Concrete Slab Calculator

Calculate the volume, number of bags and cost of concrete for slabs, driveways, patios and floors — enter length, width and thickness.

Slab dimensions

ft
ft
in
Typical slab: 4 in (102 mm) residential · 6 in (152 mm) driveway
%
10 % covers spillage and slight over-pour on most jobs.
$

Calculation results

Volume to order

— yd³

Bags needed

— bags

Slab area

— ft²

Volume (no waste)

— yd³

Bag yields from manufacturer data sheets (Quikrete/Sakrete). Order one extra bag per 50 to cover a torn bag or dropped load.

Informational only. These results are estimates for planning and ordering. For structural slabs, consult a licensed engineer for thickness, reinforcement and subgrade requirements.

Understanding the formula

Three steps turn a slab drawing into a concrete order. First compute the net volume, then add the waste, then convert to bags.

V = L × W × (T / 12)  ·  ordered = V × (1 + waste %)  ·  bags = ⌈ ordered / yield ⌉

Thickness is entered in inches because that’s how residential slabs are specified — the calculator divides by 12 to convert to feet before multiplying. Ordered volume in cubic yards is V_ft³ ÷ 27.

Worked example

A 20 × 20 ft driveway at 4-in thickness, 10 % waste, 80 lb bags:

  • Volume = 20 × 20 × (4 / 12) = 133.3 ft³ = 4.94 yd³
  • Ordered = 133.3 × 1.10 = 146.7 ft³ = 5.43 yd³
  • Bags = ⌈ 146.7 / 0.60 ⌉ = 245 bags

At 5.4 yd³, ready-mix almost always wins — mixing 245 bags by hand is a multi-day project and the truck price is usually comparable or lower.

When to use this calculator

  • Residential floor slab: basement floor, garage floor (4-in standard).
  • Driveway: 4 in for cars; 6 in for heavy trucks or RVs.
  • Patio or walkway: 3–4 in for foot traffic only.
  • Equipment pad: HVAC compressor, generator or hot-tub pad — consult the equipment manual for thickness.
  • Pool deck: typically 4 in, but slope and drainage require an engineering review.

This calculator handles rectangular slabs only. For L-shaped or irregular slabs, break the shape into rectangles, calculate each, and sum the results. Don’t subtract for anchor bolts or dowels — they’re a rounding error against the waste factor.

Common mistakes & tips

  • Always order extra. 10 % waste is the industry consensus minimum. On a large pour or a hot, dry day, bump to 12–15 %.
  • Confirm subgrade before ordering. An uneven sub-base can add 1–2 inches of effective thickness across the whole slab — a significant volume difference.
  • Switch to ready-mix above 1 yd³. One cubic yard of bagged concrete is about 45 × 80-lb bags. Above that, the truck almost always wins on cost and curing quality.
  • Reinforce slabs correctly. A concrete slab without rebar or wire mesh will crack. The calculator only handles volume — use the Rebar Calculator to size the reinforcement grid.
  • Allow for cure time. Never load a slab before 28 days of curing, and minimize foot traffic in the first 24 hours.

Frequently asked questions

How much does a 12 × 12 concrete slab cost?
A 12 × 12 ft slab at 4 in thick is about 1.78 yd³ of concrete. At $150/yd³ for ready-mix plus $400–600 for labour and finishing, expect $600–900 total installed. Using 80-lb bags (about 80 bags) adds material cost of roughly $400 at $5/bag, but doubles the labour.
How many bags of concrete for a 10 × 10 slab?
A 10 × 10 ft slab at 4 in thick needs about 37 ft³ of concrete (1.37 yd³ with 10 % waste). At 0.60 ft³ per 80-lb bag, that’s 62 bags. At 0.45 ft³ per 60-lb bag, that’s 83 bags.
How thick should a concrete slab be?
4 in (100 mm) is standard for residential floors, patios and walkways. Driveways that take heavy trucks or RVs should be 6 in. Pool decks and structural slabs may need an engineer-specified thickness.
How many bags of 80 lb concrete per cubic yard?
An 80 lb bag yields 0.60 ft³. One cubic yard is 27 ft³, so 27 / 0.60 = 45 bags per yard.
When should I use ready-mix instead of bags?
Most contractors switch to ready-mix above 1 yd³ (45 × 80-lb bags). Below that threshold, bags are convenient. Between 1 and 3 yd³, compare quotes — the minimum-load surcharge on a small ready-mix order can tip the math either way.
Can I pour a concrete slab myself?
Yes, for small slabs (patio, walkway, small shed floor). For anything load-bearing, large, or subject to frost heave, hire a licensed contractor. Key DIY tips: compact the subbase, set forms square and level, order 10 % extra concrete, and have helpers ready — concrete cannot wait.
Do I need rebar in a concrete slab?
Yes for most slabs. At minimum, 6-in × 6-in welded wire mesh provides crack control. Structural slabs (garage floors, driveways) should have #4 rebar on 18-in centres. Use the Rebar Calculator to size the grid.