Formula
Cubic feet = length (ft) × width (ft) × depth (in) ÷ 12; cubic yards = cubic feet ÷ 27. Bag count uses about 0.6 cubic feet per 80 lb bag — check your bag label, as yields vary by mix. Example: a 10 ft × 4 ft slab at 4 in deep needs 13.3 cubic feet, or 0.49 cubic yards, about 23 bags.
How to use it, step by step
Measure the slab area and the depth you plan to pour — 4 inches suits most garden paths and shed bases, while driveways need a deeper slab and reinforcement. Calculate volume first, then decide bags vs ready-mix: bags suit small jobs, ready-mix suits slabs over about 1 cubic yard, because mixing that much by hand is slow and the first batch starts setting before the last is poured.
Worked example in detail
A shed base 10 ft × 8 ft at 4 inches deep needs 10 × 8 × 4 ÷ 12 = 26.7 cubic feet, or 0.99 cubic yards — just under 1 yard, about 45 × 80 lb bags. Ordering ready-mix for just under 1 yard may incur a small-load charge, so compare the bag total (including your time) with the ready-mix delivered price before deciding.
What affects real concrete needs
Sub-base, formwork and waste all matter. A slab poured on uneven ground uses more concrete in low spots. Formwork that bulges under the weight of wet concrete also increases volume. Order about 10% extra for slabs, and have a use ready for any surplus — a small path or stepping stones — because wet concrete cannot be stored.
Frequently asked questions
How thick should a garden path be?
About 4 inches for foot traffic, with a compacted gravel base underneath. Thinner slabs crack under frost and root movement.
Bags or ready-mix?
Bags for under about 0.5 cubic yards, ready-mix above about 1 cubic yard. Between those, compare delivered price, your time, and whether you can pour and finish the whole slab before it starts setting.
Do I need reinforcement?
For a small shed base, usually not if the base is well compacted. For larger slabs and driveways, mesh or rebar helps control cracking — follow local building guidance for structural slabs.