Exact 2026 calculation: cubic yards, bags, cost, base materials, and contractor tips for a 16x20 concrete slab at 4 inches thick.
A 16x20 concrete slab is a versatile size perfect for medium-sized patios, large shed bases, garage extensions, or workshop floors. This guide gives you the exact concrete calculation for a 16x20 slab at the standard 4-inch thickness, with variations for other thicknesses, bagged concrete options, cost estimates, base material requirements, and professional tips to avoid common mistakes.
This assumes a standard 4-inch slab. Thicker slabs require more material-see the table below for 5-inch and 6-inch options.
Use our concrete calculator for other sizes and thicknesses.
Let's walk through the math so you can double-check it yourself or adjust for your specific project:
Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)
For a 16x20 slab at 4 inches thick:
You'll order concrete by the cubic yard (1 yard = 27 cubic feet).
Calculation: 106.67 / 27 = 3.95 cubic yards
Rounded: 3.95 cubic yards (suppliers often round to 4.0 yards)
In practice, you'll need extra concrete for:
A good rule of thumb: 10% for experienced DIYers, 15% for beginners.
Calculation with 10% waste: 3.95 × 1.10 = 4.35 cubic yards
Rounded up: 4.35 cubic yards (typically ordered as 4.5 yards).
Not all slabs are 4 inches thick. Here's a quick reference table for a 16x20 slab at common thicknesses:
| Thickness | Cubic Feet | Cubic Yards | Cubic Yards (10% waste) | 80-lb Bags Needed |
|---|---|---|---|---|
| 3 inches | 80 cubic feet | 2.96 cubic yards | 3.26 cubic yards | 133 bags |
| 4 inches | 106.67 cubic feet | 3.95 cubic yards | 4.35 cubic yards | 178 bags |
| 5 inches | 133.33 cubic feet | 4.94 cubic yards | 5.43 cubic yards | 222 bags |
| 6 inches | 160 cubic feet | 5.93 cubic yards | 6.52 cubic yards | 267 bags |
Note: Bag counts are without waste factor. Add 10% for waste.
In practice, most DIYers and even some pros need to adjust the textbook calculations:
Pro tip: Schedule your concrete delivery for the morning when temperatures are cooler and you have a full day to work.
Here's a realistic 2026 cost breakdown for a 16x20 concrete slab at 4 inches thick:
| Item | Quantity | Cost Range | Notes |
|---|---|---|---|
| Concrete (ready-mix) | 4.5 yards | $520-$680 | Includes short-load fee |
| Gravel base | 6-8 tons | $180-$320 | 4-6 inches compacted |
| Rebar or wire mesh | 320 sq ft | $130-$260 | Optional but recommended |
| Form lumber | 72 linear feet | $50-$100 | 2x4s, stakes, screws |
| Total materials (DIY) | - | $880-$1,360 | Before labor/equipment |
Labor cost: Professional installation adds $4-$8 per square foot = $1,280-$2,560.
Total project cost: $1,050-$2,000 (DIY) or $2,160-$3,920 (professional).
For a 16x20 slab at 4 inches thick, you'll need approximately 178 bags of 80-lb concrete (without waste). With a 10% waste factor, that increases to 196 bags. If using 60-lb bags, you'll need 238 bags (or 262 with waste).
For most patios, walkways, and shed bases, yes-4 inches is sufficient. However, if you plan to park vehicles (cars, trucks, RVs) or place heavy machinery, you should increase to 5-6 inches. A 4-inch slab can crack under heavy loads or with poor subgrade preparation.
For a typical storage shed (no vehicles), 4 inches is adequate. For a workshop shed with heavy tools or a garage shed for lawn equipment, consider 5-6 inches. Always check your shed manufacturer's recommendations.
Total project cost ranges from $1,050-$2,000 for DIY (materials only) to $2,160-$3,920 for professional installation. Concrete alone costs $520-$680 for 4.5 yards.
Yes, reinforcement (rebar or wire mesh) is highly recommended for slabs this size. It prevents cracking from ground movement and temperature changes. Plan for 320 sq ft of wire mesh or rebar grid at 16-24 inch spacing.
Concrete reaches 70% strength in 7 days and full strength in 28 days. You can walk on it after 24-48 hours, but avoid heavy loads for 7 days and vehicles for 28 days. Proper curing (keeping moist) is critical for strength.
For more detailed calculations and related projects:
Use our free Concrete Calculator for precise measurements tailored to your specific project. Enter any dimensions, thickness, and waste factor for instant cubic yards and bag counts.
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