Plank & Plot

Concrete Calculator

Enter the size of your slab, footings, or post holes to see how many bags of concrete to buy in each bag size, which size costs least, and how many cubic yards to order if you go with ready-mix.

Your pour

Length
ft
in
Width
ft
in
in
Patios and walkways: usually 4 in
Slabs
1

1.23 cu yd (33 cu ft) of concrete

Extra and bag prices
%
The ready-mix industry suggests 4 to 10%. Uneven ground and hand-dug holes need the high end.

Price of one bag at your store

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$
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Leave a price blank to skip that bag size.

62 bags of 80 lb concrete

You need 1.23 cu yd (33 cu ft). With 10% extra that's 1.36 cu yd (37 cu ft).

  • 80 lb bags$43462 bags at 0.60 cu ft each, 2.48 tons (4,960 lb) in all. Cheapest at these prices.
  • 90 lb bags$45455 bags at 0.675 cu ft each, 2.48 tons (4,950 lb) in all.
  • 60 lb bags$47282 bags at 0.45 cu ft each, 2.46 tons (4,920 lb) in all.
  • 50 lb bags$51598 bags at 0.375 cu ft each, 2.45 tons (4,900 lb) in all.
  • 40 lb bags$554123 bags at 0.30 cu ft each, 2.46 tons (4,920 lb) in all.
  • Ready-mix1.36 cubic yards. Rule of thumb: over a yard, get a ready-mix quote. Ask about minimum orders and short-load fees.

Prices are examples. Change them under “Extra and bag prices” to match your store.

Show the math
  1. Volume: 10 ft × 10 ft × 4 in = 33.33 cu ft
  2. Plus 10% extra: 33.33 cu ft × 1.1 = 36.67 cu ft
  3. 80 lb bags: 36.67 cu ft ÷ 0.60 cu ft per bag = 61.11, rounded up to 62 bags
  4. Ready-mix: 36.67 cu ft ÷ 27 cu ft per cubic yard = 1.36 cu yd

How to use this concrete calculator

  1. Pick what you’re pouring. A slab covers patios, walkways, pads, and shed floors. Footing or wall is for long, narrow pours. Round column or post hole is for tube forms and dug holes.
  2. Enter the size. Lengths and widths of slabs go in feet and inches. Thickness, depth, and diameter go in inches. For a wall, enter its thickness as the width and its height as the depth.
  3. Set the count. Four identical deck footings are one entry with a count of 4.
  4. Add another pour for each different shape. The calculator adds them up before it rounds to whole bags, so you don’t pay for a partial bag on every piece.
  5. Open “Extra and bag prices” to change the waste allowance or enter the prices at your store. Leave a price blank if a size isn’t sold near you, and that size drops out of the list. The 90 lb bag is only sold in some regions.

The result lists each bag size you have a price for, starting with our pick, with the total weight you’ll be hauling. Below that is the same volume in cubic yards, in case you’d rather have a truck deliver it.

How the calculator works

Every pour is a box or a cylinder. Inches are divided by 12 to get feet, and then:

slab           = length × width × thickness
footing / wall = length × width × depth
round column   = π ÷ 4 × diameter² × depth × count
bags           = total × 1.10 ÷ yield per bag, rounded up
cubic yards    = total × 1.10 ÷ 27

The yields come from Quikrete’s data sheet for its standard Concrete Mix (No. 1101). A 40 lb bag makes about 0.30 cubic feet, a 50 lb bag 0.375, a 60 lb bag 0.45, an 80 lb bag 0.60, and a 90 lb bag 0.675. If you buy another brand, check the yield printed on the bag.

The 1.10 is a 10% allowance. The National Ready Mixed Concrete Association tells contractors to order at least 4 to 10 percent more than the calculated volume, because forms bow, ground is uneven, and some always spills. A form that’s off by 1/8 inch on a 4 inch slab is already 3 percent short. We use the top of that range because hand-dug ground and holes are rarely exact. You can lower it if your forms are square and the base is flat.

To pick a bag size, we multiply the bags of each size by its price. If two sizes come within 3% of each other, we suggest the one with fewer bags to open and mix, and the result says which size is cheapest when that isn’t the pick.

A worked example

Say you’re pouring a 10 × 12 ft patio 4 inches thick, plus four round footings for a pergola, each 12 inches across and 36 inches deep.

  • Patio: 10 × 12 × (4 ÷ 12) = 40 cubic feet
  • Footings: π ÷ 4 × 1² × 3 × 4 = 9.42 cubic feet
  • Total: 49.42 cubic feet, or 54.37 with 10% extra
  • 80 lb bags: 54.37 ÷ 0.60 = 90.6, so 91 bags
  • 60 lb bags: 54.37 ÷ 0.45 = 120.8, so 121 bags

At our example prices, 91 bags of 80 lb concrete cost $637 and 121 bags of 60 lb cost $696. Either way you’d be lifting about 7,300 pounds. The job comes to 2.01 cubic yards, which is past our rule of thumb for ready-mix. One option is to pour the footings from bags a few days early and have a truck pour the patio.

