How Much Concrete Does a Shed Foundation Actually Need? The Numbers Most People Get Wrong

A concrete pad calculator solves three problems that trip up almost every first-time project: wrong thickness for the load, not enough material ordered, and no overage buffer. Three easy mistakes. All of them solved before you drive to the hardware store or pick up the phone.

Pads look simple — a rectangle, four sides, fill it and wait. The errors happen before the first bag opens.

The Thickness Decision Comes Before Everything Else

Thickness is not a cosmetic choice. It's a structural one, and it's determined by what the pad will carry.

• Standard patio: 4 inches minimum
• Parking pad or driveway approach: 6 inches minimum
• Equipment pad (generator, AC unit, hot tub): 6 inches minimum
• Shed foundation with no vehicle access: 4 inches is adequate

A 4-inch pad under a standard backyard shed holds fine. That same 4-inch pad under a riding mower that gets driven on regularly is undersized. When in doubt, pour 6 inches — the material cost difference on small pads is minor, and the structural difference is not.

On a 12×16 shed foundation: 4 inches requires 2.37 cubic yards. Six inches requires 3.56 cubic yards. The difference is $178 in material at average prices. The difference in performance over 20 years is not comparable.

The Real-World Numbers for Common Pad Sizes

Common pad sizes at 4-inch thickness:

The 10% overage rule applies to every number in that list. A 12×12 patio orders at 1.96 yards, not 1.78. A 20×20 orders at 5.43 yards, not 4.94. Running short on a pad pour — especially a small one you mixed yourself — means stopping, buying more material, and resuming a pour that has already begun to set in the forms. That creates a cold joint: a visible seam and a structural weak point.

A 20×20 pad requires 296 bags of 60-pound concrete. At that volume, bags are a full day of physical labor. This is where the bags-vs-readymix decision becomes critical.

Calculate your pad dimensions and bag count →

Bags vs. Readymix — Where This Decision Lives for Pads

Most pad projects land right on the bags-vs-readymix border. A 12×12 patio at 1.78 cubic yards is doable with bags if you have a mixer or a strong back and a Saturday. A 20×20 pad at 4.94 yards is a readymix project.

The practical threshold:

• Under 1 cubic yard — bags, no question
• 1 to 2 cubic yards — bags are possible; readymix is faster and cleaner
• Over 2 cubic yards — order readymix

One cost factor that changes the math: the short load fee. If your pad project falls under a supplier's minimum order (typically 5 to 8 yards), you'll pay a surcharge of $50 to $150 on top of the per-yard price. Factor that into the comparison. A 2-yard project with a $150 short load fee and $300 delivery might cost nearly as much as buying bags — especially for a 12×12 patio where bags are genuinely practical.

Pouring Larger Pads in Sections

For patios over roughly 200 square feet, pouring in sections is common practice. This approach lets you work with manageable volumes at a time, is compatible with bags, and gives you natural control joint locations where the sections meet.

The section boundaries become the control joints. On a 20×20 patio, dividing into four 10×10 sections gives you joints every 10 feet in both directions — right at the recommended maximum panel size for crack control.

If pouring in sections with readymix, coordinate timing carefully. Each section should be fully placed and screeded before the next truck arrives. Cold joints between sections that have partially cured create a different kind of seam than a planned control joint.

The 10% Overage Rule — Why It Exists

Concrete calculations assume perfectly level forms, consistent subgrade, and zero spillage. None of those assumptions are true on a real job site.

A slightly low area in the subgrade adds volume. A form that runs a quarter-inch deep across 20 feet adds volume. Spillage during placement adds to the waste. The 10% buffer covers all of it.

Every professional concrete finisher adds overage before calling for material. The cost of an extra 10% is always less than the cost of a second delivery or a cold joint in a finished pad.

Frequently Asked Questions

How do you calculate how much concrete you need for a patio?

Use the formula: cubic yards = (length × width × (depth ÷ 12)) ÷ 27. A 12×16 patio at 4 inches: 12 × 16 × 0.333 = 63.96 cubic feet ÷ 27 = 2.37 cubic yards. Add 10% overage and order 2.61 cubic yards. At 60 bags per cubic yard, the bag count is 142 before overage.

How many yards of concrete do I need for a 20×20 pad?

A 20×20 pad at 4 inches thick requires 4.94 cubic yards. With 10% overage, the order is 5.43 cubic yards. At 6 inches thick, the same pad requires 7.41 cubic yards, ordering 8.15 yards with overage. The 4-inch figure is appropriate for standard patio use; 6 inches is required for any parking or vehicle load.

Should I use bags or readymix for a concrete pad?

Below 1 cubic yard, bags are practical. The 10×10 pad at 1.23 yards sits right at the decision point — manageable with bags and a mixer, but readymix is faster. Above 2 cubic yards, readymix is the right call for quality and efficiency. Factor in the short load fee when comparing — a 2-yard project with a $150 short load fee narrows the gap.

What thickness do I need for a concrete pad for a shed?

Four inches is the standard for a shed foundation without vehicle access. If a riding mower or cart will be driven into the shed regularly, 6 inches is more appropriate. The material cost difference on a 10×12 shed foundation is approximately $50 at average readymix prices — a minor upgrade for meaningfully better performance.

How does pouring a pad in sections affect the concrete calculation?

Pouring in sections doesn't change the total volume — the math is the same. What changes is how you order and time the material. Each section should be calculated separately with its own 10% overage, and the pours should be timed so each section is placed fully before the adjacent section begins, creating a clean control joint at the boundary.