Concrete Calculator
Concrete is worked out as a volume and bought as a count of bags. Measure the pour, multiply the dimensions together to get cubic feet or cubic metres, add a waste allowance for spillage and an over-dug hole, then divide by what one bag yields — and round that up, because a part-used bag is still a whole bag on the receipt. This does all of it, and shows the rounding rather than hiding it.
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Concrete needed
1.36 yd³
Cubic metres
1.038 m³
Details
Updates as you typeEach pour reads only some of the dimensions below. The working under the result names every one it used, and says which it ignored.
Switching this restates the measurements you already typed rather than reinterpreting them, so the pour stays the same size. Cubic yards, cubic metres and every bag count are shown either way.
Slab: the longer side. Footing: the whole run of the strip, corners included, measured along its centre line. Round pours ignore this.
Slab: the shorter side. Footing: how wide the trench is. Round pours ignore this — a circle is as wide as its own diameter.
Slab: 4 in (100 mm) for a patio or shed base, 5 to 6 in (125-150 mm) where a vehicle will stand. Footing: how deep the trench is filled. Round pours ignore this.
The distance straight across the tube or the hole, not the radius. Slab and footing ignore this.
Column: how tall the finished pier stands. Post hole: how deep the hole is, from the bottom up to where the concrete stops. Slab and footing ignore this.
How many identical pours of the shape above — one slab, or twelve post holes. Whole numbers only.
Added on top for spillage, an over-dug hole and a sub-base that is not perfectly level. 10% is the usual allowance; 15% suits dug trenches and post holes, where the ground rarely matches the drawing.
Summary
Concrete needed
1.36 yd³
Concrete needed
1.36 yd³
- Concrete in the pour1.235 yd³91%
- Waste allowance0.123 yd³9%
- Cubic yards
- 1.36 yd³
- Cubic metres
- 1.038 m³
- Cubic feet
- 36.7 ft³
- 40 lb bags
- 123 bags
- 60 lb bags
- 82 bags
- 80 lb bags
- 62 bags
- 20 kg bags
- 104 bags
- 25 kg bags
- 84 bags
- A slab is worked out from its length, width and thickness. The diameter and height fields are ignored for this pour.
- A slab is measured to the inside face of the forms. Thicken the edges of a driveway or a shed base if the ground is soft — an extra 2 in around the perimeter is common, and it is not included here.
- Bags are rounded UP, never averaged: a pour needing 61.1 bags takes 62. Running short mid-pour is far worse than a spare bag — concrete placed against concrete that has already stiffened leaves a cold joint, which is a plane of weakness right through the finished work.
- Bag yields are the manufacturers’ published figures: QUIKRETE Concrete Mix No. 1101 gives 0.30, 0.45 and 0.60 cubic feet for its 40, 60 and 80 lb bags, and Tarmac Blue Circle Multi-Purpose Concrete gives 0.01 cubic metres for a 20 kg bag. The 25 kg figure is that same 0.01 scaled by mass, because Blue Circle publishes the 20 kg size only. Check the bag you are actually buying — mixes differ by a few per cent.
- Past about one cubic yard (0.75 m³) mixing by the bag stops being sensible: that is roughly sixty 80 lb bags, half a tonne of lifting, and a mix that has to stay wet and continuous. A ready-mix truck is usually both cheaper and stronger at that point.
- This is the concrete only. Reinforcing mesh, rebar, a compacted sub-base and the expansion joints a slab needs are separate materials, and none of them are in this figure.
How this is calculated
- Pouring
- Slab
- Length (as typed)
- 10.00 ft
- Width (as typed)
- 10.00 ft
- Thickness (as typed)
- 4.00 in
- Length in metres
- 3.0480 m
- Width in metres
- 3.0480 m
- Thickness in metres
- 0.1016 m
- Volume of one slab = length × width × thickness
- 0.94389 m³
- How many (slabs)
- 1
- Concrete in the pour = one × how many
- 0.94389 m³
- Waste allowance
- 10 %
- Total = pour × (1 + 10%)
- 1.03828 m³
- Cubic yards = m³ ÷ 0.764554857984
- 1.358 yd³
- Cubic feet = m³ ÷ 0.028316846592
- 36.67 ft³
- One 80 lb bag yields
- 0.6000 ft³
- Bags needed exactly = 36.667 ft³ ÷ 0.6 ft³
- 61.111 bags
- Bags to buy = rounded UP to a whole bag
- 62 bags
- Concrete purchased = 62 × 0.6 ft³
- 37.200 ft³
- Left over in the last bag
- 0.533 ft³
Waste allowance (%)
Compare scenarios
See how one change moves the result
- CurrentYour inputs as they stand1.36 yd³Current
- Length — slab and footing onlyft 12.51.70 yd³
- Width — slab and footing onlyft 12.51.70 yd³
- Thickness — slab and footing onlyin 51.70 yd³
Frequently asked questions
How many bags of concrete are in a cubic yard?
Forty-five 80 lb bags, sixty 60 lb bags, or ninety 40 lb bags. That follows straight from QUIKRETE’s published yields: an 80 lb bag makes about 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 divided by 0.60 is 45. In cubic metres, a 20 kg Blue Circle bag makes about 0.01 m³, so a cubic metre takes roughly 100 of them.
Why are the bag counts rounded up?
Because you cannot buy most of a bag, and because running short mid-pour is much worse than having one spare. Concrete placed against concrete that has already begun to stiffen forms a cold joint — a plane of weakness right through the finished slab or footing, which tends to become the crack. A calculation saying 61.1 bags means 62 bags, and the leftover 0.9 of a bag is the price of not stopping halfway.
How thick should a concrete slab be?
Four inches (100 mm) is the standard for a patio, path or shed base carrying foot traffic and garden furniture. Go to five or six inches (125–150 mm) where a car, a trailer or a heavy workshop machine will stand, and thicken the edges of a driveway further. Thickness matters more than most people expect: taking a 10 by 10 ft slab from 4 in to 6 in adds half again to the concrete and half again to the bag count.
What waste allowance should I add?
Ten per cent for anything poured into forms you built yourself, such as a slab or a column tube, where the shape is what you made it. Fifteen per cent for anything dug — footings and post holes — because a trench is always a little wider and a little deeper in places than the drawing, and the extra concrete goes in without asking. On a small pour the allowance is often less than one bag, which the rounding then absorbs anyway.
When should I stop using bags and order ready-mix?
Around one cubic yard, or about three quarters of a cubic metre. That is roughly sixty 80 lb bags: half a tonne to lift, and a mix that has to be kept going continuously so no part of it stiffens before the next batch arrives. Beyond that a ready-mix truck is usually cheaper per yard, more consistent in strength, and finishes in one pour instead of an afternoon of mixing.
Does the post itself count when filling a post hole?
It displaces concrete, so the real requirement is a little less than the figure here, which is the volume of the empty hole. A 4 by 4 post in a 10 in hole takes up about a seventh of it. Being generous is the right way to be wrong on a post hole, since the hole is usually a bit over-dug and any surplus simply crowns the top so rain runs off the post rather than into it.