How to Pour a Small Concrete Slab (Shed Base, AC Pad, or Utility Pad)

This guide is specifically for slabs under 100 square feet (about 1.25 cubic yards). If your project is larger, see our complete first concrete pour guide for ready-mix delivery considerations.

What You Need

Materials

Item	Notes
80-lb bagged concrete mix	See bag count table below
2×4 lumber (form boards)	Perimeter length ÷ 8 = number of 8-foot boards
Wood stakes (18")	1 stake per 3–4 linear feet of form
Deck screws (2.5")	One box
Gravel (3/4" crushed stone)	4" base = ~0.5 tons per 100 sq ft
Wire mesh (6×6 W1.4)	One sheet per 25 sq ft
Mesh chairs or bricks	Lifts mesh to center of slab
6-mil poly sheeting	Curing cover

Tools

• Wheelbarrow and concrete hoe
• Plate compactor (rent: $50–75/half day)
• Bull float or darby
• Screed board (straight 2×4)
• Edging tool
• Hand float
• 4-foot level
• Tape measure

How Many Bags Do You Need?

Use the table below, then add 10% for waste:

Slab Size	Thickness	Cubic Yards	80-lb Bags
4×4 ft	4"	0.20	9
4×8 ft	4"	0.40	18
6×6 ft	4"	0.44	20
6×8 ft	4"	0.59	27
8×8 ft	4"	0.79	36
8×10 ft	4"	0.99	45
10×10 ft	4"	1.23	56

Open the full Concrete Slab Calculator →

Step-by-Step

Step 1: Lay Out and Excavate

Mark corners with stakes and string lines. Check for square using the 3-4-5 triangle method (measure 3 feet one direction, 4 feet perpendicular — the diagonal should be 5 feet).

Excavate to accommodate your gravel base plus slab thickness. For a standard 4" slab on 4" of gravel, dig 8 inches below your desired finished height. Remove all topsoil and organic material.

Step 2: Compact the Subgrade

This is the step most beginners skip — and it's why slabs crack. Run a plate compactor over the excavated soil in overlapping passes until the surface feels solid underfoot. See our subgrade preparation guide for what a properly prepared base looks like.

Spread gravel in 2-inch layers, compacting each layer before adding the next. The finished gravel surface should be flat and firm.

Step 3: Build the Forms

Cut 2×4s to your slab dimensions. Stake them outside your layout lines so the inside face of the lumber defines the slab edge. Screw (don't nail) stakes to boards every 3–4 feet.

Set a drainage slope: the form boards on the low side should be 1/8" per foot lower than the high side. Check with a level.

Apply cooking spray or form release oil to the inside of all form boards.

Place wire mesh on bricks or mesh chairs so it sits at mid-depth (2 inches for a 4-inch slab). Overlap mesh sections by 6 inches and tie with wire.

Step 4: Mix and Pour

For slabs under 0.5 cubic yards (about 20 bags), mix in a wheelbarrow in batches. Add water first, then dry mix. Target a stiff, workable consistency — not soupy.

For slabs 0.5–1.25 cubic yards (20–56 bags), rent a concrete mixer. Feed bags in one at a time and batch continuously. Have a helper screed finished sections while you mix the next batch.

Pour concrete starting from the far corner and work toward your exit. Don't walk on the mix — dump from the wheelbarrow and spread with a square shovel.

See our concrete mixing guide for exact technique and the water ratio guide for common mix mistakes.

Step 5: Screed

Immediately after filling each section, rest a straight 2×4 across both form boards and pull it toward you in a sawing motion. Fill low spots and re-screed. The goal is a surface level with the tops of the forms.

Step 6: Finish

After screeding, float the surface with a bull float to push aggregate down and bring paste up. Wait for bleed water to disappear (surface goes from shiny to matte — 20–60 minutes depending on temperature). Never finish while bleed water is visible.

Run the edging tool along all form edges. Cut a control joint across the middle of any slab longer than 8 feet. Drag a broom in straight passes for a slip-resistant finish.

Step 7: Cure

Cover with 6-mil plastic sheeting within 1 hour of finishing. Weigh edges down to prevent wind lifting. Keep covered for 7 days, misting if the weather is hot.

Drainage Slope Quick Reference

Slab width	Minimum height difference (1/8" per foot)
4 ft	1/2 inch
6 ft	3/4 inch
8 ft	1 inch
10 ft	1-1/4 inch

Common Mistakes

Pouring on uncompacted base. The most common cause of small slab failure. Always compact the subgrade and base gravel — don't assume it's firm enough. Rent the plate compactor.

Mixing too wet. Extra water feels helpful while pouring but permanently weakens the slab. Target the consistency shown on the bag instructions.

No drainage slope. Water pooling on a flat slab wicks under the base over time, eroding support and causing cracking. Even a slight slope prevents this.

Related Guides

• First Concrete Pour Guide — Full beginner walkthrough including ready-mix options
• Concrete Subgrade Preparation — Base preparation step-by-step
• How to Build Concrete Forms — Form construction and staking technique
• Concrete Fire Pit Pad — A small slab variation for outdoor fire pits