Concrete Bag Calculator
How many 40, 50, 60 or 80 lb bags of pre-mix your pour needs, and what one bag covers.
Quick estimating rule of thumb
For concrete bags: area (sq ft) × thickness (in) ÷ 12 = cubic feet, then divide by the bag yield: 0.6 ft³ for an 80 lb bag, 0.45 for a 60 lb, 0.3 for a 40 lb. A cubic yard is 45 bags of 80 lb. Past about 25 bags, ready-mix is cheaper and far less work. Add 10% so the pour does not run short.
This concrete bag calculator answers the question you actually have in the aisle: how many bags. Enter what you are pouring and it returns a bag count for 40, 50, 60 and 80 lb pre-mix, with the cubic yards shown underneath for when you need to compare against a ready-mix quote.
Switch to Bags I have to run it backwards. Tell it how many bags are already in the truck and it works out how far they go at your slab thickness.
For the full pour including depth and sub-base guidance, use the concrete calculator, the slab calculator, or the post hole calculator.
What each bag of concrete yields
Pre-mix is sold by the weight of the dry mix, not the volume of wet concrete it makes, which is why bag maths trips people up. An 80 lb bag yields about 0.60 cubic feet, a 60 lb bag about 0.45, a 50 lb bag about 0.375, and a 40 lb bag about 0.30. Those are the Quikrete and Sakrete published figures and the ones this calculator uses.
Divide 27 by the yield and you get bags per cubic yard: 45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb, or 90 bags of 40 lb. If a supplier quotes you a yard and you are buying bags, those are the numbers to hold them to. The bags per cubic yard page carries the same table for mulch, soil and sand.
How far one bag goes
At 4 inches thick (the standard for a patio, walkway or shed floor), one 80 lb bag covers about 1.8 square feet. A 60 lb bag covers about 1.35 square feet. That is a square roughly 16 inches on a side per bag, which is why slabs eat bags so fast: a modest 8 ft × 8 ft pad at 4 inches is 64 square feet and takes about 36 bags of 80 lb.
At 6 inches thick those figures drop to about 1.2 square feet per 80 lb bag. Footings and post holes are usually quoted per hole instead: a 10-inch hole 3 feet deep takes roughly 1.6 cubic feet, about three 80 lb bags.
When to stop buying bags
The crossover is around half a cubic yard, or roughly 22 to 25 bags of 80 lb. Below that, bags win on convenience and there is no short-load fee. Above it, three things turn against you at once: cost per yard climbs well past ready-mix, the mixing time runs into hours, and every batch boundary is a potential cold joint in the finished pour.
There is also a plain physical limit. Twenty-five 80 lb bags is half a ton of material to carry, open and mix by hand. Most people underestimate that badly. If the calculator returns a number in the thirties or higher, price a ready-mix delivery or a mixing trailer before committing.
Buy spares, not exactly enough
Always add two or three bags beyond the calculated count. Sub-grade is never perfectly flat, forms bulge under wet concrete, and some mix is lost to the mixer and the wheelbarrow. Unopened bags return easily; a half-finished pour waiting on a hardware run does not. The 10% allowance in the tool covers this, and the bag count shown is already rounded up to whole bags.
Frequently asked questions
How many bags of concrete do I need?
Work out the volume in cubic feet (length × width × thickness in feet), then divide by the bag yield: 0.6 for an 80 lb bag, 0.45 for a 60 lb, 0.3 for a 40 lb. A 10 ft × 10 ft slab at 4 inches is 33.3 cubic feet, about 56 bags of 80 lb.
How many 80 lb bags of concrete make a cubic yard?
About 45. An 80 lb bag yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet.
How many 60 lb bags of concrete make a cubic yard?
About 60, since a 60 lb bag yields roughly 0.45 cubic feet.
How many 40 lb bags of concrete make a cubic yard?
About 90. A 40 lb bag yields roughly 0.3 cubic feet.
How much area does one bag of concrete cover?
At 4 inches thick, about 1.8 square feet for an 80 lb bag and 1.35 square feet for a 60 lb bag. At 2 inches thick those double.
How many bags of concrete for a 10x10 slab?
At 4 inches thick a 10 ft × 10 ft slab needs about 1.23 cubic yards, or roughly 56 bags of 80 lb or 74 bags of 60 lb. That is well past the point where ready-mix is cheaper and faster.
How many bags of concrete for a fence post?
A 4-inch post in a 10-inch hole 2 feet deep needs about 0.75 cubic feet: one to two 60 lb bags, or one 80 lb bag plus a little. Most fence posts take one to three bags each depending on hole size and depth.
At how many bags should I switch to ready-mix?
Around 22 to 25 bags of 80 lb, which is about half a cubic yard. Past that, ready-mix costs less per yard, saves hours of mixing, and avoids cold joints between batches.
How much does a bag of concrete cost?
Roughly $5 to $8 for an 80 lb bag and $4 to $6 for a 60 lb in most of the US. At 45 bags per yard that works out far above the $125 to $175 per yard of ready-mix.
How much water does a bag of concrete need?
About 3 quarts for an 80 lb bag and 2.5 quarts for a 60 lb, but always follow the bag. Mixes vary and too much water permanently weakens the cured concrete.
Should I buy extra bags of concrete?
Yes, two or three spare. Forms bulge, sub-grade is uneven, and some mix is always lost. Unopened bags are easy to return; running short mid-pour is not easy to fix.
How many bags of concrete fit on a pallet?
Typically 42 to 45 bags of 80 lb, 56 to 60 of 60 lb, and about 80 of 40 lb. A full pallet of 80 lb bags weighs roughly 3,500 lb, so check your vehicle and delivery access.
Is bagged concrete as strong as ready-mix?
Standard pre-mix cures to about 4,000 psi, comparable to typical residential ready-mix, provided you follow the water ratio and mix each batch thoroughly. The risk with bags is inconsistency between batches and cold joints, not the mix itself.