Post Hole Concrete Calculator
Work out how much concrete you need to set fence, gate or deck posts – per post, in total and in premix bags.
An estimate of material only. Bag yields vary between products – check the bag. Posts that carry a structure (decks, carports, gates) need footings sized for the load and ground conditions, below the frost line where the ground freezes.
About this tool
Enter the size and depth of the post holes, the gravel layer at the bottom and the size of the post. The calculator subtracts the space the post takes up and tells you how much concrete each hole needs, how much to order for all posts with a waste allowance, and how many 25 kg, 80 lb or 60 lb premix bags that is. Works for round or square holes and square or round posts, in metric or imperial units.
Frequently asked questions
Work out the volume of the hole above the gravel and subtract the post. A round hole 30 cm wide and 80 cm deep with 15 cm of gravel and a 10 × 10 cm post needs (π/4 × 0.30² − 0.10²) × 0.65 = 0.039 m³, about 39 litres or 3.2 bags of 25 kg. A 12 in hole, 36 in deep with 6 in of gravel around a 4×4 post needs about 1.75 ft³ – just under three 80 lb bags.
Quikrete’s guidance is a hole three times as wide as the post and about one third of the post’s total length deep, plus 6 in (150 mm) for a gravel base. A 4×4 post (3.5 in actual size) gets a hole about 10½–12 in wide; a 2.4 m post would be set about 80 cm deep, plus 15 cm for gravel.
A layer of tamped gravel or crushed stone lets water drain away from the end of the post, which helps timber posts last longer. Quikrete recommends 6 in (150 mm). The gravel layer does not need concrete, so the calculator subtracts it from the hole depth.
Where the ground freezes, frost can lift posts that stop above the frost line. For posts that carry a structure, such as deck or carport posts, Quikrete says the hole must go at least 6 in (150 mm) below the frost line. Frost depth depends on where you live – check local building guidance.
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