Concrete Slab Calculator — Volume, Bags & Thickened Edges
Calculate concrete for a slab, patio, shed base or garage floor from length, width and thickness, including thickened edges, waste, premix bags and cost.
Use this slab calculator for patios, shed bases, garage floors, driveways and any other flat pour. It gives the volume to order in cubic yards or cubic metres, the floor area, how far one yard or metre goes at your thickness, and the number of premix bags. If the slab has a thickened edge — a deeper strip around the perimeter poured with the slab, common under garage and shed walls — enter its width and depth and the extra concrete is added.
How to use this tool
- Choose US or metric units.
- Enter the slab length, width and thickness.
- For a thickened edge, enter how wide the deeper strip is and the full depth at the edge; leave the width at 0 for a flat slab.
- Set the waste allowance and, optionally, the ready-mix price.
- Compare other thicknesses in the table below the results.
Formula
V = L × W × T + [L × W − (L − 2e) × (W − 2e)] × (D − T)
L and W are the slab length and width, T is the slab thickness, e is the width of the thickened edge and D is the full depth at the edge. The second term is the ring of extra concrete under the perimeter and is zero for a flat slab. The waste percentage is added on top.
Example
A 20 × 12 ft patio at 4 in covers 240 ft² and holds 80 ft³, or 2.96 yd³. With 10% extra you would order 3.26 yd³ — or 147 bags of 80 lb premix. A 24 × 24 ft garage slab 4 in thick with a 12 in wide edge that is 12 in deep needs 192 ft³ for the slab plus 92 ft² × 8 in = 61.3 ft³ for the edge: 253.3 ft³, or 9.38 yd³ before waste.
Frequently asked questions
How much area does a yard of concrete cover?
At 4 in thick, one cubic yard covers 81 ft²; at 6 in it covers 54 ft². In metric, one cubic metre covers 10 m² at 10 cm thick.
Is it cheaper to use bags or ready-mix?
Compare your supplier’s delivered price and any short-load charge with premix cost and handling. A 3 yd³ pour takes 135 bags at 0.60 ft³ per bag before any waste allowance.
Where do I measure the thickness?
From the top of the compacted gravel base to the top of the forms. If the base is uneven, measure in several places, use the average and raise the waste allowance.
Does a thicker slab need much more concrete?
For a flat slab with the same footprint, increasing thickness from 4 to 6 in increases volume by 50%. With thickened edges, recalculate the entire geometry because the edge depth is entered separately.
Sources
Last updated: 2026-10-30