How many bags of concrete are in a yard

A cubic yard is 27 cubic feet. Divide it by the yield of one bag and you get these counts for Quikrete Concrete Mix:

Bag size Yield per bag Bags per cubic yard Total weight
40 lb 0.30 cu ft 90 3,600 lb
50 lb 0.375 cu ft 72 3,600 lb
60 lb 0.45 cu ft 60 3,600 lb
80 lb 0.60 cu ft 45 3,600 lb
90 lb 0.675 cu ft 40 3,600 lb

Every size adds up to about 3,600 pounds of dry mix per yard. The counts are exact, so add your waste allowance on top.

Concrete for common slab sizes

The cubic yards are the exact volume. The bag counts include 10% extra.

Slab Cubic yards 80 lb bags 60 lb bags
4 × 4 ft, 4 in 0.20 10 14
10 × 10 ft, 4 in 1.23 62 82
10 × 12 ft, 4 in 1.48 74 98
12 × 12 ft, 4 in 1.78 88 118
12 × 16 ft, 4 in 2.37 118 157
16 × 20 ft, 4 in 3.95 196 261
20 × 20 ft, 4 in 4.94 245 326
24 × 24 ft, 4 in 7.11 352 470
10 × 10 ft, 6 in 1.85 92 123

When ready-mix makes sense

This is our rule of thumb, not a published standard. Under half a cubic yard, about 22 bags of 80 lb mix, bags mixed by hand in a tub or wheelbarrow are the easy way. From half a yard to a full yard, bags still work, but rent a drum mixer. Quikrete says to allow at least 1 cubic foot of mixer capacity for each 80 lb bag you mix at once. Over a yard, call a ready-mix plant for a quote before you buy bags. Ask about the minimum order, any short-load fee, and how close the truck has to park.

Tips for pouring

  • Have every bag on site before you start. Quikrete warns against partial-depth fills, which leave a weak cold joint where fresh concrete meets concrete that has started to set.
  • Dampen the ground or gravel base before you pour, but don’t leave puddles. Tamp the base firm first.
  • Cut control joints about 1 inch deep every 6 to 8 feet so the slab cracks along your lines instead of its own.
  • Keep new concrete damp. Quikrete recommends curing for 5 days in weather at 70°F or warmer and 7 days between 50°F and 70°F. Protect it from freezing for the first 48 hours.
  • Wear gloves, eye protection, and a dust mask. Wet concrete can burn skin, and the dust is harmful to breathe.
  • Contact 811 a few business days before you dig footings or post holes. It’s a free service that marks buried utility lines. Ask your building department whether the slab or footings need a permit.

Frequently asked questions

How many bags of concrete do I need for a 10 × 10 slab?

At 4 inches thick, a 10 × 10 slab is 33.3 cubic feet, or 1.23 cubic yards. That takes 56 bags of 80 lb concrete or 75 bags of 60 lb concrete with nothing to spare. With 10% extra, which we recommend, buy 62 bags of 80 lb or 82 bags of 60 lb.

How many 80 lb bags of concrete make a yard?

Forty-five. A cubic yard is 27 cubic feet and an 80 lb bag of Quikrete Concrete Mix makes about 0.60 cubic feet. The same yard takes 60 bags of 60 lb, 90 bags of 40 lb, or 40 bags of 90 lb. Those counts are exact, so add a few bags for waste.

Should I buy 60 lb or 80 lb bags?

Compare the price per bag against the yield. 80 lb bags usually cost less per cubic foot, and you open and mix fewer of them. 60 lb bags are easier to lift into a cart, a truck, and a mixer. The calculator shows the cost of every size at the prices you enter.

How much concrete do I need for a fence post?

It depends on the hole, not the post. Quikrete says two 50 lb bags of its Fast-Setting Concrete Mix set a 3 inch post in a hole 9 inches across and 30 inches deep. Larger holes take more. For post holes, use the column option here, or the fence calculator, which also subtracts the space the post takes up.

How thick should a concrete slab be?

Patios, walkways, and shed floors are usually poured 4 inches thick. Quikrete Concrete Mix is made for pours at least 2 inches thick. Driveways and slabs that carry heavy loads are often thicker and may need reinforcement, so check with your building department before you pour one.

Sources

  1. Concrete Mix (No. 1101), product data sheet, Quikrete. Yield per bag: 0.30 cu ft (40 lb), 0.375 (50 lb), 0.45 (60 lb), 0.60 (80 lb), 0.675 (90 lb). Minimum 2 in thick, control joints every 6 to 8 ft, curing times.
  2. CIP 8, Discrepancies in Yield, National Ready Mixed Concrete Association. Order 4 to 10 percent more than the calculated volume; a 1/8 in error on a 4 in slab is a 3 percent shortage.
  3. Fast-Setting Concrete Mix (No. 1004), product data sheet, Quikrete. Two 50 lb bags set a 3 in post in a 9 in hole 30 in deep.
  4. 811 Before You Dig, Common Ground Alliance. Free national service; contact 811 a few business days before digging.

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