Concrete Slab Bag Calculator
Find out how many bags of pre-mixed concrete you need to pour a slab. Enter the slab length, width, and thickness, pick your bag size, and add a waste allowance to get the exact bag count for footings, patios, shed bases, and walkways.
Last updated: May 2026
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About this calculator
This calculator converts your slab dimensions into a concrete volume and then into the number of pre-mixed bags required. The formula is: Bags = ⌈ (Length × Width × (Thickness ÷ 12) × (1 + Waste ÷ 100)) ÷ BagYield ⌉. Length and width are in feet and thickness is entered in inches, so dividing thickness by 12 converts it to feet and gives the raw volume in cubic feet. The waste multiplier (1 + Waste ÷ 100) adds material for spillage, uneven subgrade, and over-excavation — 5–10% is typical for a well-prepared base. Dividing the adjusted volume by the per-bag yield (0.30 ft³ for a 40 lb bag, 0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb) gives the bag count, and the ceiling function rounds up because you can only buy whole bags. For large pours, compare this bag total against the cost of ready-mix delivery, which usually wins above roughly 1 cubic yard (27 ft³).
How to use
Suppose you are pouring a 10 ft × 10 ft shed base, 4 inches thick, using 80 lb bags (0.60 ft³ yield) with a 10% waste allowance. Step 1 — Convert thickness: 4 ÷ 12 = 0.333 ft. Step 2 — Raw volume: 10 × 10 × 0.333 = 33.33 ft³. Step 3 — Add waste: 33.33 × (1 + 10 ÷ 100) = 33.33 × 1.10 = 36.67 ft³. Step 4 — Divide by bag yield: 36.67 ÷ 0.60 = 61.1 bags. Step 5 — Round up: you need 62 bags of 80 lb concrete. Switching to 60 lb bags (0.45 ft³) would instead require 82 bags.
Frequently asked questions
How many 80 lb bags of concrete do I need for a 10x10 slab?
For a 10 ft × 10 ft slab poured 4 inches thick, you need about 33.3 cubic feet of concrete, or roughly 37 cubic feet once a 10% waste allowance is added. Since each 80 lb bag yields about 0.60 cubic feet, that works out to 62 bags. If you pour the same slab 6 inches thick, the volume rises by half and you would need about 92 bags, which is the point where ordering ready-mix concrete usually becomes cheaper than mixing bags by hand.
How much waste allowance should I add when buying bagged concrete?
A waste allowance of 5–10% is standard for slab work. The extra material covers concrete lost to spillage during mixing and pouring, an uneven or over-excavated subgrade that is deeper than planned in spots, and the small amount that sticks to tools and the mixer. For footings poured against rough soil walls, bump the allowance up to 10–15%. Running out of concrete mid-pour creates a cold joint and a weak point, so it is always safer to have one extra bag than to stop a pour short.
Is it cheaper to use bagged concrete or order ready-mix?
Bagged concrete is usually cheaper and more convenient for small projects under about 1 cubic yard (27 cubic feet), such as post footings, a small pad, or repairs. Above roughly 1 cubic yard, the labor of mixing dozens of bags by hand and the higher per-yard cost of bags make ready-mix delivery the better value, even after the delivery fee and any short-load surcharge. This calculator gives you the bag count so you can price both options and compare them directly.