.

Ready-mix truck delivery

• Best for: Projects over 1 cubic yard, or any pour where timing and consistency matter.
• Cost: About $125–175 per cubic yard delivered, plus a short-load fee (typically $30–60 per yard) if you order less than the truck minimum (usually 3–5 yards depending on your area).
• Pros: Consistent mix quality. Much faster — the truck does the mixing. A 3-yard pour takes 30 minutes instead of 3 hours.
• Cons: Minimum order requirements. You need to be ready the moment the truck arrives. Excess concrete is your problem.

The breakeven point is usually around 1 cubic yard. Below that, bags are cheaper and simpler. Above it, the labor savings of ready-mix outweigh the higher per-yard cost — especially once you factor in the value of your time and your spine.

The 5–10% Extra Rule

Here's a rule every experienced concrete worker follows: always order 5–10% more than your calculated volume. Here's why:

• The ground isn't level. Even a well-prepared sub-base has dips and variations. A spot that's 1/2 inch deeper than planned across a 10×10-foot area adds 4 cubic feet of concrete you didn't account for.
• Forms flex. Wooden forms bow outward slightly under the weight of wet concrete. That extra 1/4 inch across a 20-foot form adds up fast.
• Spillage happens. Concrete drops off the chute, overflows the wheelbarrow, or splashes out of the form. It's not a lot, but it's not zero.
• You can't undo a short pour. If you run out of concrete mid-pour, you get a cold joint — a weak seam where fresh concrete meets partially cured concrete. Cold joints are structural weak points and they look terrible. This is the one thing you want to avoid above all else.

For a standard slab on flat, well-prepped ground, 5% extra is usually sufficient. For uneven terrain, deep footings, or your first DIY pour, go with 10%. The extra cost is trivial compared to the cost of a failed pour.

Standard Thickness by Project

Using the wrong thickness is one of the fastest ways to waste a concrete project. Too thin and the slab cracks under load. Too thick and you've overspent on material for no structural benefit. Here are the standard thicknesses for common residential projects:

Notice that driveways need 25–50% more thickness than patios. That extra inch or two means significantly more concrete. A 12×20-foot driveway at 5 inches requires 3.7 cubic yards — almost a full yard more than the same slab at 4 inches (2.96 cu yd). Don't try to save money by going thinner than recommended; a cracked driveway costs far more to replace than the concrete you "saved."

If you're unsure about thickness for your specific project, check your local building code. Frost lines, soil type, and expected loads all factor in, and your municipality may require specific minimums.

Frequently Asked Questions

How many bags of concrete do I need for a 10×10 slab?

At 4 inches thick, a 10×10-foot slab requires about 1.23 cubic yards of concrete — that's roughly 56 80-lb bags. Add 10% for waste and you're looking at about 62 bags. At that quantity, it's worth calling a ready-mix supplier for a price quote, since you're right at the crossover point where truck delivery becomes practical.

How much concrete do I need for a fence post hole?

For a standard 4×4 post in a 10-inch wide, 36-inch deep hole, you need about 1.6 cubic feet of concrete — roughly one 80-lb bag per hole after accounting for the post displacing some volume. For a 6×6 post in a 12-inch wide, 42-inch deep hole, plan on 2 bags per hole. Multiply by your number of posts and add a few extra bags.

What is the difference between 60-lb and 80-lb bags of concrete?

Same mix, different quantity. An 80-lb bag yields about 0.6 cubic feet of concrete; a 60-lb bag yields about 0.45 cubic feet. The 80-lb bags cost less per cubic foot and mean fewer bags to handle overall. But if you're working alone or have a bad back, the 60-lb bags are much easier to carry and dump into a mixer. Either way, the finished concrete is identical.

Should I order extra concrete beyond my calculated amount?

Yes, every time. Add 5% for straightforward slabs on well-prepared ground, and 10% for uneven sites, first-time pours, or projects with footings. The ground is never perfectly flat, forms flex under pressure, and running short mid-pour creates a cold joint — a structural weak point that's far more expensive to fix than a few extra bags of concrete.

Last updated: March 2026