Concrete bag calculator

How many bags your pour needs in 40, 60, and 80 lb sizes, side by side, with the total weight you'd carry for each. The bag count swings hard with size; the weight you move barely does.

01 How the math works

Bag count is the pour volume divided by what one bag yields, with a waste factor added and the result rounded up to whole bags. You can't buy a partial bag, so the rounding always goes up.

Those yields are the Quikrete spec figures for standard concrete mix. The volume comes from the shape: length × width × thickness for a slab, or π × radius² × depth for a round post footing or sonotube. Pick the size, divide, round up.

02 Worked example

Take four deck post footings, each a 12 in diameter tube poured 36 in deep (305 mm diameter × 914 mm deep). Each footing is about 2.36 ft³; four of them total 9.42 ft³, or 0.35 yd³ (0.265 m³).

Run that same 9.42 ft³ through all three bag sizes and the spread is the point:

• 40 lb bags: 32 bags, about 1,280 lb (581 kg) to carry.
• 60 lb bags: 21 bags, about 1,260 lb (572 kg).
• 80 lb bags: 16 bags, about 1,280 lb (581 kg).

The bag count runs from 16 to 32 — double — but the total weight lands within 20 lb of itself every time. That's not a coincidence. It's the same concrete; the bag size only decides whether you make 16 heavy trips or 32 lighter ones. Add a 10% waste factor and the 80 lb count rounds up to 18.

03 When this calculator is wrong

Bagged premix stops making sense above about 1 yd³ (0.76 m³). A cubic yard is 45 bags of 80 lb premix — 3,600 lb to lift, mix, and pour by hand inside the working time of the concrete. Above that, ready-mix is faster, costs less per yard, and pours more uniformly. The short-load fee most suppliers charge under 2 yd³ runs $50–150, recovered before you've finished mixing the second bag. The exception is access: a basement, a second-story balcony, or a backyard the truck can't reach leaves bagged as the only option, whatever the volume.

• The count rounds up per bag, and bigger bags round looser. A pour needing 15.7 bags of 80 lb becomes 16; the same pour is 31.4 bags of 40 lb, rounded to 32. Smaller bags track the real volume more tightly, so you waste less in the last bag — but you carry twice as many.
• Yield is the spec figure at the recommended water content. The 0.60 ft³ from an 80 lb bag assumes you mix to the label. Mix wetter and usable volume drops, so treat the bag count as a floor, not a ceiling.
• It assumes uniform thickness. Slabs are usually sloped slightly for drainage and forms bow outward under pour pressure. On a small slab that's a few percent more concrete than the calculation; bump the waste factor to 15% for anything under 2 yd³.

04 What to do with the result

If the job is under 1 yd³ (45 bags of 80 lb), bagged is fine and the size question is about handling, not material. Buy 80 lb for the fewest trips if you've got help and a mixer. Drop to 60 or 40 lb if you're carrying bags any distance or mixing solo — the total weight is the same either way, split into more pieces. Add a bag or two over the count; an unopened bag goes back, a short pour doesn't wait.

If the rounded count climbs past 45 bags of 80 lb, price ready-mix before you buy. That's the line where the bag math stops being the cheap option.

05 Common questions

How many 80 lb bags of concrete make a yard?

At the 0.60 ft³ yield per 80 lb bag, one cubic yard is 45 bags (27 ft³ ÷ 0.60). With a 10% waste factor, 50 bags. That's the point where most people switch to ready-mix.

What size concrete bag should I buy?

Whichever you can handle for the number of trips involved. The total weight of concrete is fixed by the volume, so 40, 60, and 80 lb bags move nearly the same load — 80 lb means fewer, heavier lifts; 40 lb means more, lighter ones. The bag count is what changes, not the tonnage.

How many bags of concrete for a 10×12 slab?

A 10 ft × 12 ft × 4 in slab is 40 ft³. With 10% waste that's 74 bags of 80 lb — about 5,920 lb to mix by hand. At that size ready-mix wins on every count; this is past the bagged threshold.

How much does an 80 lb bag of concrete cover?

A standard 80 lb bag yields 0.60 ft³ (0.0170 m³) of cured concrete. A 60 lb bag yields 0.45 ft³; a 40 lb bag yields 0.30 ft³.

Is it cheaper to buy bags or ready-mix?

Under about 1 yd³, bags win — no delivery, no short-load fee, no minimum. Above that, ready-mix is cheaper per yard even after the $50–150 short-load fee, and it saves the hours of mixing.