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How many bags of concrete do I need?

Enter the size of the hole, slab or form to see how many bags of concrete to buy in 80, 60, 50 or 40 lb sizes, how much they weigh and what they cost. Handy for fence posts and small pours.

What are you pouring?
Units

Across the hole or tube, e.g. 8, 10 or 12 inches.

Extra for waste and spills
Bag size
80 lb bags of concrete to buy
12 bags
For 7.2 cu ft (0.27 cubic yards), with 10% extra.

Bags of concrete mix

80 lb bags (0.6 cu ft each)
12
60 lb bags (0.45 cu ft each)
16
50 lb bags (0.375 cu ft each)
20
40 lb bags (0.3 cu ft each)
24

Standard concrete mix yields. Total weight about 1,080 lb (490 kg).

Amounts

Exact volume (no extra)
0.24 cu yd
Cubic feet to order
7.2
Ready-mix order (to the next ¼ yard)
0.5 cu yd
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How to work out how many bags of concrete to buy

  1. 1

    Pick the shape. It starts on Round hole for fence posts; switch to slab or footing for pads.

  2. 2

    Enter the hole diameter and depth, and how many holes you are filling.

  3. 3

    Choose the bag size you can buy and carry: 80 lb bags are cheapest per yard, 40 lb bags are easier to lift.

  4. 4

    Read the number of bags, and add the bag price to see the total cost.

Features

  • Starts with post holes

    Fence and deck posts are the most common bag job, so a round hole with four posts is the starting point.

  • Every bag size

    Compare 80, 60, 50 and 40 lb bags at once, rounded up to whole bags.

  • Weight to haul

    See the total weight before you load a car or a small trailer.

  • Total cost

    Enter your store price per bag to see what the job costs.

Bag yields and coverage

BagYieldBags per cubic yardCovers at 4 in thick
80 lb0.6 cu ft451.8 sq ft
60 lb0.45 cu ft601.35 sq ft
50 lb0.375 cu ft721.13 sq ft
40 lb0.3 cu ft900.9 sq ft

Yields are for standard concrete mix as printed on common US bags. Always check your bag: high-strength and fast-setting mixes vary.

Post holes: concrete per hole

Hole diameter2 ft deep (80 lb bags)3 ft deep (80 lb bags)
6 in0.39 cu ft · 0.7 bags0.59 cu ft · 1 bags
8 in0.7 cu ft · 1.2 bags1.05 cu ft · 1.7 bags
10 in1.09 cu ft · 1.8 bags1.64 cu ft · 2.7 bags
12 in1.57 cu ft · 2.6 bags2.36 cu ft · 3.9 bags

Round the total up for the whole job, not hole by hole: ten 10-inch holes 3 feet deep need about 28 bags, not 30. A common rule of thumb is a hole about three times the width of the post. How deep to dig depends on the post height and your frost line, so check local guidance.

Buying and handling bags

An 80 lb bag is heavy to lift, and a car load adds up fast: 20 bags weigh 1,600 lb. Check your vehicle's payload before loading, or ask about delivery. Keep bags dry and off the ground, because damp mix starts to set in the bag. Mix with the amount of water printed on the bag: extra water makes concrete easier to pour but weaker.

Pouring more than about a cubic yard? Compare a ready-mix delivery in the full concrete calculator. For patios and driveways, the concrete slab calculator has thickness presets.

Frequently asked questions

How many bags of concrete do I need for a fence post?

A hole 10 inches across and 3 feet deep holds about 1.64 cubic feet, so it takes about 2.7 bags of 80 lb mix, or 3 bags in practice. The post itself takes up a little room, which acts as your margin.

How many 80 lb bags are in a yard of concrete?

About 45. A cubic yard is 27 cubic feet and an 80 lb bag makes about 0.6 cubic feet. With 60 lb bags you need 60, and with 40 lb bags 90.

How much area does an 80 lb bag of concrete cover?

At 4 inches thick, one 80 lb bag covers about 1.8 square feet. At 2 inches thick it covers twice that. A 60 lb bag covers about 1.35 square feet at 4 inches.

Is it cheaper to buy 80 lb or 60 lb bags?

Usually the 80 lb bag is cheaper per cubic foot, but compare the shelf prices: divide each price by the bag yield (0.6 or 0.45 cubic feet). Smaller bags can be worth it if you are carrying them a long way.

Do fast-setting post mixes yield the same?

Not always. Fast-setting mixes often come in 50 lb bags, and their yield can differ from standard concrete mix. Check the yield printed on the bag and pick the closest size here.

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